Title:
HIGH-GAIN MONITOR TO DETERMINE ELECTRO-CEREBRAL SILENCE
United States Patent 3721230
Abstract:
An apparatus for measuring physiological conditions of human and animal bodies characterized by; in addition to a preamplifier, filter, amplifier, recorder, and power supply; a common mode noise rejection circuit and a shield for the noise-sensitive components; whereby the apparatus may be employed at a patient's location, even in the presence of electrical noise. The apparatus is characterized by other improved concepts that are also disclosed; such as, front end loading of paper for the recorder; a channel selector for selecting the electrodes to be monitored; a small portable unit that includes a self-contained calibrate and test circuit to ensure correct operation regardless of the electrical noise in a given area or location; and a specific electrical schematic diagram.


Inventors:
ZIERNICKI R
Application Number:
05/091629
Publication Date:
03/20/1973
Filing Date:
11/23/1970
Assignee:
Overton III, Marvin C. (Ft. Worth, TX)
Primary Class:
Other Classes:
128/902, 600/523
International Classes:
A61B5/0476; (IPC1-7): A61B5/00
Field of Search:
128/2
View Patent Images:
US Patent References:
3572324N/A1971-03-23Petersen
3572321N/A1971-03-23Bloomfield et al.
3565060BIOPOTENTIAL SENSOR EMPLOYING INTEGRATED CIRCUITRY1971-02-23Sipple
3520295CARDIAC R-WAVE DETECTOR WITH AUTOMATIC GAIN CONTROL1970-07-14Kelley
3453554HIGH PERFORMANCE CIRCUIT INSTRUMENTATION AMPLIFIER WITH HIGH COMMON MODE REJECTION1969-07-01Shoemaker
3426150SYSTEM FOR FM TRANSMISSION OF CARDIOLOGICAL DATA OVER TELEPHONE LINES1969-02-04Tygart
3369249Electrocardiograph1968-02-13Aray et al.
3082970Electrocardiograph1963-03-26Rasmussen
2400583N/A1946-05-21White
2087935Electric switch1937-07-27Cannon
Primary Examiner:
Kamm, William E.
Claims:
What is claimed is

1. Apparatus for determining physiological conditions in human and animal bodies, said apparatus having a sensitivity 1,000 times greater than that normally necessary for electrocardiographs and sufficient to provide the criteria of death through indication of electro-cerebral silence, comprising:

2. The apparatus of claim 1 wherein said filter means comprises a twin tee notch filter.

3. The apparatus of claim 1 wherein a power supply is connected to said power conductor means; said power supply providing the requisite voltage when plugged into a conventional electrical outlet and having discrete voltage taps with decoupling means to minimize noise signals thereon.

4. The apparatus of claim 1 wherein said feedback network comprises a parallel connected resistor-capacitor network for feedback.

5. The apparatus of claim 4 wherein said preamplifier section includes a conversion means that is connected with said output conductors from said operational amplifiers and has a single second output conductor for converting said differential output of said common mode noise rejection circuit means to a single output; said single second output conductor being connected with said filter means; said conversion means comprising an additional operational amplifier having a plurality of input terminals that are connected with said output conductors from said operational amplifiers; a parallel connected resistor-capacitor network; said additional operational amplifier having its single second output terminal serially connected with one of its input terminals via said parallel connected resistor-capacitor network for feedback.

6. The apparatus of claim 5 wherein said operational amplifiers and said additional operational amplifier are formed by at least medium scale integrated circuit techniques such that at least each operational amplifier is contained on a single monolithic semiconductor chip, whereby size is reduced sufficiently that said shield means can be employed and said unit still be practical and hand portable.

7. The apparatus of claim 6 wherein said operational amplifiers and said additional operational amplifier have an input off-set voltage less than about 5 millivolt, input off-set current less than about 200 nanoampere, an input resistance greater than about 150,000 ohms, open loop gain greater than about 25,000, and a common mode noise rejection ratio greater than about 70 decibels.

8. Apparatus for determining physiological conditions in human and animal bodies, said apparatus having a sensitivity 1,000 times greater than that normally necessary for electrocardiographs and sufficient to provide the criteria of death through indication of electro-cerebral silence, comprising:

9. The apparatus of claim 8 wherein a parallel connected resistor-capacitor network is provided for each said operational amplifier; and each operational amplifier in said differential amplifier configuration has its output conductor serially connected with its inverting input terminal via said parallel connected resistor-capacitor network for feedback.

10. The apparatus of claim 8 wherein said preamplifier section includes a conversion means that is connected with said output conductors from said operational amplifiers and has a single second output conductor for converting said differential output of said common mode noise rejection circuit means to a single output; said single second output conductor being connected with said filter means.

11. The apparatus of claim 10 wherein said conversion means comprise an additional operational amplifier having a plurality of input terminals that are connected with said output terminals from said operational amplifiers; a third parallel connected resistor-capacitor network is provided; and said additional operational amplifier having its single second output conductor serially connected with one of its input terminals via said parallel connected resistor-capacitor network for feedback.

12. The apparatus of claim 11 wherein said operational amplifiers, including said additional operational amplifier, are formed by at least medium scale integrated circuit techniques such that at least each operational amplifier is contained on a single monolithic semiconductor chip, whereby size is reduced sufficiently that said shield means can be employed and said unit be practical and hand portable.

13. The apparatus of claim 12 wherein said operational amplifiers have an input off-set voltage less than about 5 millivolt, input off-set current less than about 200 nanoampere, an input resistance greater than about 150,000 ohms, open loop gain greater than about 25,000, and a common mode noise rejection ratio greater than about 70 decibels.

14. Apparatus for determining physiological conditions in human and animal bodies, said apparatus having a sensitivity 1,000 times greater than that normally necessary for electrocardiographs and sufficient to provide the criteria of death through indication of electro-cerebral silence, comprising:

15. The apparatus of claim 14 wherein said pen is a thermal stylus and said chart is a heat sensitive paper.

16. Apparatus for determining physiological conditions in human and animal bodies, said apparatus having a sensitivity 1,000 times greater than that normally necessary for electrocardiographs and sufficient to provide the criteria of death through indication of electro-cerebral silence, comprising:

17. The apparatus of claim 16 wherein a pair of short conductor means are provided with said portable unit and said channel selector means is carried by said portable unit and has its output terminals connected therewith by said pair of short conductor means.

18. The apparatus of claim 16 wherein a test switch means is connected with said input terminals of said common mode noise rejection circuit means for testing and calibrating said apparatus; and said portable unit has a self-contained calibrate and test means that has test output terminals that are connected with said test switch means.

19. The apparatus of claim 18 wherein said test switch means is also connected with said output terminals of said channel selector means for connecting said input terminals of said common mode noise rejection circuit means with either said test output terminals or said output terminals of said channel selector means.

Description:
BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to apparatus for detecting physiological conditions in human and animal bodies; as in monitoring organs connected with their nervous systems. This invention is particularly useful in detecting conditions in the human brain and in the human heart.

2. Description of the Prior Art

Several types of apparata are known in the prior art for measuring physiological conditions by measuring bioelectric disturbances on areas of the body adjacent the organ of interest, or even on the organ itself. For example, a variety of electroencephalographs (EEG's) and electrocardiographs (EKG's) are known. In these devices, primary useage was made of thermionic emission type electronic components as the active elements in a circuit network. The prior art devices used heavily shielded enclosures with several stages of high gain amplification, employing discrete vacuum tubes, to achieve the necessary amplification. Aside from the extensive shielding of the instrument, special shielding, such as metal stretchers, were employed for the patient. Special hum-free power supplies, filters and decoupling networks along with complicated ground networks were required to provide the low electrical noise environment for successful operation of prior art circuits. Even with such precautions, the electrical noise in a normal environment, such as in a room with neon lights, effected localized excitations that gave spurious readings. Accordingly, a primary need has been apparatus that could measure and respond to the very low signal levels while screening out the frequently greater noise signals in a normal environment; without having to be so careful with circumstances such as lighting and movement about a patient being tested. One of the ways in which the prior art devices have attempted to solve the problem is to employ a push-pull amplifier arrangement connected to respective electrode means and employing staged amplification, up to four stages or more, such that the signals that are in phase with each other are not amplified whereas the signals which are out of phase with each other are amplified more and more.

Such an arrangement of amplification has not been totally satisfactory, however, and has required the shielding of the patient referenced hereinbefore. With the advent of organ transplants an urgent need has arisen for a device that can be taken to a patient's location and yet have sufficient sensitivity, at the patient's location and in the presence of the noise there, to determine whether or not death has occurred. An excellent article that illustrates the need for such an EEG is "Irreversible Coma Associated with Electrocerebral Silence," Daniel Silverman, Richard L. Maslin, Mitchell G. Saunders, and Robert S. Schwab, NEUROLOGY, Vol. 20, June 1970, page 525-533. Other similar and illustrative articles include "Cerebral Death and the Electroencephalogram," Daniel Silverman, Mitchell G. Saunders, Robert S. Schwab and Richard L. Maslin in JAMA (Journal of the American Medical Association), Sept. 8, 1969, Vol. 209, No. 10; and an editorial by Irving H. Page, Editor, Walter C. Alverez, MODERN MEDICINE, Apr. 20, 1970, page 119. A sensitivity sufficient to establish the criterion of death is 1 microvolt per millimeter. The prior art devices that have had sufficient sensitivity have not been portable and have not been usable in the absence of special shielding precautions such as are available in a central room in a hospital. Moreover, with such a portable unit, it is desirable that a self-test and calibrate feature be employed on the unit to ensure its accuracy in the environment in which it is to be taken and used with a given patient. The shielding and noise elimination circuits must obviously be sufficient to enable it to be used in the environment about the patient, even if electrical noise be present.

A portable instrument which is usable both as an EEG and an EKG for monitoring either the brain, the heart, or any other organ for that matter, is highly desirable and has not been provided by the prior art.

Once the unit is made portable and taken from a central room in a hospital, several other features become important. It becomes important that the unit be conveniently loaded with paper since the paper cannot be stacked beneath the unit as in a central room in a hospital. Accordingly, it is desirable that easy loading, such as front end loading of the paper, be effected. The portable unit should have channel selectors where a plurality of electrode means can be affixed to the patient's body and connected with the unit and the output of selected pairs of electrode means monitored. Such selector switch capability is made advisable because, in a portable unit, a single pen recorder means will frequently be employed. In any event, the plurality of six or seven pens is ordinarily not practical in a portable unit and, hence, the selector switch capability is advisable. In some instances, it may be desirable to provide a plurality of plug-ins and a selector switch means that can be placed near the patient whereas the portable unit may be positioned remote therefrom to allow diagnosis with minimal distraction of the patient. It is desirable that the unit, even though portable, have the convenience features of the more elaborate central facilities such as variable chart drive speeds, and universal binding post connectors that are adaptable to conventional EEG equipment.

DESCRIPTION OF PREFERRED EMBODIMENTS

It is an object of this invention to provide apparatus for determining physiological conditions in human and animal bodies by detecting bioelectric outputs from spaced locations on the body and to provide the needs and features delineated hereinbefore and not provided by the prior art devices.

It is an object of this invention to provide a circuit configuration that accomplishes common mode noise rejection in combination with suitable shielding such that it can be employed in any location, even if electrical noise be present, for monitoring a patient.

It is a specific object of this invention to provide a portable apparatus for determining physiological conditions in response to bioelectric outputs with the common mode noise rejection and shielding sufficient to be employed at the patient's location.

A variety of other objects will become apparent to one skilled in the art when considering the following descriptive matter in combination with the drawings; an illustrative object being to provide a readily portable instrument having an overall instrument sensitivity that is continuously variable and able to yield a minimum sensitivity of one microvolt per millimeter, which is sufficiently sensitive to establish the criterion of death during patient monitoring.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of the device being employed as an electroencephalograph in accordance with one embodiment of this invention.

FIG. 2 is a partial isometric view showing the device with a separable selector switch in accordance with another embodiment of this invention.

FIG. 3 is a partial side elevational view, partly in section, illustrating the feature of the front end loading of the recorder paper in accordance with one embodiment of this invention.

FIG. 4 is an electrical schematic diagram of one embodiment of this invention.

Referring to FIG. 1, the portable unit 11 is being employed as an electroencephalograph (EEG). The portable unit 11 is a single-channel apparatus for detecting physiological conditions in a patient's body. Specifically, the portable unit 11 is connected to the scalp of the patient by a plurality of electrode means 13 for detecting bioelectric outputs from predetermined spaced locations. The predetermined locations are well known for a variety of organs and for a variety of portions of a given organ such as the brain. For example, a total of twenty electrodes and conductors may be emplaced about the head of a patient to monitor the brain. The typical placements of the electrode means are discussed in patents such as U.S. Pat. No. 2,409,033 and U.S. Pat. No. 3,195,533. While as many as ten channels may be employed, work with the portable unit of this invention has indicated that six channels are sufficient for routine observation. For example, one channel comprising a pair of electrode means may monitor the bioelectric output, or signals, between a left frontal and a left central electrode; a second channel monitors the bioelectric output between a right frontal and a right central electrode; a third channel monitor the bioelectric output between a left parietal and a left occipital electrode; a fourth channel monitor the bioelectric output between a right parietal and a right occipital electrode; a fifth channel monitor the output between a left interior temporal and a left mid temporal; and a sixth channel monitor the bioelectric output between a right anterior temporal and a right mid temporal. Each physician or user of portable unit 11 may emplace the electrode means at the locations carried by the body of the patient as he desires. Ordinarily, the electrode means 13 are emplaced on the surface of the selected locations by way of a conductive grease; such as, a silicon lubricant; and a suitable adhesive patch emplaced thereover. As illustrated in FIG. 1, the plurality of conductors 15 from the respective electrode means 13 are connected to respective receptacles 17. The receptacles are in turn connected with a selector switch means 19. The selector switch means 19 is then movable to monitor the desired pair of electrodes. The output from selector switch means 19 is fed into the preamplifier section 21, FIG. 4; and, thence, through a filter means 23 to an amplifier section 25. The output from the amplifier section 25 is sent to a recorder means 27, having a compatible stylus 29 and chart paper 31. The paper 31 will be driven at one of a plurality of variable rates with respect to time; for example, by pinch roller 33 and power roller 49 frictionally engaging the paper. Preferably, the paper 31 is held by means of a bracket in a front-end loading panel, as illustrated in FIG. 3 and described in more detail with respect thereto.

If desired, the plurality of conductors 15 from the electrode means 13 may engage receptacles 17 in a separable selector switch cabinet 35, FIG. 2. The selector switch (not shown) may then be dialed to a given pair of electrodes. The given pair of electrodes are connected with a simplified portable unit 37 by a pair of conductors in shielded coaxial cable 39.

As illustrated in FIG. 3, the paper 31 may comprise a suitable roll 41 that is connected with a panel 43 that opens to the front by way of suitable mounting bracket (not shown). The paper 31 traverses over the exterior of a writing surface 45 and beneath stylus 29. Stylus 29 may be appropriately moved by any means 47 that is responsive to the magnitude of an electrical output from the amplifier 25. For example, the stylus may be moved linearly along a rotating and reversible shaft or it may be moved in one direction against a spring force in the other, as in a galvanometer action. Such means are conventional and need not be described herein. The paper 31 passes between and in frictional engagement with pinch roller 33 and a power roller 49, which is driven at one of a plurality of speeds by a variable speed motor. The chart speed is determined by chart speed dial 51, FIG. 1. The significant element is that panel 43 may be opened from the front and allow easily replacing a roll 41 of paper 31, regardless of where the portable unit is located.

The electrical schematic diagram employed in one embodiment of this invention is illustrated in FIG. 4. The pair of conductors 53 and 55 are connected with contacts 57 and 59 of switch S1. Switch S1 is a double throw-double pole switch used to switch the preamplifier input to either the external input conductors 53 and 55 or to a built-in calibrate and test circuit 67 that is connected with contacts 61 and 63. Specifically, the contact 61 is connected with the electrical common, or ground; and the contact 63 is connected with conductor 65. Conductor 65 is, in turn, connected with the calibrate and test circuit 67. The output terminals 69 and 71 of switch S1 are serially and respectively connected with operational amplifiers A1 and A2 via conductors 73 and 75 and resistors R1 and R2. The operational amplifiers A1 and A2 are connected in differential configuration and comprise the key elements in the common mode noise rejection circuit 77. The amplifiers A1 and A2 are operationally connected with respective power sources 79 and 81. The power sources may be, for example, 15 volts direct current (DC). The output from amplifier A1 is connected with terminal 83. The terminal 83 is connected with another, or inverting, input terminal 85 of amplifier A1 via parallel connected capacitor C4 and resistor R5 for feedback. Similarly, the output of amplifier A2 is connected with terminal 87. The terminal 87 is connected to another, or inverting, input terminal 89 of amplifier A2 by parallel connected capacitor C5 and resistor R6. The input terminals 85 and 89 are serially connected via resistor R4. The terminal 83 is serially connected with input terminal 91 of amplifier A3 via coupling resistor R9 and coupling capacitor C7. The terminal 87 is serially connected with the other input terminal 93 of amplifier A3 via coupling resistor R10 and coupling capacitor C8. The amplifier A3 is a key element in the conversion means 95 serving to convert the two outputs from the differentially connected amplifiers A1 and A2 into a single ended output. The output of amplifier A3 is connected with terminal 97. The input terminal 91 and the output terminal 97 are serially connected by way of parallel coupled resistor R13 and capacitor C11. The input terminal 93 of amplifier A3 is serially connected with ground via capacitor C9 and input resistor R11. The amplifier A3 is operationally connected with power source 96.

The terminal 97 is serially connected with the filter means 23 via coupling capacitor C13. The output terminal 99 of the filter means 23 is serially connected with the input terminal 101 of amplifier A4 via resistor R17. As illustrated, the filter means 23 is a twin tee notch filter. The twin tee notch filter 23 comprises the serially connected capacitor C14 and C15 connected in parallel with serially connected resistors R14 and R15; with the juncture of capacitors C14 and C15 being connected with ground via resistor R16 and the juncture of resistors R14 and R15 connected with ground via capacitor C16. The twin tee notch filter is tuned at 60 Hertz (Hz) to eliminate "60-cycle hum." Expressed otherwise, the filter means 23 will pass useful information below about 48 Hertz and above about 70 Hertz but will reduce intermediate frequencies and will substantially eliminate the 60-cycle per second electrical noise of conventional electrical outlets. An illustrative relationship which affords an operable twin tee notch filter is:

R14 = R15 = 2R16 and C14 = C15 = 1/2 C16, where

R14 = 1/(2πfC14).

The amplifier A4 is operationally connected with power source 103. The output of amplifier A4 is connected with terminal 107. The terminal 107 is serially connected with input terminal 101 via resistor R20 for feedback. The output terminal 107 is serially connected with ground via resistor R21, capacitor C19 and gain control resistor R22. The resistor R21 is a low value, series compensation resistor for isolating the amplifier A4 output from the capacitive load presented by C19. The potentiometer R22 is an output gain control used to control the signal level applied to the recorder pen drive amplifier. Specifically, a tap wiper arm 109 is connected with a terminal 111 that is connected to the recorder pen drive amplifier, which is conventional and therefore not shown.

The power supply 113 is connected with a conventional electrical outlet via conductors 115 and 117, including serially coupled "on-off" switch 119. The power supply 113 is also connected with the ground at the juncture of capacitor C9 and R11. A suitable dropping resistor 121 and pilot light 123 are employed to indicate when power is being supplied to the power source. The power source has suitable positive and negative voltage taps 125 and 127. The taps 125 and 127 are serially connected with ground via simple respective decoupling and filtering capacitors C21 and C22 to prevent hum and electrical noise. The power supply is a solid state modular power supply providing equal magnitude supply voltages of opposite polarities on the respective voltage taps 125 and 127, as required by the operational amplifiers A1, A2, A3 and A4 and the recorder pen drive amplifier. Expressed otherwise, the taps 125 and 127 supply power to power sources 79, 81, 96. 103, etc. The power supply has a line regulation of ±0.02 percent and load regulation of ±0.05 percent. The maximum RMS ripple is limited to 500 microvolts.

The calibrate and test circuit 67 is basically an astable multivibrator using two NPN transistors T1 and T2. The resistors R23 and R24 are connected serially with the transistors T1 and T2 as collector load resistors. The RC networks R25 C21 and R26 C20 in conjunction with resistor R27 control the pulse width and frequency of the built-in test signal. The resistor R28 and potentiometer R29 act as a voltage divider and calibrator for the built-in test signal to keep its range in the low microvolt region. The resistor R29 is ordinarily internally set by being fixed in production to determine the frequency and signal level. The resultant free running multivibrator circuit 67 puts out a square wave, as a calibrate and test signal of predetermined amplitude and frequency. I have found that a 50 microvolt signal at 30 cycles per second affords an operable calibrate and test signal.

The amplifiers A1, A2, A3 and A4 are linear integrated circuit operational amplifiers, presently constructed using medium scale integration techniques. They may be fabricated using large scale integration techniques, if desired. These operational amplifiers have the properties of input offset voltage less than 5 millivolts, input offset current less than 200 nanoampere, an input resistance greater than 150,000 ohms, transient rise time response less than 1 microsecond, open loop gain greater than 25,000 and a common mode rejection ratio greater than 70 decibels. Each of the respective operational amplifiers A1, A2, A3 and A4 have respective RC networks R7 C1, R8 C2, R12 C10, R19 C17 and respective capacitors C3, C6, C12 and C18 connected with respective terminals for frequency compensation to stabilize the amplifier operating points, as recommended by the operational amplifier manufacturer. The capacitors C4, C5, C9 and C11 set the upper limit of the amplifier pass band at about 100 Hertz. The resistor R4 cross couples the input amplifiers A1 and A2 to set the common mode gain to unity and by its ratio with the resistors R4 and R6 determine the differential signal gain. I have found that a gain of about 100 is adequate for amplifiers A1 and A2 when the operational amplifier A3 provides a voltage amplification of about 100 also. The capacitors C7, C8, C13, C20 and C21 are large value; for example, at least 100 microfarads; coupling capacitors. The resistors R9, R10 and R21 are low value coupling resistors; for example, about 50 ohms; recommended by the manufacturer to avoid saturating the respective amplifiers.

The portable unit 11 has the usual ground receptacle 131 and the reference receptacle 133 in the event the unit is to be employed at a location; such as, a patient's bed where grounding is difficult or where a desired standardized reference electrode means is available. In addition to the on-off power switch 119, an external plug-in receptacle 135 is afforded if remote monitoring is desired. In addition to the chart drive speed dial 51, the stylus 29 may be set to zero by a suitable zeroing knob and rheostat 139. The gain control rheostat R22 with its wiper arm 109 is operated by the gain knob 141. The function of the switch S1 has been explained in connection with FIG. 4.

Since the portable unit will be taken into many different environments such as hospital rooms having neon lighting, a shield means is employed about the sensitive electronic components to further alleviate any problems with electrical noise.

While the operation is believed apparent from the foregoing description, the following brief summary will clarify any questions. The electrode means are connected with the patient and the portable unit at the patient's bedside without having to disturb the patient. A pair of the electrode means 13 are connected with conductors 53 and 55 via selector switch 19. The switch S1 is moved to connect terminals 57 and 59 with terminals 69 and 71 for transmitting the respective bioelectric disturbances and any noise signals that are sensed to the amplifiers A1 and A2. The noise signals that are sensed are generally of the same polarity. Because of the differential connection of amplifiers A1 and A2 and because of their input circuit configuration, the noise signals will appear on both sides of coupling resistor R4 and therefore present zero potential difference to the following amplifier A3. On the other hand, bioelectrical signals that are not in phase and not of the same polarity with respect to each other will be respectively amplified and sent to the respective input terminals 91 and 93 for conversion to a single ended output that is impressed on terminal 97. The filter means 23 removes the 60 Hz noise due to the power source at the patient's bedside. Since the useful information will ordinarily have a frequency of less than about 30 cycles per second, the filter means 23 does not remove any useful information. Accordingly, the filtered, single ended output is sent to amplifier A4. There it is suitably amplified to interface with the pen drive amplifier that is connected with terminal 111, though not shown in the electrical schematic. Accordingly, the bioelectric signals detected by emplacement of the electrode means 13 about the body of the patient are sent via their respective conductors to the receptacles and via selector switch 19 to the electronic circuit. Signals from a given pair of electrodes are converted to single ended output and the magnitude of the differential between the bioelectric outputs is recorded with respect to time on paper 31.

The fabrication of the operational amplifiers by integrated circuit techniques enable making a unit small enough that it can be shielded and yet be hand portable. Expressed otherwise, the shielding is not so bulky as to prohibit hand portability, since the electronic components, though relatively complex, are small enough to be readily shielded without adding impractical bulk and weight. The operational amplifiers operate differently electrically from single stage amplifiers with discrete components, as is well recognized in the art, having exceptionally high gain. The high gain is tempered with suitable capacitance and resistance network feedback to effect stability. When the operational amplifiers are connected in a common mode noise rejection configuration as described and illustrated herein, the noise rejection and the operational stability is exceptionally good. The resistor R4 connected with input terminals 85 and 89 makes practical common mode noise rejection and increases the noise rejection many fold.

The combination of the shielding and the differential amplifier configuration have been proven satisfactory in every environment in which the unit has been employed to date. Moreover, the unit has been tested by neurosurgeons against a standard unit employed in a central facility in a hospital and has been found to afford a sensitivity and accuracy that is as good as the elaborate central unit. Yet the portable unit is only about 11 × 11 × 14 inches and weighs less than 16 pounds for easy portability. It sits readily on a bedside stand. The calibrate and test unit 67, that is included directly in the portable unit, ensures that the technician employing the unit can check the response of the unit to a known signal, and thereby monitor for the presence of spurious signals such as would be induced by electrical noise. Medical technicians have employed the unit and have pronounced it acceptable in accuracy in a wide variety of different environments. They have agreed that it affords a satisfactory output with sufficient sensitivity to determine death even when carried to a patient's bedside. Expressed otherwise, the portable unit has sufficient sensitivity that it can effect a millimeter of pen travel for a bioelectric response of one microvolt.

Once the principle of the invention is made clear to one skilled in the art, a variety of different embodiments and structures will become apparent. For example, if desired, a recorder means having a plurality of pens and pen drive amplifiers may be employed. Ordinarily, a single pen recorder has advantages in decreasing size and increasing the portability of the unit.

From the foregoing it can be seen that this invention accomplishes the objects set forth hereinbefore and provides the features indicated to be desirable and not heretofore provided by the prior art devices. Specifically, this invention provides a unit that has shielding and amplifiers connected in a differential configuration such that it can be employed in environments in the presence of electrical noises that prohibit use of prior art devices for monitoring bioelectric output from a plurality of locations on the body of the patient. Moreover, this invention provides a unit that is small and portable and can be readily taken to the location of the patient with the resultant convenience afforded thereby. The unit, even though portable, provides a sensitivity sufficient to establish the criterion of death, even in the presence of electrical noises that make prohibitive operation of prior art devices in such an environment. Moreover, the unit has convenience features to facilitate its use in the plurality of environments and at a plurality of locations; typical of such convenience features is the front end loading of the paper for the recorder means. In addition, the unit provides all of the features that have been employed heretofore and are desirable.

Although the invention has been described with a certain degree of particularity, it is understood that the present disclosure is made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and the scope of the invention.