Title:
COMPLIANCE COMPENSATED VENTILATION SYSTEM
United States Patent 3729000


Abstract:
A method and apparatus for maintaining the volume of gas delivered to a patient by a volume-limited ventilator substantially constant, regardless of changes in the delivery pressure which induce corresponding changes in the compression of the gas and the size of various machine elements in the delivery system such as the delivery tubing and the ventilator bellows. The delivery pressure to the patient is monitored and is used to compute, together with ventilator machine compliance, a compliance compensation volume, i.e., the volume trapped within the machine dead space, which is first subtracted from the measured delivery volume prior to comparison of the latter with a reference proportional to desired delivery volume, to effect ventilator machine control for terminating further volume delivery.



Inventors:
BELL S
Application Number:
05/144521
Publication Date:
04/24/1973
Filing Date:
05/18/1971
Assignee:
PURITAN BENNETT CORP,US
Primary Class:
Other Classes:
417/274
International Classes:
A61M16/00; (IPC1-7): A62B7/00
Field of Search:
128/145
View Patent Images:
US Patent References:
3633576VOLUMETRIC RESPIRATOR1972-01-11Gorsuch
3628042CONTROL SYSTEM1971-12-14Jacobus
3599633VOLUME-LIMITED VENTILATOR WITH COMPLIANCE COMPENSATOR1971-08-17Beasley



Primary Examiner:
Gaudet, Richard A.
Assistant Examiner:
Dunne G. F.
Claims:
I claim

1. A method of compliance compensation for a volume-limited ventilator, comprising the steps of:

2. A method of compliance compensation for a volume-limited ventilator, comprising the steps of:

3. A method of compliance compensation as set forth in claim 2, wherein said step of computing a compensation volume includes determining said machine compliance Cm in accordance with the relationship:

4. A method of preserving substantially constant tidal volume exchange using a volume-limited ventilator, comprising the steps of:

5. A method of compliance compensation for a volume-limited ventilator, comprising the steps of:

6. A method of preserving substantially constant tidal volume exchange using a volume-limited ventilator, comprising the steps of:

7. A method of preserving substantially constant tidal volume exchange using a volume-limited ventilator, comprising the steps of:

8. A method as set forth in claim 7, wherein said step of computing a compensation volume includes determining said machine compliance Cm in accordance with the relationship:

9. A compliance compensated ventilation system for delivering gas to a receiver, comprising:

10. A compliance compensated ventilation system as set forth in claim 9, wherein said means for terminating displacement of gas includes a comparator for comparing said actual volume with said desired volume.

11. A compliance compensated ventilation system for delivering gas to a receiver, comprising:

12. A system as set forth in claim 11, wherein said means for calculating machine compliance Cm includes:

13. A compliance compensated ventilation system for delivering gas to a receiver, comprising:

14. A system as set forth in claim 13, wherein said means for continuously measuring the rate of change of said apparent volume includes a differentiator.

15. A compliance compensated ventilation system for delivering gas to a receiver, comprising:

16. A system as set forth in claim 15, wherein said means for continuously computing includes:

17. A system as set forth in claim 16, wherein said means for measuring the rate of change of said apparent volume includes differentiation means.

Description:
BACKGROUND OF THE INVENTION

This invention relates generally to respiration systems and, more particularly, to improvements in volume-limited ventilators wherein a measured volume of gas is delivered to a patient in administering intermittent positive pressure breathing therapy and the like.

Respiration apparatus used in administering intermittent positive pressure breathing therapy and related applications is well known in the art, and it is common practice to measure at the respirator the volume of air or other gas to be delivered to the patient. However, where the delivery pressure changes, due to a change in the patient's condition or an obstruction in the delivery system, the tidal volume delivered to the patient does not remain constant, unless it is adjusted for such pressure changes. This occurs because of compression or expansion of the gas in the delivery system as well as change in the size of the delivery system components. For example, a common value for the tubing compliance of the delivery system is 5 c.c. per centimeter of water change in pressure. Thus, if the ventilator machine is set to deliver 200 c.c. of air or gas to the patient while the pressure in the delivery tubing system is 10 cm H2 O, the patient actually receives 200 c.c. less 50 c.c., or only 150 c.c. of gas. Moreover, should the patient's condition change so that 20 cm H2 O pressure are required in the delivery system, the patient would then receive a volume of 200 c.c. less 100 c.c. or only 100 c.c. of gas. Such changes in patient tidal volume can result in relatively rapid and substantial abnormalities in a patient's blood chemistry.

It will be apparent, therefore, that some of the difficult problems confronting medical personnel in administering respiration therapy have been those of accurately and reliably compensating for changes in tidal volume due to ventilator machine compliance and variations in delivery pressure. In this regard, various mechanical expedients have been developed for introducing approximate corrections to offset tidal volume errors caused by changes in delivery pressure. However, such compensation systems have not been entirely satisfactory under all operating conditions.

Hence, those concerned with the development and use of volume-limited ventilator equipment have recognized the need for relatively simple, reliable and accurate machine compliance compensation for such ventilation systems. The present invention fulfills this need.

SUMMARY OF THE INVENTION

Briefly, and in general terms, the present invention provides a new and improved method and apparatus for compliance compensation in a volume ventilation system, wherein the delivery pressure and apparent volume delivered by the ventilator machine are both monitored, the apparent volume being subsequently modified by the computed value of the volume trapped within the machine dead space, to determine the actual volume delivered to the patient, this actual volume being compared with a preselected desired volume to determine when volume delivery by the machine is to be terminated. Hence, the present invention enables anticipation of and compensation for tidal volume errors due to machine compliance and changes in delivery pressure, and automatically and continuously corrects the output of the volume ventilator so as to maintain constant volume delivery to the patient under all conditions.

In a presently preferred embodiment, by way of example, a volume generator in any suitable form, such as a collapsible bellows or a movable piston, displaces an appropriate gas, such as air, for delivery to a patient. The gas displaced by the volume generator is continuously monitored to provide instantaneous data regarding apparent volume delivered by the ventilator machine. The delivery pressure (P1) is also monitored and is multiplied by a separately computed factor representing total machine compliance (Cm) to arrive at a value of the volume at any instant trapped within the dead space of the machine. This trapped volume is subtracted from the previously monitored apparent volume to arrive at the actual volume of gas delivered by the machine to the patient. The latter value of actual volume is compared with a preselected value of desired volume to be delivered and, when the two volume values are equal, volume delivery and the inspiration cycle are terminated, as by generation of an "end inspiration" signal.

The value of machine compliance used in the computation of volume trapped within the machine may be constant or, in the case of a system having components where compliance varies as a function of delivered volume, a more complex function of machine compliance may be specially generated for purposes of providing accurate dead space volume determinations.

In addition, "lag compensation" may be derived from the rate of change of the apparent volume value to modify the actual volume prior to comparison of the latter with the preselected desired volume, so that any errors due to time delay from the initiation of the "end inspiration" signal to the actual cessation of gas flow are corrected.

The new and improved compliance compensated ventilation system of the present invention is extremely accurate and reliable in maintaining constant tidal volume exchange is patients undergoing respiration therapy and satisfies a real need in the art for such a system.

The above and other objects and advantages of the invention will become apparent from the following more detailed description, when taken in conjunction with the accompanying drawings of illustrative embodiments.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a typical volume-limited ventilator without compliance compensation;

FIG. 2 illustrates one embodiment of a volume-limited ventilator apparatus embodying features of the present invention;

FIG. 3 illustrates another embodiment of a volume-limited ventilator apparatus embodying the present invention; and

FIG. 4 is a flow diagram illustrating the steps in the new and improved method of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

One example of a volume ventilator machine of the type to which the present invention may be applied is diagramatically illustrated in FIG. 1 of the drawings. The ventilator machine is shown in its simplest form and, for more detailed descriptions of the overall operation of such systems, reference is made, by way of example, to U.S. Pat. Nos. 3,221,734; 3,368,555 and 3,385,295, which explain in detail the operation of various respirators and ventilators and their physiological effects, and which are incorporated by reference in this application by way of general background, but are not required for an understanding of the present invention.

As shown in FIG. 1, a ventilator 10 comprises a closed, outer housing 11 in which a collapsible bellows 13 has its upper open end sealed to the interior surface of the uppermost wall 12 of the housing. The interior of the bellows 13, together with the wall 12 of the housing 11, defines a chamber 14 provided with a gas supply conduit 15 adapted to be selectively closed by an outwardly operating check valve 16. As pointed out in the aforementioned patents, the conduit 15 may deliver air, air enriched with oxygen, or such gases with vaporized or nebulized medication. The chamber 14 also connects to a delivery system represented schematically by tubing 17 leading to a receiver 18 establishing fluid communication between the ventilator and a patient's respiratory system by any device known in the respiration art including, without limitation, face masks and mouthpieces. Reverse flow from the tubing 17 is prevented by a downwardly operating check valve 19.

A chamber 21 beneath the bellows 13 is periodically and cylically pressurized through an appropriate operating pressure supply conduit 22 connecting the chamber 21 to a cycling pressure source 23 of well-known construction.

The chamber 21 is provided with a vent 24 to atmosphere under the control of a conventional bellows valve 25 connected by a conduit 26 to the supply conduit 22, so that the vent 24 closes upon the application of pressure from the source 23 into the conduit 22 and opens when this operating pressure is released. Reverse flow from the chamber 21 into conduit 22 is prevented by a downwardly operating check valve 27.

Any desired volume-limiting means may be used to control the volume delivered by the ventilator 10. By way of example, the base of the bellows 13 is mechanically coupled in any suitable manner to the slider arm 30a of a potentiometer 30 across which a reference voltage from a power supply 31 has been applied. Since the physical travel of the bellows 13 along its axis within the housing 11 is directly proportional to the amount of gas displaced from the chamber 14, the mechanical position of the slider arm 30a and, hence, the electrical potential on line 32 is likewise proportional to the amount of gas displaced, or apparent volume delivered by the ventilator. The same reference voltage is also applied across a second potentiometer 33 having a slider arm 33a which is selectively adjusted by the operator in accordance with the desired volume to be delivered, and provides a corresponding output potential proportional to this preselected delivery volume on line 34. The electrical signals proportional to apparent volume and desired volume are directed over lines 32, 34, respectively, as a pair of electrical inputs to an electrical comparator 35 of well-known design. When the input voltages to the comparator 35 are equal, the comparator produces an "end inspiration" signal over line 36 to the cycling pressure source 23 which, in turn, causes removal of operating pressure from the ventilator 10 and, therefore, terminates further gas delivery to the patient.

While a bellows 13 is illustrated in FIG. 1 as the volume generator for the ventilator 10, it will be apparent that any other form of volume generator well-known in the art, such as a moving piston or the like, may be employed to displace gas for purposes of delivery to a patient, without altering the basic operation of the system shown in FIG. 1.

However, the system shown in FIG. 1 depends for its successful operation upon precise correlation between the apparent volume delivered, as measured by the travel of the bellows 13, and the actual volume delivered to the patient, and no compensation is provided for those errors caused by machine compliance and changes in delivery pressure which may introduce substantial differences between the apparent volume delivered and the actual tidal volume for the patient. Hence, selection of a desired volume, via adjustment of the potentiometer 33 by an operator, is not necessarily an accurate measure of the volume actually delivered by the ventilator to the patient under all conditions of operation.

Referring now more particularly to FIG. 2 of the drawings, there is shown a compliance compensated ventilation system characterized by increased accuracy and reliability in maintaining constant tidal volume for a patient. In the embodiment of the ventilation system illustrated in FIG. 2, reference numerals 110 through 135 designate elements corresponding to those indicated by the reference numerals 10 through 35, respectively, in the system of FIG. 1. In this regard, it will be understood that the volume generator 113, while shown for purposes of illustration as a collapsible bellows, may take the form of any suitable volume generating apparatus known in the art.

In the system of FIG. 2, the pressure P1 in the delivery system, between the volume generator 113 and the patient, is monitored by an appropriate pressure transducer 137, and the output of the transducer is directed over line 138 to any suitable network 139 for computing the dead space volume trapped within the ventilator machine 110. The dead space volume within the machine is equal to the algebraic product of the delivery pressure P1 multiplied by the total machine compliance Cm.

If the ventilation machine 110 is of a type wherein machine compliance can be considered constant, regardless of the delivered volume, then the P1 Cm compute network 139 may be simply a fixed gain amplifier. On the other hand, if the machine compliance varies with the volume displaced by the volume generator, then a more complex function of the machine compliance must first be generated prior to multiplication by the delivery pressure. This is indicated schematically by an electrical input to the compute network 139 over line 141, the latter being connected to the potentiometer tap 130a and, therefore, providing an input proportional to apparent volume delivered.

An electrical signal representing the compliance compensation volume P1 Cm is fed over line 142 as negative input to a summing junction 143 which simultaneously receives as positive input over line 132 an electrical signal representing the apparent volume delivered. The electrical output from the summing junction 143 represents the actual compensated volume delivered and is directed over line 144, as one input to the comparator 135, the other input to the comparator over line 134 being the preselected value of desired volume to be delivered to the patient. Hence, the compliance compensated ventilation system of FIG. 2 automatically and continuously corrects the output of the ventilator machine 110 so as to maintain constant volume delivery to the patient under all conditions and avoid the problems of undesired change in tidal volume.

In the embodiment of the invention shown in FIG. 3, a ventilator compensation system is shown which illustrates one example of typical subsystems for computing compliance compensation volume P1 Cm, as well as providing appropriate lag compensation to minimize any errors due to the time delay between the initiation of the "end inspiration" signal and the actual cessation of gas displacement by the volume generator. The reference numerals 210 through 244 in FIG. 3 designate elements corresponding to those designated by the reference numerals 110-144, respectively, in the embodiment of the invention shown in FIG. 2.

It can be shown that total machine compliance Cm may be expressed as follows:

Ct + Cvg = Cm

where:

Ct is the compliance of the delivery tubing; and

Cvg is the compliance of the volume generator.

It can also be shown that the compliance Ct is, for all practical purposes, substantially constant over the range of operating pressures and volumes encountered by the typical volume ventilator machine, whereas the compliance Cvg may vary as a function of volume, as in the case where a bellows or piston is the volume generator. As a result, the total machine compliance Cm may be expressed as:

Cm = K1 - K2 V

where:

K1 is a constant representing delivery tubing compliance Ct ; and

K2 is a constant multiplying apparent volume V to obtain volume generator compliance Cvg.

Referring again to FIG. 3, the full reference potential from the supply 231 is directed over line 246 as one input to a summing junction 248, the latter input being constant and representing the K1 tubing compliance constant. A second input to the summing junction 248 is received over line 249 which is the output of an amplifier 251 having a gain equal to the constant K2 and receiving as electrical input a signal over line 252 representing the apparent volume V delivered by the volume generator 213. Hence, the output from the summing junction 248 is a voltage proportional to the computed value of total machine compliance Cm in accordance with the previously described mathematical relationships. This latter voltage is directed as input to a conventional scaling amplifier 252, the output voltage from this amplifier being applied across a potentiometer 253.

The potentiometer 253 includes a slider arm 253a having its mechanical position continuously adjusted by the output of pressure transducer 237, to provide an electrical output over line 242 representing the compliance compensation volume P1 Cm. The latter signal representing compensation volume is directed to the negative input of the summing junction 243. The positive input of the summing junction 243 receives a signal over line 232 representing the apparent volume delivered and also receives at the positive input a signal over line 254 from a lag compensation network 255 responsive to rate of change of volume.

The lag compensation network 255 may take any form well-known in the control systems art and typically includes a differentiator 255a receiving as input over line 256 the signal from the potentiometer slider 230a representing apparent volume delivered. Hence, the signal to the summing junction 243 over line 254 is a function of the rate of change of the apparent volume delivered and, therefore, provides an output signal over line 244 to the comparator 245 which is already compensated for the anticipated time delay between the initiation of the "end inspiration" signal by the comparator and the actual cessation of gas flow induced by the volume generator 213.

While the embodiments of the invention shown in FIGS. 2 and 3 illustrate two basic systems for electromechanically compensating a volume ventilation system, it will be appreciated that any equivalent electrical, pneumatic, hydraulic, or other means known in the art may be substituted for the monitoring and computing subsystems shown in FIGS. 2 and 3 without in any way departing from the present invention. In this connection, the method of the present invention embodied by the apparatus of FIGS. 2 and 3 is illustrated by the flow chart shown in FIG. 4 which sets forth the various steps in the compensation process.

After volume delivery has been initiated, step I in FIG. 4 involves the process of monitoring the apparent volume delivered by the ventilator machine. Step II of the compensation method involves the process of computing the dead space volume P1 Cm trapped within the ventilator machine. In step III, the trapped volume is subtracted from the apparent volume delivered by the machine to obtain the actual volume delivered which, in step IV, is continuously compared with the preselected desired volume to be delivered to the patient and, when the two volumes are equal, volume delivery is terminated, typically by generation of an "end inspiration" signal.

If desired, an additional step II' may be included wherein the rate of change of the apparent volume delivered is measured and is used to compute a lag compensation factor which is added to the computation of actual volume in step III to compensate for anticipated time delay errors.

The method and apparatus of the present invention provides an extremely accurate and reliable means for compensating volume-limited ventilation systems so that constant tidal volume exchange is maintained in patients undergoing respiration therapy, regardless of changes in the patient's condition and in the delivery pressure of the ventilation system.

It will be apparent from the foregoing that, while particular forms of the invention have been illustrated and described, various modifications can be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited except as by the appended claims.