Title:
ELECTRICALLY ISOLATED SIGNAL PATH MEANS FOR A PHYSIOLOGICAL MONITOR
United States Patent 3690313
Abstract:
A circuit for providing electrical isolation between a patient and physiological monitoring apparatus comprising an amplifier, having applied to the input thereof a physiological signal from the patient, and a first circuit branch including an isolation transformer connected between a modulator and a demodulator for connecting a d.c. voltage source of relatively low magnitude to the amplifier. Connected to the amplifier output is a second circuit branch comprising an isolation transformer connected between a modulator and a demodulator, and the output of the demodulator is coupled to the input of physiological monitoring apparatus. Each of the isolation transformers includes an extremely low capacitance between the primary and secondary windings thereof to present an extremely high reactance to low frequency, including line frequency, alternating current and each is capable of withstanding relatively high voltages without breakdown.
US Patent References:
/2673559.html
Fawcett - March 1954 - 2673559
CURRENT LIMITING CIRCUIT
Lombardi - July 1970 - 3521087
ENERGY BARRIER LIMITING TRANSFER OF ENERGY FROM ONE AREA TO ANOTHER
Flanagan et al. - September 1970 - 3527984
/3587562.html
Williams - June 1971 - 3587562
/2673559.html
Fawcett - March 1954 - 2673559
CURRENT LIMITING CIRCUIT
Lombardi - July 1970 - 3521087
ENERGY BARRIER LIMITING TRANSFER OF ENERGY FROM ONE AREA TO ANOTHER
Flanagan et al. - September 1970 - 3527984
/3587562.html
Williams - June 1971 - 3587562
Inventors:
Weppner, Benjamin H. (Snyder, NY)
Lefebvre, Leo P. (Tonawanda, NY)
Miller, Victor R. (Clarence, NY)
Lefebvre, Leo P. (Tonawanda, NY)
Miller, Victor R. (Clarence, NY)
Application Number:
05/079516
Publication Date:
09/12/1972
Filing Date:
10/09/1970
View Patent Images:
Export Citation:
Assignee:
Mennen-Greatbatch Electronics, Inc. (Clarence, NY)
Primary Class:
Other Classes:
361/42, 128/908, 361/1, 128/902
International Classes:
A61B5/0428; G01R19/18; H03F3/387; A61B5/0402; H03F3/38; A61B5/04
Field of Search:
128/2.5R,2.6B,2.8R,2.1A,2.1R 307/92 317/9AC,9B,9R
Primary Examiner:
Kamm, William E.
Claims:
We claim
1. A circuit for providing electrical isolation between a patient and physiological monitoring apparatus comprising:
2. A circuit according to claim 1 wherein each of said first and second isolation means comprises a transformer having primary and secondary windings, the capacitance between said primary and secondary windings in each transformer being the only stray capacitance providing a path for leakage currents and being sufficiently low without earth ground shielding to present an extremely high reactance to low frequency and line frequency alternating current so as not to permit a patient hazard from leakage currents at low and line frequency.
3. A circuit according to claim 1 wherein said means defining an electrical path comprises:
4. A circuit according to claim 3 wherein the frequency of said periodic signal is greater than about 1,000 c.p.s.
5. A circuit according to claim 3 wherein said first isolation means comprises a transformer having a primary winding connected to the output of said modulator means and a secondary winding connected to the input of said demodulator means, the capacitance between said primary and secondary windings being the only stray capacitance providing a path for leakage currents and being sufficiently low without earth ground shielding to present an extremely high reactance to low frequency and line frequency alternating current so as not to permit a patient hazard from leakage currents at low and line frequency.
6. A circuit according to claim 3 further including means connecting the output of said modulator means to said signal transmission path for synchronizing the operation of said signal transmission path with the operation of said electrical path.
7. A circuit according to claim 1 wherein said means defining a signal transmission path comprises:
8. A circuit according to claim 7 wherein said second isolation means comprises a transformer having a primary winding connected to the input of said demodulator means and a secondary winding connected to the output of said modulator means, the maximum capacitance between said primary and secondary windings being the only stray capacitance providing a path for leakage currents and being sufficiently low without earth ground shielding to present an extremely high reactance to low frequency and line frequency alternating current so as not to permit a patient hazard from leakage currents at low and line frequency.
9. A circuit according to claim 7 wherein the frequency of said periodic signal is greater than about 1,000 c.p.s.
10. A circuit according to claim 7 further including means connecting said modulator means and said demodulator means to said electrical path for synchronizing the operation of said modulator means and demodulator means with that of said electrical path.
11. A circuit for providing electrical isolation between a patient and physiological monitoring apparatus comprising:
12. A circuit according to claim 11 wherein the capacitance between said primary and secondary windings is the only stray capacitance providing a path for leakage currents and is sufficiently low so as not to permit a patient hazard from leakage currents at low and line frequency.
13. A circuit according to claim 11 wherein said modulators in said first and second circuit branches each operate at a frequency substantially greater than line frequency.
14. A circuit according to claim 11 further including means connecting the output of said modulator in said first circuit branch to said modulator and demodulator in said second circuit branch whereby the operation of the latter are synchronized with the frequency of modulation in said first branch.
15. A circuit according to claim 11 wherein said modulator in said second branch comprises a switching transistor connected between said modulator input and a ground or reference potential and having a control terminal connected to said first circuit branch whereby amplified physiological signals applied to the input of said modulator are chopped at a rate equal to the frequency of modulation of said modulator in said first branch.
16. A circuit according to claim 11 wherein said coupling means comprises an amplifier having an RC network connected across the input and output thereof and wherein said demodulator in said second branch comprises a transistor switch connected between said isolation transformer and said coupling means and having a control terminal connected to said modulator in said first circuit branch.
1. A circuit for providing electrical isolation between a patient and physiological monitoring apparatus comprising:
2. A circuit according to claim 1 wherein each of said first and second isolation means comprises a transformer having primary and secondary windings, the capacitance between said primary and secondary windings in each transformer being the only stray capacitance providing a path for leakage currents and being sufficiently low without earth ground shielding to present an extremely high reactance to low frequency and line frequency alternating current so as not to permit a patient hazard from leakage currents at low and line frequency.
3. A circuit according to claim 1 wherein said means defining an electrical path comprises:
4. A circuit according to claim 3 wherein the frequency of said periodic signal is greater than about 1,000 c.p.s.
5. A circuit according to claim 3 wherein said first isolation means comprises a transformer having a primary winding connected to the output of said modulator means and a secondary winding connected to the input of said demodulator means, the capacitance between said primary and secondary windings being the only stray capacitance providing a path for leakage currents and being sufficiently low without earth ground shielding to present an extremely high reactance to low frequency and line frequency alternating current so as not to permit a patient hazard from leakage currents at low and line frequency.
6. A circuit according to claim 3 further including means connecting the output of said modulator means to said signal transmission path for synchronizing the operation of said signal transmission path with the operation of said electrical path.
7. A circuit according to claim 1 wherein said means defining a signal transmission path comprises:
8. A circuit according to claim 7 wherein said second isolation means comprises a transformer having a primary winding connected to the input of said demodulator means and a secondary winding connected to the output of said modulator means, the maximum capacitance between said primary and secondary windings being the only stray capacitance providing a path for leakage currents and being sufficiently low without earth ground shielding to present an extremely high reactance to low frequency and line frequency alternating current so as not to permit a patient hazard from leakage currents at low and line frequency.
9. A circuit according to claim 7 wherein the frequency of said periodic signal is greater than about 1,000 c.p.s.
10. A circuit according to claim 7 further including means connecting said modulator means and said demodulator means to said electrical path for synchronizing the operation of said modulator means and demodulator means with that of said electrical path.
11. A circuit for providing electrical isolation between a patient and physiological monitoring apparatus comprising:
12. A circuit according to claim 11 wherein the capacitance between said primary and secondary windings is the only stray capacitance providing a path for leakage currents and is sufficiently low so as not to permit a patient hazard from leakage currents at low and line frequency.
13. A circuit according to claim 11 wherein said modulators in said first and second circuit branches each operate at a frequency substantially greater than line frequency.
14. A circuit according to claim 11 further including means connecting the output of said modulator in said first circuit branch to said modulator and demodulator in said second circuit branch whereby the operation of the latter are synchronized with the frequency of modulation in said first branch.
15. A circuit according to claim 11 wherein said modulator in said second branch comprises a switching transistor connected between said modulator input and a ground or reference potential and having a control terminal connected to said first circuit branch whereby amplified physiological signals applied to the input of said modulator are chopped at a rate equal to the frequency of modulation of said modulator in said first branch.
16. A circuit according to claim 11 wherein said coupling means comprises an amplifier having an RC network connected across the input and output thereof and wherein said demodulator in said second branch comprises a transistor switch connected between said isolation transformer and said coupling means and having a control terminal connected to said modulator in said first circuit branch.
Description:
BACKGROUND OF THE INVENTION
This invention relates to physiological monitoring apparatus and, more particularly, to electrical isolation of a patient being monitored by such apparatus.
Modern medicine is experiencing widespread use of electrical physiological monitoring apparatus wherein electrical signals derived from the patient indicative of his physiological behavior, such as electrical waveforms produced by various body organs, are displayed and processed electrically. In recent times there has developed an increased awareness of the potential hazard of electrical shock from such apparatus, and coupled with this response from doctors, patients and regulatory agencies that the necessary precautions be taken in the construction and operation of such apparatus to insure patient safety.
The primary potential electrical hazard associated with present apparatus is from leakage current, which is current applied to the patient undergoing examination either by the monitoring apparatus or an electrical fault related thereto. In particular, it is possible with some types of existing apparatus for the patient along with the equipment to become part of a ground path for electrical current from an external source or for current to leak from the apparatus to the patient. This problem is compounded in some apparatus wherein the patient is connected electrically to the equipment ground or electrical reference point.
SUMMARY OF THE INVENTION
It is, therefore, an object of this invention to provide an improved circuit for electrically isolating a patient from physiological monitoring apparatus.
It is a more particular object of this invention to provide such a circuit for protecting a patient from leakage currents from an external source or from the physiological monitoring apparatus.
It is a further object of the present invention to provide such a circuit wherein the patient's electrical activity is floated relative to the electrical ground of the physiological monitoring apparatus.
The present invention provides a circuit for electrically isolating a patient from physiological monitoring apparatus wherein physiological signals derived from the patient are amplified and transmitted through a path to the input of physiological monitoring apparatus. The circuit includes another path connecting the circuit amplifier to a d.c. voltage source of relatively low magnitude. Both paths include isolation means for isolating electrically the patient from the monitoring apparatus and from the d.c. source as well as external sources. Each of the isolation means includes an extremely low capacitance to present an extremely high reactance to low frequency and line frequency alternating current as well as being capable of withstanding relatively high voltages without breakdown.
The foregoing and additional advantages and characterizing features of the present invention will become clearly apparent from a reading of the ensuing detailed description together with the included drawing wherein.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
FIG. 1 is a block diagram of a circuit for providing electrical isolation between a patient and physiological monitoring apparatus according to the present invention;
FIG. 2 is a schematic diagram of a preferred form of the circuit of FIG. 1;
FIG. 3 is a schematic diagram of a conventional isolation transformer having an earth ground shield; and
FIG. 4 is a schematic diagram of an isolation transformer according to the present invention.
DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT
FIG. 1 shows in block diagram form a circuit 10 according to the present invention for providing electrical isolation between a patient and physiological monitoring apparatus. Circuit 10 comprises an amplifier 11 having a pair of input terminals 12, 13 and an output terminal 14. A physiological signal is obtained from a patient by electrodes attached to portions of the patient's body (not shown) and applied by means including leads 15, 16 to the amplifier input terminals 12 and 13, respectively.
Circuit 10 further comprises means defining an electrical path for connecting amplifier 11 to a d.c. voltage source of relatively low magnitude. A d.c. voltage source preferably delivering about 12 volts is designated generally at 20 in FIG. 1, one terminal of which is connected to earth ground. The other terminal of source 20 is connected by means of a lead 23 to the input terminal of a modulator circuit indicated generally at 25. The electrical path or circuit branch connecting source 20 to amplifier 11 further comprises a demodulator circuit indicated generally at 28, the output of which is connected by means of a lead 30 to amplifier 11. The path also includes isolation means in the form of isolation transformer 32 having a primary winding 33 and a secondary winding 34 for isolating electrically the patient from d.c. source 20 or any other external electrical source which might become connected or coupled to this path. Transformer primary winding 33 is connected by means of leads 35, 36 to the output of modulator 25. Similarly, transformer secondary winding 34 is connected by means of leads 37, 38 to the input of demodulator circuit 29. Transformer 32 is constructed to include an extremely low capacitance between primary and secondary without earth ground shielding to present an extremely high reactance to low frequency and line frequency alternating currents in this path as well as to be capable of withstanding relatively high voltages applied across primary and secondary windings without breakdown.
Circuit 10 further comprises means defining a signal transmission path for connecting the output of amplifier 11 to the input of physiological monitoring apparatus. Output terminal 14 of amplifier 11 is connected by means of lead 40 to the input terminal of a modulator circuit designated generally at 41 in FIG. 1. The signal transmission path further comprises a demodulator circuit 44, and the output signal from demodulator 44 is coupled by means of a lead 45 and amplifier 46 to a circuit output terminal 50 which, in turn, is connected to the input of physiological monitoring apparatus (not shown). The latter can comprise, for example, a standard chart recorder, an oscilloscope, or other similar apparatus for displaying and, in some instances, electrically processing the physiological signals. The signal transmission path or circuit branch also comprises isolation means in the form of isolation transformer 51 having a secondary winding 52 and primary winding 53. Secondary winding 52 is connected by means of leads 54 and 55 to corresponding input terminals of demodulator circuit 44. Likewise, primary winding 53 is connected by means of leads 56 and 57 to the output terminals of modulator circuit 41. Isolation transformer 51, like isolation transformer 32, is constructed to include an extremely low capacitance between primary and secondary windings without earth ground shielding to present an extremely high reactance to low frequency and line frequency alternating currents which may be present in the signal transmission path and to be capable of withstanding relatively high voltages applied across windings 52 and 53 without breakdown.
Circuit 10 further comprises means including lead 60 connecting the carrier output of modulator 25 to the signal transmission path together with means including lead 61 connecting the carrier input of demodulator 28 to another point in the signal transmission path for synchronizing the operation of the path connecting source 20 to amplifier 11 with the operation of the signal transmission path as will be described in detail presently. Bias voltage for amplifier 46 is obtained through lead 62 from d.c. source 20. It is apparent, therefore, that the electrical ground or reference level for the output signal appearing on terminal 50 is earth ground.
Fig. 2 shown in detail a preferred construction for the circuit 10 of FIG. 1. Modulator 25 comprises first and second transistors 70 and 71, the collector terminals of which are connected to corresponding ends of primary winding section 33a of isolation transformer 32. The base terminals of transistors 70, 71 are connected through resistors 72 and 73, respectively, to primary winding sections 33b and 33c, respectively, of transformer 32. The transistor emitter terminals are connected to the other sides of the winding portions 33b and 33c. A smoothing capacitor 74 is connected in parallel with winding portion 33a, and a signal or voltage level input to modulator 25 is applied to the midpoint of winding 33a by a lead 75 connected thereto and to the base terminal of transistor 70 through a resistor 76.
Source 20 delivers a d.c. voltage of a relatively low magnitude, preferably about 12 volts. When the voltage of source 20 is connected to the input of modulator 25 through lead 23 and a resistor 77, this voltage is applied from a divider comprising resistor 72 and resistor 76 to the base terminal of transistor 70 to turn that transistor on thereby causing a flow of current in one direction through primary winding portion 33a. The relative magnitudes of resistors 72 and 76 are selected to insure that the 12 volt input will turn on transistor 70. Transistor 71 initially is nonconducting. The turning on of transistor 70 and consequent flow of current through winding portion 33a, by virtue of the inductive coupling induces a flow of current in winding portions 33b and 33c. The sense of the winding portions 33b and 33c is such as to cause current flows in respective directions which turn transistor 70 off and turn transistor 71 on. The conduction of transistor 71, in turn, causes a flow of current through primary winding portion 33a in an opposite direction and again induces a flow of current in the winding portions 33b and 33c. As a result, an alternate reversal of current is produced in primary winding portion 33a which induces an alternating voltage in the transformer secondary winding 34. The frequency of alternation or oscillation is a function of the voltage input to modulator 25 and of the characteristics of transformer 32. According to this illustration, the frequency of oscillation is selected to be about 3,000 cycles per second.
Modulator circuit 25 functions to convert the d.c. voltage input to a periodic, i.e., pulsating or alternating, voltage signal available across output leads 37, 38. Circuit 25 thus functions to chop the d.c. voltage input applied thereto, and it is in this general sense that the term modulator is employed to designated circuit 25 and circuit 41 as well.
The modulated signal voltage present on secondary winding 34 of transformer 32 is applied through leads 37, 38 to the input of demodulator circuit 28. Circuit 28 includes a conventional diode rectifier network, which is connected through an RC filter network for smoothing the rectified signal to a pair of leads 85, 86 for connection to appropriate portions of amplifier 11 as will be described presently. In addition, a pair of Zener diodes can be connected in series across the output of the smoothing filter for overvoltage protection between leads 85, 86.
Physiological signals derived from electrodes attached to portions of a patient's body are applied through leads 15, 16 to the input of network 90. In addition, a reference electrode can be attached to a reference or ground potential portion of the patient's body and connected through a lead 17 to the network 90. For example, in a typical electrocardiogram examination, lead 15 would be connected to an electrode on the patient's left arm, lead 16 connected to an electrode on the patient's right arm, and lead 17 connected to an electrode on the patient's right leg. Network 90 is a protective network for circuit 10 and the monitoring apparatus, and it includes inductive and capacitive components to trap or filter out spurious radio frequency signals and further includes neon bulbs and diodes to prevent a defibrillation electrical pulse applied to the patient during cardiac arrest from damaging the equipment.
The output of network 90 is connected to the input of amplifier 11 which, according to this illustration of the present invention, comprises three differential amplifiers 93, 94 and 95. Alternatively, circuit 10 could include a five lead input wherein each lead is connected to a corresponding one of five amplifiers. The physiological signals from network 90 are applied through leads 96 and 97 to corresponding input terminals of amplifiers 93 and 94. A reference potential line for network 90 and amplifier 11 is indicated at 98, and is coupled through lead 17 to the patient's electrical reference electrode and to various other portions of circuit 10 as will presently be described.
The amplified physiological signals are available on amplifier output terminal 14 which is connected through a resistor 100 to the input of modulator 41 in the signal transmission path of circuit 10. Modulator 41 includes a transistor 101 of the MOS, field effect type having a control or gate terminal 102 which is connected through a lead 103 and a resistor 104 to lead 61 connected to secondary winding 34 of isolation transformer 32. Transistor 101 thus is turned on and off at a rate equal to the frequency of modulator 25 which, according to this illustration of the present invention is 3,000 c.p.s. A drain terminal 105 of transistor 101 is connected to resistor 100, and a source terminal 106 of transistor 101 is connected to reference potential line 98. The voltage at drain terminal 105 thus is driven to the reference level at the rate of the frequency of modulator 25 by virtue of the fact that transistor 101 is connected in controlling relation between these two voltage level points and in controlled relation to modulator 25. As a result, the amplified physiological signals are chopped or modulated by this operation of transistor 101. A substrate terminal 107 of transistor 101 is connected through a resistor 108 to a low negative voltage source.
The modulated signal is applied to primary winding 53 of transformer 51, with a resistor 109 in combination with resistor 100 functioning as a voltage divider for this signal. This signal applied to transformer winding 53 is single-ended. Due to the action of transformer 51 the signal appearing on secondary winding 52 is double-ended, that is a mirror image of the signal applied to winding 53. The double-ended signal from transformer winding 52 is applied through a resistor 111 to demodulator 44, including a transistor 112 in which the mirror portions are removed.
The base terminal of transistor 112 is connected through a resistor 113 and lead 60 to primary winding 33 of transformer 32, and as a result, transistor 112 is turned on and off at a rate equal to the frequency of modulator 25 which, according to this illustration of the present invention, is 3,000 c.p.s. The emitter terminal of transistor 112 is connected to earth ground, and the double-ended signal appearing at the collector terminal of transistor 112 is grounded at a rate equal to the modulator frequency. Transistor 112 thus is connected in controlled relation to modulator 25 and in controlling relation between transformer winding 52 and the remainder of the circuit. As a result, the signal is converted back to the single ended form and is applied through a variable, gain adjustment resistor 114 and a fixed resistor 115 to the input of amplifier 46. A network comprising resistor 116 and capacitor 117 connected across the input and output of amplifier 46 functions to remove the 3,000 c.p.s. chopping signal component. The signal appearing on output terminal 50 is a substantial replica of the amplified physiological signal at the output 14 of amplifier 11.
The output signal on terminal 50, which is connected to the input of physiological monitoring apparatus, is referenced to the equipment ground by virtue of the connection of amplifier 46 through lead 62 to d.c. source 20. It will be noted that the electrical reference potential of the patient, connected to circuit 10 by lead 17, is connected through line 98 to windings 34 and 53 of transformer 32 and 51, respectively, and hence isolated or floated from the equipment electrical grounds such as those of d.c. source 20 and of the physiological monitoring apparatus. In addition, while the three electrode system of FIG. 2 is preferred in some monitoring procedures, lead 17 can be eliminated to provide a two electrode, differential system. Isolation transformers 32 and 51 still would serve to isolate or float the patient relative to the equipment grounds and in a manner preventing the flow of leakage current.
Isolation transformers 32 and 51 each are constructed to include an extremely low capacitance between primary and secondary windings without earth ground shielding to present an extremely high reactance to low frequency and especially line frequency alternating current as well as to be capable of withstanding relatively high voltages between primary and secondary without breakdown. In particular, transformer 32 is constructed so that the capacitance between primary winding 33 and secondary winding 34 without earth ground shielding, according to the present illustration, is approximately 12 picofarads. In addition, transformer 32 is constructed to have a dielectric withstanding voltage of approximately 7,000 volts rms at 60 cycles measured between the secondary winding 34 to the primary winding 33. The dielectric withstanding voltage between primary winding portion 33a and portions 33b and 33c is about 100 volts rms. In addition, with 2 volts at 400 cycles applied to primary winding portion 33a the following voltages shall be measured: 1.0 volt plus or minus 1 percent from the midpoint of winding portion 33a to the either of the other winding terminals, 0.248 volts plus or minus 3 percent between the winding portions 33b and 33c, and 1.39 volts plus or minus 3 percent measured between the midpoint and either terminal of winding 34. These various operating characteristics of isolation transformer 32 which are required for the illustrated operation of circuit 10 are of course obtainable by means of known transformer design procedures and determined by such factors as wire size, number of turns in the windings, core material, and insulation as is readily apparent to those skilled in the transformer design art.
Isolation transformer 51 according to the present illustration is constructed to have a capacitance between the secondary 52 and the primary 53 windings without earth ground shielding of approximately 12 picofarads. The dielectric withstanding voltage is approximately 7,000 volts rms at 60 cycles between the primary and secondary windings. In addition, with 12 volts at 3 kilocycles applied to secondary winding 52, the voltage at primary winding 53 shall be about 12 volts. With 12 volts at 3 kilocycles applied to secondary winding 52 and with the primary winding 53 open circuited, the transformer input current shall equal about 6 milliamperes. As with isolation transformer 32, these characteristics of transformers 51 which are required for the illustrated operation of circuit 10 are obtainable through known transformer design techniques by controlling such factors as wire size, number of turns of the windings, core material and insulation as is readily apparent to those skilled in the transformer art.
FIGS. 3 and 4 illustrate further a distinguishing characteristic of isolation transformers 32 and 51 as compared with a common type of isolation transformer provided with an earth ground shield. In particular, FIG. 3 shows the latter type of transformer, designated 130, having primary and secondary windings 131 and 132, respectively. Transformer 130 is provided with a shield, indicated by the dashed line 133, which is electrically connected to earth ground. The stray capacitance between primary and secondary windings is represented schematically at 134. Those skilled in the art are, of course, readily familiar with stray capacitance in transformers. In transformer 130 there will exist also a stray capacitance 135 between primary winding 131 and shield 133 and a stray capacitance 136 between secondary winding 132 and shield 133.
Transformer 130 can be designed whereby capacitance 134 between primary winding 131 and secondary winding 132 has an extremely low value. As a result, transformer 130 can provide satisfactory common mode rejection. Transformer 130 is not desirable, however, for isolating a patient from leakage currents because a leakage current path exists from earth ground connected to shield 133 through either stray capacitance 135 and 136 to the corresponding transformer windings 131 or 132. In other words, were transformer 130 included in an isolation circuit with primary winding 131 connected through an amplifier and electrodes to a patient, and secondary winding 132 connected to the monitoring equipment, a leakage current path exists from earth ground connected to shield 133 through stray capacitance 135 and primary winding 131 to the patient. Therefore, although capacitance 134 between primary and secondary can be made small, the leakage current in transformer 130 having earth ground shield 133 remains a function of the capacitance between each winding and the shield, i.e., capacitances 135 and 136.
FIG. 4 shows an isolation transformer according to the present invention which has an extremely low capacitance between primary and secondary windings and no earth ground shield. For convenience in illustration, transformer 51 is shown although transformer 31 has the same characteristics as described. The stray capacitance between primary winding 53 and secondary winding 52 is designated 140 in FIG. 4. According to the present invention, this capacitance is made extremely low to present an extremely high reactance to low frequency and line frequency alternating currents. Transformer 51, and similarly transformer 31, has no earth ground shield and, consequently, there are no stray capacitances between windings 52 and 53 and earth ground. Referring now to FIG. 4, one terminal of primary winding 53 is connected through line 98 to the patient's electrical reference point, and the other terminal of primary winding 53 is connected through resistor 109 to the circuitry wherein the physiological signal is present. One terminal of secondary winding 52 is connected to earth ground, and the other terminal of winding 52 is connected through resistor 111 and other circuitry to the monitoring equipment.
The absence of an earth ground shield in the isolation transformers of the present invention results in the absence of any leakage current paths from the primary and secondary windings through stray capacitances to earth ground. The only stray capacitance providing a path for leakage current is the capacitance between primary and secondary windings, such as capacitance 140 shown in Fig. 4. According to the present invention, this capacitance is made extremely low to present an extremely high reactance to low frequency and line frequency alternating currents.
Referring now to FIG. 1, circuit 10 operates in the following manner. Physiological signals from a patient undergoing an examination are applied through leads 15, 16 and 17 to amplifier 11. Amplifier 11, in turn, is energized from d.c. source 20, which delivers an output of about 12 volts, and through the electrical path or circuit branch including modulator 25, isolation transformer 32, and demodulator 28. The d.c. voltage from source 20 is chopped or modulated in circuit 25, the frequency in this illustration being about 3,000 cycles but in any event significantly greater than 60 cycles or line frequency. Transformer 32 includes an extremely low capacitance between primary and secondary without earth ground shielding to present an extremely high reactance to any low frequency, in particular line frequency, alternating currents which might inadvertently be applied to the path. In addition, the capability of transformer 32 to withstand relatively high voltages applied across the primary and secondary windings without breakdown further precludes the possibility of any electrical shock hazard to the patient through this path.
The physiological signals from the patient then are amplified by amplifier 11 and applied to modulator 41 wherein the amplified signals are chopped or modulated at the frequency established by circuit 25. The amplitude modulated signals are transmitted through isolation transformer 51 to demodulator circuit 44, the output of which is amplified and available at output 50 for utilization by the physiological monitoring apparatus. Isolation transformer 51 includes an extremely low capacitance between primary and secondary without earth ground shielding to present an extremely high reactance to low frequency, in particular line frequency, alternating current which might inadvertently be applied to terminal 50. In addition, the capability of transformer 51 to withstand relatively high voltages applied across the primary and secondary windings without breakdown further precludes the possibility of any electrical shock hazard to the patient from this path.
Circuit 10 of the present invention thus functions to protect a patient from the hazard of leakage currents, which might otherwise be applied to the patient by the testing equipment or an electrical fault related thereto. The most probable hazard is from standard 60 cycle or line frequency alternating current which by accident might be present in either path. The relatively low, i.e., less than about 12 picofarad, capacitance between the primary and secondary windings of each of the isolation transformers 32, 51 presents an extremely high reactance to alternating current at this frequency. This protection afforded to the patient is enhanced by the fact that each transformer 32, 51 has a relatively high dielectric withstanding voltage.
A circuit 10 constructed according to this illustration of the present invention can withstand about 10 kilovolts applied to the input thereof without breakdown. In addition, when a source delivering 220 volts rms at 60 cycles is connected between amplifier input terminals 12, 13 and 17 and earth ground, the current flowing through the paths including isolation transformers 32 and 51 is less than about 5.0 microamperes. Furthermore, it has been determined that the relationship of leakage current to applied voltage appears to be linear, to values approaching the dielectric withstanding voltage of transformers 32 and 51.
Circuit 10 thus functions to isolate leakage currents from a patient which otherwise would be passed to him if the isolation transformers 32, 51 were not present. The operation of circuit 10 utilizes the isolation effect of transformers 32 and 51 along with their minimum capacitance specifications from primary to secondary without earth ground shielding. A transformer providing leakage protection must of necessity withstand high voltage differentials between primary and secondary which in the present instance are about 10,000 volts. Another advantageous characteristic of circuit 10 is its capability of operating from a d.c. voltage source of relatively low magnitude, i.e., source 20 which delivers 12 volts. This approach does not necessitate the use of 60 cycle line voltage for operation. This of course further insures that the patient will be protected from the possibility of electrical shock. Another advantageous characteristic of circuit 10 is that the electrical reference point of the patient is isolated or floated relative to the equipment electrical ground such as that of the source 20 or of the physiological monitoring apparatus connected at terminal 50. In particular and referring to FIG. 2, it will be noted that lead 17 connected to the patient's reference electrode is connected through lead 98 to the reference terminals of winding 34 in isolation transformer 32 and of winding 53 in isolation transformer 51. In other words, transformers 32 and 51 isolate or float the patient's electrical reference point from the earth ground of source 20 and of the apparatus connected to terminal 50. This same floating or isolation of the patient's reference point is provided by isolation transformers 32 and 51 in a multi-lead or differential system as previously described in connection with FIG. 2.
The modulation frequency utilized in circuit 10 according to this illustration is about 3,000 cycles per second but in any event is significantly greater than 60 cycles per second. Experimental evidence has indicated that the effect of current on the body (except for heating) decreases with increasing frequency. The selection of an operating frequency for circuit 10 is made from an inspection of the relationship between frequency and let-go current, which is defined as the highest value of current which still permits the subject to release a wire. In the frequency range around 60 cycles, this current has the lowest value, beginning to increase relatively slowly in the range between 500 and 1,000 cycles whereupon the rate of increase than becomes much steeper.
Circuit 10 of the present invention has been described with particular reference to heartbeat monitoring wherein the physiological signals are electrocardio signals. It is to be understood, however, that circuit 10 can operate effectively with other physiological signals, for example blood pressure, electroencephalo, or fetal signals.
It is therefore apparent that the present invention accomplishes its intended object. While a single embodiment of the present invention has been described in detail, this has been done by way of illustration without thought of limitation.
This invention relates to physiological monitoring apparatus and, more particularly, to electrical isolation of a patient being monitored by such apparatus.
Modern medicine is experiencing widespread use of electrical physiological monitoring apparatus wherein electrical signals derived from the patient indicative of his physiological behavior, such as electrical waveforms produced by various body organs, are displayed and processed electrically. In recent times there has developed an increased awareness of the potential hazard of electrical shock from such apparatus, and coupled with this response from doctors, patients and regulatory agencies that the necessary precautions be taken in the construction and operation of such apparatus to insure patient safety.
The primary potential electrical hazard associated with present apparatus is from leakage current, which is current applied to the patient undergoing examination either by the monitoring apparatus or an electrical fault related thereto. In particular, it is possible with some types of existing apparatus for the patient along with the equipment to become part of a ground path for electrical current from an external source or for current to leak from the apparatus to the patient. This problem is compounded in some apparatus wherein the patient is connected electrically to the equipment ground or electrical reference point.
SUMMARY OF THE INVENTION
It is, therefore, an object of this invention to provide an improved circuit for electrically isolating a patient from physiological monitoring apparatus.
It is a more particular object of this invention to provide such a circuit for protecting a patient from leakage currents from an external source or from the physiological monitoring apparatus.
It is a further object of the present invention to provide such a circuit wherein the patient's electrical activity is floated relative to the electrical ground of the physiological monitoring apparatus.
The present invention provides a circuit for electrically isolating a patient from physiological monitoring apparatus wherein physiological signals derived from the patient are amplified and transmitted through a path to the input of physiological monitoring apparatus. The circuit includes another path connecting the circuit amplifier to a d.c. voltage source of relatively low magnitude. Both paths include isolation means for isolating electrically the patient from the monitoring apparatus and from the d.c. source as well as external sources. Each of the isolation means includes an extremely low capacitance to present an extremely high reactance to low frequency and line frequency alternating current as well as being capable of withstanding relatively high voltages without breakdown.
The foregoing and additional advantages and characterizing features of the present invention will become clearly apparent from a reading of the ensuing detailed description together with the included drawing wherein.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
FIG. 1 is a block diagram of a circuit for providing electrical isolation between a patient and physiological monitoring apparatus according to the present invention;
FIG. 2 is a schematic diagram of a preferred form of the circuit of FIG. 1;
FIG. 3 is a schematic diagram of a conventional isolation transformer having an earth ground shield; and
FIG. 4 is a schematic diagram of an isolation transformer according to the present invention.
DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT
FIG. 1 shows in block diagram form a circuit 10 according to the present invention for providing electrical isolation between a patient and physiological monitoring apparatus. Circuit 10 comprises an amplifier 11 having a pair of input terminals 12, 13 and an output terminal 14. A physiological signal is obtained from a patient by electrodes attached to portions of the patient's body (not shown) and applied by means including leads 15, 16 to the amplifier input terminals 12 and 13, respectively.
Circuit 10 further comprises means defining an electrical path for connecting amplifier 11 to a d.c. voltage source of relatively low magnitude. A d.c. voltage source preferably delivering about 12 volts is designated generally at 20 in FIG. 1, one terminal of which is connected to earth ground. The other terminal of source 20 is connected by means of a lead 23 to the input terminal of a modulator circuit indicated generally at 25. The electrical path or circuit branch connecting source 20 to amplifier 11 further comprises a demodulator circuit indicated generally at 28, the output of which is connected by means of a lead 30 to amplifier 11. The path also includes isolation means in the form of isolation transformer 32 having a primary winding 33 and a secondary winding 34 for isolating electrically the patient from d.c. source 20 or any other external electrical source which might become connected or coupled to this path. Transformer primary winding 33 is connected by means of leads 35, 36 to the output of modulator 25. Similarly, transformer secondary winding 34 is connected by means of leads 37, 38 to the input of demodulator circuit 29. Transformer 32 is constructed to include an extremely low capacitance between primary and secondary without earth ground shielding to present an extremely high reactance to low frequency and line frequency alternating currents in this path as well as to be capable of withstanding relatively high voltages applied across primary and secondary windings without breakdown.
Circuit 10 further comprises means defining a signal transmission path for connecting the output of amplifier 11 to the input of physiological monitoring apparatus. Output terminal 14 of amplifier 11 is connected by means of lead 40 to the input terminal of a modulator circuit designated generally at 41 in FIG. 1. The signal transmission path further comprises a demodulator circuit 44, and the output signal from demodulator 44 is coupled by means of a lead 45 and amplifier 46 to a circuit output terminal 50 which, in turn, is connected to the input of physiological monitoring apparatus (not shown). The latter can comprise, for example, a standard chart recorder, an oscilloscope, or other similar apparatus for displaying and, in some instances, electrically processing the physiological signals. The signal transmission path or circuit branch also comprises isolation means in the form of isolation transformer 51 having a secondary winding 52 and primary winding 53. Secondary winding 52 is connected by means of leads 54 and 55 to corresponding input terminals of demodulator circuit 44. Likewise, primary winding 53 is connected by means of leads 56 and 57 to the output terminals of modulator circuit 41. Isolation transformer 51, like isolation transformer 32, is constructed to include an extremely low capacitance between primary and secondary windings without earth ground shielding to present an extremely high reactance to low frequency and line frequency alternating currents which may be present in the signal transmission path and to be capable of withstanding relatively high voltages applied across windings 52 and 53 without breakdown.
Circuit 10 further comprises means including lead 60 connecting the carrier output of modulator 25 to the signal transmission path together with means including lead 61 connecting the carrier input of demodulator 28 to another point in the signal transmission path for synchronizing the operation of the path connecting source 20 to amplifier 11 with the operation of the signal transmission path as will be described in detail presently. Bias voltage for amplifier 46 is obtained through lead 62 from d.c. source 20. It is apparent, therefore, that the electrical ground or reference level for the output signal appearing on terminal 50 is earth ground.
Fig. 2 shown in detail a preferred construction for the circuit 10 of FIG. 1. Modulator 25 comprises first and second transistors 70 and 71, the collector terminals of which are connected to corresponding ends of primary winding section 33a of isolation transformer 32. The base terminals of transistors 70, 71 are connected through resistors 72 and 73, respectively, to primary winding sections 33b and 33c, respectively, of transformer 32. The transistor emitter terminals are connected to the other sides of the winding portions 33b and 33c. A smoothing capacitor 74 is connected in parallel with winding portion 33a, and a signal or voltage level input to modulator 25 is applied to the midpoint of winding 33a by a lead 75 connected thereto and to the base terminal of transistor 70 through a resistor 76.
Source 20 delivers a d.c. voltage of a relatively low magnitude, preferably about 12 volts. When the voltage of source 20 is connected to the input of modulator 25 through lead 23 and a resistor 77, this voltage is applied from a divider comprising resistor 72 and resistor 76 to the base terminal of transistor 70 to turn that transistor on thereby causing a flow of current in one direction through primary winding portion 33a. The relative magnitudes of resistors 72 and 76 are selected to insure that the 12 volt input will turn on transistor 70. Transistor 71 initially is nonconducting. The turning on of transistor 70 and consequent flow of current through winding portion 33a, by virtue of the inductive coupling induces a flow of current in winding portions 33b and 33c. The sense of the winding portions 33b and 33c is such as to cause current flows in respective directions which turn transistor 70 off and turn transistor 71 on. The conduction of transistor 71, in turn, causes a flow of current through primary winding portion 33a in an opposite direction and again induces a flow of current in the winding portions 33b and 33c. As a result, an alternate reversal of current is produced in primary winding portion 33a which induces an alternating voltage in the transformer secondary winding 34. The frequency of alternation or oscillation is a function of the voltage input to modulator 25 and of the characteristics of transformer 32. According to this illustration, the frequency of oscillation is selected to be about 3,000 cycles per second.
Modulator circuit 25 functions to convert the d.c. voltage input to a periodic, i.e., pulsating or alternating, voltage signal available across output leads 37, 38. Circuit 25 thus functions to chop the d.c. voltage input applied thereto, and it is in this general sense that the term modulator is employed to designated circuit 25 and circuit 41 as well.
The modulated signal voltage present on secondary winding 34 of transformer 32 is applied through leads 37, 38 to the input of demodulator circuit 28. Circuit 28 includes a conventional diode rectifier network, which is connected through an RC filter network for smoothing the rectified signal to a pair of leads 85, 86 for connection to appropriate portions of amplifier 11 as will be described presently. In addition, a pair of Zener diodes can be connected in series across the output of the smoothing filter for overvoltage protection between leads 85, 86.
Physiological signals derived from electrodes attached to portions of a patient's body are applied through leads 15, 16 to the input of network 90. In addition, a reference electrode can be attached to a reference or ground potential portion of the patient's body and connected through a lead 17 to the network 90. For example, in a typical electrocardiogram examination, lead 15 would be connected to an electrode on the patient's left arm, lead 16 connected to an electrode on the patient's right arm, and lead 17 connected to an electrode on the patient's right leg. Network 90 is a protective network for circuit 10 and the monitoring apparatus, and it includes inductive and capacitive components to trap or filter out spurious radio frequency signals and further includes neon bulbs and diodes to prevent a defibrillation electrical pulse applied to the patient during cardiac arrest from damaging the equipment.
The output of network 90 is connected to the input of amplifier 11 which, according to this illustration of the present invention, comprises three differential amplifiers 93, 94 and 95. Alternatively, circuit 10 could include a five lead input wherein each lead is connected to a corresponding one of five amplifiers. The physiological signals from network 90 are applied through leads 96 and 97 to corresponding input terminals of amplifiers 93 and 94. A reference potential line for network 90 and amplifier 11 is indicated at 98, and is coupled through lead 17 to the patient's electrical reference electrode and to various other portions of circuit 10 as will presently be described.
The amplified physiological signals are available on amplifier output terminal 14 which is connected through a resistor 100 to the input of modulator 41 in the signal transmission path of circuit 10. Modulator 41 includes a transistor 101 of the MOS, field effect type having a control or gate terminal 102 which is connected through a lead 103 and a resistor 104 to lead 61 connected to secondary winding 34 of isolation transformer 32. Transistor 101 thus is turned on and off at a rate equal to the frequency of modulator 25 which, according to this illustration of the present invention is 3,000 c.p.s. A drain terminal 105 of transistor 101 is connected to resistor 100, and a source terminal 106 of transistor 101 is connected to reference potential line 98. The voltage at drain terminal 105 thus is driven to the reference level at the rate of the frequency of modulator 25 by virtue of the fact that transistor 101 is connected in controlling relation between these two voltage level points and in controlled relation to modulator 25. As a result, the amplified physiological signals are chopped or modulated by this operation of transistor 101. A substrate terminal 107 of transistor 101 is connected through a resistor 108 to a low negative voltage source.
The modulated signal is applied to primary winding 53 of transformer 51, with a resistor 109 in combination with resistor 100 functioning as a voltage divider for this signal. This signal applied to transformer winding 53 is single-ended. Due to the action of transformer 51 the signal appearing on secondary winding 52 is double-ended, that is a mirror image of the signal applied to winding 53. The double-ended signal from transformer winding 52 is applied through a resistor 111 to demodulator 44, including a transistor 112 in which the mirror portions are removed.
The base terminal of transistor 112 is connected through a resistor 113 and lead 60 to primary winding 33 of transformer 32, and as a result, transistor 112 is turned on and off at a rate equal to the frequency of modulator 25 which, according to this illustration of the present invention, is 3,000 c.p.s. The emitter terminal of transistor 112 is connected to earth ground, and the double-ended signal appearing at the collector terminal of transistor 112 is grounded at a rate equal to the modulator frequency. Transistor 112 thus is connected in controlled relation to modulator 25 and in controlling relation between transformer winding 52 and the remainder of the circuit. As a result, the signal is converted back to the single ended form and is applied through a variable, gain adjustment resistor 114 and a fixed resistor 115 to the input of amplifier 46. A network comprising resistor 116 and capacitor 117 connected across the input and output of amplifier 46 functions to remove the 3,000 c.p.s. chopping signal component. The signal appearing on output terminal 50 is a substantial replica of the amplified physiological signal at the output 14 of amplifier 11.
The output signal on terminal 50, which is connected to the input of physiological monitoring apparatus, is referenced to the equipment ground by virtue of the connection of amplifier 46 through lead 62 to d.c. source 20. It will be noted that the electrical reference potential of the patient, connected to circuit 10 by lead 17, is connected through line 98 to windings 34 and 53 of transformer 32 and 51, respectively, and hence isolated or floated from the equipment electrical grounds such as those of d.c. source 20 and of the physiological monitoring apparatus. In addition, while the three electrode system of FIG. 2 is preferred in some monitoring procedures, lead 17 can be eliminated to provide a two electrode, differential system. Isolation transformers 32 and 51 still would serve to isolate or float the patient relative to the equipment grounds and in a manner preventing the flow of leakage current.
Isolation transformers 32 and 51 each are constructed to include an extremely low capacitance between primary and secondary windings without earth ground shielding to present an extremely high reactance to low frequency and especially line frequency alternating current as well as to be capable of withstanding relatively high voltages between primary and secondary without breakdown. In particular, transformer 32 is constructed so that the capacitance between primary winding 33 and secondary winding 34 without earth ground shielding, according to the present illustration, is approximately 12 picofarads. In addition, transformer 32 is constructed to have a dielectric withstanding voltage of approximately 7,000 volts rms at 60 cycles measured between the secondary winding 34 to the primary winding 33. The dielectric withstanding voltage between primary winding portion 33a and portions 33b and 33c is about 100 volts rms. In addition, with 2 volts at 400 cycles applied to primary winding portion 33a the following voltages shall be measured: 1.0 volt plus or minus 1 percent from the midpoint of winding portion 33a to the either of the other winding terminals, 0.248 volts plus or minus 3 percent between the winding portions 33b and 33c, and 1.39 volts plus or minus 3 percent measured between the midpoint and either terminal of winding 34. These various operating characteristics of isolation transformer 32 which are required for the illustrated operation of circuit 10 are of course obtainable by means of known transformer design procedures and determined by such factors as wire size, number of turns in the windings, core material, and insulation as is readily apparent to those skilled in the transformer design art.
Isolation transformer 51 according to the present illustration is constructed to have a capacitance between the secondary 52 and the primary 53 windings without earth ground shielding of approximately 12 picofarads. The dielectric withstanding voltage is approximately 7,000 volts rms at 60 cycles between the primary and secondary windings. In addition, with 12 volts at 3 kilocycles applied to secondary winding 52, the voltage at primary winding 53 shall be about 12 volts. With 12 volts at 3 kilocycles applied to secondary winding 52 and with the primary winding 53 open circuited, the transformer input current shall equal about 6 milliamperes. As with isolation transformer 32, these characteristics of transformers 51 which are required for the illustrated operation of circuit 10 are obtainable through known transformer design techniques by controlling such factors as wire size, number of turns of the windings, core material and insulation as is readily apparent to those skilled in the transformer art.
FIGS. 3 and 4 illustrate further a distinguishing characteristic of isolation transformers 32 and 51 as compared with a common type of isolation transformer provided with an earth ground shield. In particular, FIG. 3 shows the latter type of transformer, designated 130, having primary and secondary windings 131 and 132, respectively. Transformer 130 is provided with a shield, indicated by the dashed line 133, which is electrically connected to earth ground. The stray capacitance between primary and secondary windings is represented schematically at 134. Those skilled in the art are, of course, readily familiar with stray capacitance in transformers. In transformer 130 there will exist also a stray capacitance 135 between primary winding 131 and shield 133 and a stray capacitance 136 between secondary winding 132 and shield 133.
Transformer 130 can be designed whereby capacitance 134 between primary winding 131 and secondary winding 132 has an extremely low value. As a result, transformer 130 can provide satisfactory common mode rejection. Transformer 130 is not desirable, however, for isolating a patient from leakage currents because a leakage current path exists from earth ground connected to shield 133 through either stray capacitance 135 and 136 to the corresponding transformer windings 131 or 132. In other words, were transformer 130 included in an isolation circuit with primary winding 131 connected through an amplifier and electrodes to a patient, and secondary winding 132 connected to the monitoring equipment, a leakage current path exists from earth ground connected to shield 133 through stray capacitance 135 and primary winding 131 to the patient. Therefore, although capacitance 134 between primary and secondary can be made small, the leakage current in transformer 130 having earth ground shield 133 remains a function of the capacitance between each winding and the shield, i.e., capacitances 135 and 136.
FIG. 4 shows an isolation transformer according to the present invention which has an extremely low capacitance between primary and secondary windings and no earth ground shield. For convenience in illustration, transformer 51 is shown although transformer 31 has the same characteristics as described. The stray capacitance between primary winding 53 and secondary winding 52 is designated 140 in FIG. 4. According to the present invention, this capacitance is made extremely low to present an extremely high reactance to low frequency and line frequency alternating currents. Transformer 51, and similarly transformer 31, has no earth ground shield and, consequently, there are no stray capacitances between windings 52 and 53 and earth ground. Referring now to FIG. 4, one terminal of primary winding 53 is connected through line 98 to the patient's electrical reference point, and the other terminal of primary winding 53 is connected through resistor 109 to the circuitry wherein the physiological signal is present. One terminal of secondary winding 52 is connected to earth ground, and the other terminal of winding 52 is connected through resistor 111 and other circuitry to the monitoring equipment.
The absence of an earth ground shield in the isolation transformers of the present invention results in the absence of any leakage current paths from the primary and secondary windings through stray capacitances to earth ground. The only stray capacitance providing a path for leakage current is the capacitance between primary and secondary windings, such as capacitance 140 shown in Fig. 4. According to the present invention, this capacitance is made extremely low to present an extremely high reactance to low frequency and line frequency alternating currents.
Referring now to FIG. 1, circuit 10 operates in the following manner. Physiological signals from a patient undergoing an examination are applied through leads 15, 16 and 17 to amplifier 11. Amplifier 11, in turn, is energized from d.c. source 20, which delivers an output of about 12 volts, and through the electrical path or circuit branch including modulator 25, isolation transformer 32, and demodulator 28. The d.c. voltage from source 20 is chopped or modulated in circuit 25, the frequency in this illustration being about 3,000 cycles but in any event significantly greater than 60 cycles or line frequency. Transformer 32 includes an extremely low capacitance between primary and secondary without earth ground shielding to present an extremely high reactance to any low frequency, in particular line frequency, alternating currents which might inadvertently be applied to the path. In addition, the capability of transformer 32 to withstand relatively high voltages applied across the primary and secondary windings without breakdown further precludes the possibility of any electrical shock hazard to the patient through this path.
The physiological signals from the patient then are amplified by amplifier 11 and applied to modulator 41 wherein the amplified signals are chopped or modulated at the frequency established by circuit 25. The amplitude modulated signals are transmitted through isolation transformer 51 to demodulator circuit 44, the output of which is amplified and available at output 50 for utilization by the physiological monitoring apparatus. Isolation transformer 51 includes an extremely low capacitance between primary and secondary without earth ground shielding to present an extremely high reactance to low frequency, in particular line frequency, alternating current which might inadvertently be applied to terminal 50. In addition, the capability of transformer 51 to withstand relatively high voltages applied across the primary and secondary windings without breakdown further precludes the possibility of any electrical shock hazard to the patient from this path.
Circuit 10 of the present invention thus functions to protect a patient from the hazard of leakage currents, which might otherwise be applied to the patient by the testing equipment or an electrical fault related thereto. The most probable hazard is from standard 60 cycle or line frequency alternating current which by accident might be present in either path. The relatively low, i.e., less than about 12 picofarad, capacitance between the primary and secondary windings of each of the isolation transformers 32, 51 presents an extremely high reactance to alternating current at this frequency. This protection afforded to the patient is enhanced by the fact that each transformer 32, 51 has a relatively high dielectric withstanding voltage.
A circuit 10 constructed according to this illustration of the present invention can withstand about 10 kilovolts applied to the input thereof without breakdown. In addition, when a source delivering 220 volts rms at 60 cycles is connected between amplifier input terminals 12, 13 and 17 and earth ground, the current flowing through the paths including isolation transformers 32 and 51 is less than about 5.0 microamperes. Furthermore, it has been determined that the relationship of leakage current to applied voltage appears to be linear, to values approaching the dielectric withstanding voltage of transformers 32 and 51.
Circuit 10 thus functions to isolate leakage currents from a patient which otherwise would be passed to him if the isolation transformers 32, 51 were not present. The operation of circuit 10 utilizes the isolation effect of transformers 32 and 51 along with their minimum capacitance specifications from primary to secondary without earth ground shielding. A transformer providing leakage protection must of necessity withstand high voltage differentials between primary and secondary which in the present instance are about 10,000 volts. Another advantageous characteristic of circuit 10 is its capability of operating from a d.c. voltage source of relatively low magnitude, i.e., source 20 which delivers 12 volts. This approach does not necessitate the use of 60 cycle line voltage for operation. This of course further insures that the patient will be protected from the possibility of electrical shock. Another advantageous characteristic of circuit 10 is that the electrical reference point of the patient is isolated or floated relative to the equipment electrical ground such as that of the source 20 or of the physiological monitoring apparatus connected at terminal 50. In particular and referring to FIG. 2, it will be noted that lead 17 connected to the patient's reference electrode is connected through lead 98 to the reference terminals of winding 34 in isolation transformer 32 and of winding 53 in isolation transformer 51. In other words, transformers 32 and 51 isolate or float the patient's electrical reference point from the earth ground of source 20 and of the apparatus connected to terminal 50. This same floating or isolation of the patient's reference point is provided by isolation transformers 32 and 51 in a multi-lead or differential system as previously described in connection with FIG. 2.
The modulation frequency utilized in circuit 10 according to this illustration is about 3,000 cycles per second but in any event is significantly greater than 60 cycles per second. Experimental evidence has indicated that the effect of current on the body (except for heating) decreases with increasing frequency. The selection of an operating frequency for circuit 10 is made from an inspection of the relationship between frequency and let-go current, which is defined as the highest value of current which still permits the subject to release a wire. In the frequency range around 60 cycles, this current has the lowest value, beginning to increase relatively slowly in the range between 500 and 1,000 cycles whereupon the rate of increase than becomes much steeper.
Circuit 10 of the present invention has been described with particular reference to heartbeat monitoring wherein the physiological signals are electrocardio signals. It is to be understood, however, that circuit 10 can operate effectively with other physiological signals, for example blood pressure, electroencephalo, or fetal signals.
It is therefore apparent that the present invention accomplishes its intended object. While a single embodiment of the present invention has been described in detail, this has been done by way of illustration without thought of limitation.