Compliance compensation for electronically controlled volume respirator systems

Abstract

A volume-cycled respirator for delivering a prescribed volume of gas to a patient utilizing pressure responsive apparatus to monitor the gas pressure developed at the patient interface. The apparatus produces a signal proportional to gas pressure and includes a variable gain amplifier to proportion the signal to volume. The signal is then used to compensate a volume cycled control signal for the volume of gas required to fill the interconnecting hoses and equipment between the volume measuring apparatus and the patient interface to assure that the prescribed volume of gas is delivered to the patient.

Government Interests

The invention herein described was made under a contract with the Department of the Navy, Office of Naval Research.

Description

BACKGROUND OF THE INVENTION

The present invention relates to respirators and, more particularly, to a control system for a fixed volume delivery respirator.

Forced breathing respirators are generally either of a type for providing a breathable gas to a patient at a prescribed pressure or of a type for providing a prescribed volume of breathable gas to a patient. These types of respirators are referred to respectively as pressure-cycled and volume-cycled. A third type of respirator is a time-cycled respirator in which breathable gas is made available to a patient at timed intervals. Of the foregoing three types, the volume-cycled is preferable for long-term patient ventilation since physiological changes in a patient require frequent adjustment of the pressure-cycled and time-cycled types of respirators.

Volume cycling is advantageous because a predetermined volume of gas is delivered to a patient regardless of any change in compliance of the patient. However, any change in compliance of the interconnecting hoses and equipment between the patient and the gas supply will result in a change in the volume of gas delivered to the patient. Since compliance with respect to the interconnecting hoses is defined as the change in the volume of gas in the hoses with respect to the change in pressure across the hoses, it can be seen that the volume of gas required to fill the hoses will vary with temperature and pressure thus resulting in a change in compliance. Even with a constant temperature, the volume of gas required to fill the hoses will vary as patient resistance varies and will change the amount of gas which is actually delivered to the patient. Obviously as the hose length increases the change in the volume of gas delivered to the patient increases and such change may reach a point where the patient's life is threatened by the change, particularly where the change is such as to significantly reduce the volume of gas delivered.

Accordingly, it is an object of the present invention to provide a volume-cycled respirator control system which can be calibrated to deliver a desired volume of breathable gas to a patient.

It is a further object of the present invention to provide a volume-cycled respirator control system which automatically compensates for changes in compliance of interconnecting hoses and equipment.

It is another object of the present invention to provide a volume-cycled respirator control system which automatically adjusts the volume of gas applied to the interconnecting hoses and equipment to assure that the desired volume of gas is delivered to the patient.

SUMMARY OF THE INVENTION

In accordance with the present invention, a volume-cycled respirator of the type including a gas source, an inhale valve, a flow sensor and interconnecting hoses and further including control apparatus responsive to the flow sensor for controlling the inhale valve, is provided with pressure responsive apparatus connected to monitor the gas pressure developed at the patient interface. The pressure responsive apparatus is conected to adjust the control apparatus to correct for changes in pressure such that the desired volume of gas is delivered to the patient. The pressure responsive apparatus includes a calibrating adjustment which allows initial calibration of the volume-cycled controlled apparatus to compensate for the volume of gas required to fill the interconnecting hoses and equipment.

BRIEF DESCRIPTION OF THE DRAWING

For a better understanding of the invention, reference may be had to FIG. 1 of the accompanying drawing showing a block diagram of a volume-cycled respirator control system incorporating a pressure responsive apparatus in accordance with the present invention.

DETAILED DESCRIPTION

Referring now to the drawing, there is shown a simplified representation of an electronically controlled volume-cycled respirator in which those elements not essential to an understanding of the present invention have been omitted. The respirator includes a pressurized source 10 for supplying a breathable gas through a passageway 11 to an inhale valve 12 and from inhale valve 12 through a passageway 13 to a flow sensor 14. From flow sensor 14, which provides a signal representative of the gas flow therethrough, the gas passes through a flexible interconnecting hose 15 to a patient indicated generally at 16. As is shown, interconnecting hose 15 is formed generally in a Y configuration with one arm of the Y connected to receive a gas from flow sensor 14 during an inhalation phase and the other arm of the Y connected to expel air through an exhale valve 17 to atmosphere during an exhalation phase. Check valves may be incorporated in both the inhale and exhale portions of the respirator, and other controls such as, e.g., pattern or flow rate controls may also be incorporated in the inhalation passageway of the respirator intermediate the gas source and the patient, all of the thus far mentioned elements being well known in the art.

In order to control the opening and closing of the inhale and exhale valves 12 and 17, respectively, to thereby provide a prescribed volume of gas to the patient 16, there is provided a control system connected to respond to an electrical signal from flow sensor 14, which signal is representative of the mass flow of gas per unit time passing through flow sensor 14, for controlling the operation of inhale and exhale valves 12 and 17. The signal from flow sensor 14 is directed into a linearizer 18 of a type well known in the art which converts the signal from flow sensor 14 to a voltage signal proportional to flow rate. The voltage signal or flow rate signal from linearizer 18 is supplied to an integrator 19 also of a type well known in the art which integrates the flow rate signal. Since the integral of flow rate is a volume equivalent, the amplitude of the output signal developed by flow integrator 19 is proportional to the volume of gas which has passed through flow sensor 14. In the prior art systems, the signal from a flow integrator such as flow integrator 19 would be compared to a volume set signal and when the two were equal would be utilized to close off an inhale valve and open an exhale valve on the assumption that the volume of air flowing through the flow sensor represented a constant volume of air being delivered to the patient under all given sets of temperature, pressure, and patient resistance.

In the present invention the output signal from flow integrator 19 is combined with a signal generated by a pressure responsive circuit to produce a resultant signal which is proportional to the actual amount of gas delivered to the patient. The pressure responsive circuit comprises a pressure transducer 20 and a pressure amplifier 21. Pressure transducer 20 is connected to monitor the gas pressure in interconnecting hose 15 and to provide an output signal to amplifier 21 which signal is proportional to the pressure of the gas in hose 15. Amplifier 21 includes a gain adjust 22 which provides calibration adjustment. Since hose 15 is of a semi-rigid quality and is not conducive to stretch, its geometric volume, i.e., length of hose multiplied by cross-sectional area, is substantially constant. Under such conditions and considering the small pressure range over which a gas is supplied to a patient, the well-known relationship of pressure being equal to a constant times volume is applicable. Therefore, a measurement of pressure in hose 15 will yield a signal directly proportional to the volume of gas in the hose.

The output signal developed by amplifier 21 is applied to a first input terminal of a differential amplifier 23 which differential amplifier also includes a second input terminal connected to receive the output signal from flow integrator 19. The output signal from differential amplifier 23 is applied to a first input terminal of a threshold detector 24. A second input terminal of threshold detector 24 is connected to receive a voltage from a volume set control 25 which voltage is proportional to the desired volume of gas to be supplied to patient 16. Threshold detector 24 is connected to supply control signals to inhale valve 12 and exhale valve 17 for controlling respectively the inhalation and exhalation phases of the respiration cycle of patient 16.

In operation, the system is initially calibrated by energizing source 10 and setting control 25 to cause threshold detector 24 to provide a signal to open inhale valve 12 and close exhale valve 17 such that a flow of gas is supplied to interconnecting hose 15. At this time the end of hose 15 which would normally be connected to a patient is blocked off, e.g., by placing one's hand over the end of the hose, and hose 15 is allowed to be pressurized to the full pressure of source 10. The amplified signal from flow sensor 14 as integrated by integrator 19 will represent the amount of gas necessary to fill the volume of interconnecting hose 15. Likewise, the output signal from pressure transducer 20 as amplified by pressure amplifier 21 will represent the pressure of the gas and thus be proportional to the volume of the gas in hose 15. These two signals are combined in subtractive relationship in differential amplifier 23.

A volt meter 26 is connected to monitor the amplitude of the signal produced by differential amplifier 23 as a result of the subtraction of the signals from integrator 19 and amplifier 21. Gain adjust 22 is then adjusted until the amplitude of the output signal from differential amplifier 23 is zero. A repeated series of steps of alternately blocking the hose and adjusting gain adjust 22 for calibration will result in zero reading of volt meter 26 when the input to the differential amplifier 23 from flow integrator 19 is equal to the input signal from pressure amplifier 21. At this point the volume of air which is required to fill the hoses has been compensated for. Any subsequent compliance change will result in a change of the output signal from transducer 20 in a direction to adjust the volume of gas supplied to the hoses 15 to assure that the prescribed volume is delivered to patient 16. This means that when a patient is connected to the respirator, the output signal developed by differential amplifier 23 will be directly proportional to the volume of air which is actually received by the patient independently of the interconnecting hoses. This output signal is then compared in threshold detector 24 with the volume set signal developed from set control 25. When the amplitude of the signal from differential amplifier 23 indicates that the prescribed volume of gas has been delivered to patient 16, threshold detector 24 produces a signal which is applied to inhale valve 12 to terminate the inhalation cycle and to exhale valve 17 to initiate the exhalation cycle. It is noted that suitable circuitry (not shown) is normally provided to control the duration of the exhalation cycle and to provide a reset signal to integrator 19 to initiate a new inhalation cycle.

It will thus be seen that the present invention provides a respirator in which the volume of gas actually delivered to the patient is accurately determined and may be compensated for by a pressure transducer to assure that the prescribed volume is applied to the patient regardless of interconnecting hose or equipment compliance or patient compliance.

Although the invention has been described with respect to a single embodiment, it is intended that the appended claims not be limited to the specific embodiment of the invention described, but that they cover modifications falling within the spirit and scope of the claims.

Claims

What is new and desired to be secured by Letters Patent of the United States is:

1. In a respirator including

an interconnecting hose assembly adapted to be connected to a patient and connected to an inhalation passageway and to an exhalation passageway and further including a pressurized gas source connected to said inhalation passageway and a flow control valve and flow rate sensor connected in said inhalation passageway intermediate said source and said interconnecting hose, a flow control valve in said exhalation passageway and control means responsive to said flow rate sensor to control operation of said flow control valves, the improvement comprising:

pressure responsive means connected to said interconnecting hose assembly for developing a first signal representative of a gas pressure in said interconnecting hose assembly, means for supplying said first signal to said control means for modifying the operation of said control means in proportion to gas pressure in said interconnecting hose assembly; and

an integrating circuit means connected to said flow rate sensor for developing a second signal proportional to a volume of gas passing through said flow sensor, means for supplying said second signal to said control means for controlling the operation of said control means in response to said volume of gas flowing through said flow sensor, wherein said control means responds to said second signal by controlling the operation of said flow control valves.

2. The apparatus as defined in claim 1 wherein said pressure responsive means includes:

a pressure transducer connected to said interconnecting hose assembly for producing said first signal representative of said gas pressure in said hose assembly;

differential amplifier means for combining said first signal with said second signal representative of said volume of gas flowing into said hose assembly to produce a third signal representative of a difference therebetween; and

means for comparing said third signal to a voltage proportional to said prescribed volume of gas for controlling said flow control valves.

3. The apparatus as defined in claim 2 and including a variable gain amplifier connected intermediate said pressure transducer and said differential amplifier means for adjusting the amplitude of said first signal to proportion said first signal to a volume signal.