Lung ventilator safety valve apparatus

Abstract: The lung ventilator safety valve circuit (10) includes a normally open, pulse width modulated solenoid safety valve (12) connected to a patient airway (14) of a ventilator system. A pressure sensor (34) monitors pressure in the patient airway (14), and a control unit (40) maintains the safety valve (12) closed when the pressure is not excessive. The safety valve (12) is opened when the sensed pressure equals or exceeds a maximum airway pressure threshold that can be set by an operator, or in the event of a power failure.

Description: BACKGROUND OF THE INVENTION Field of the Invention : This invention relates generally to breathing ventilators, and more particularly relates to a programmable safety circuit for a lung ventilator that provides ventilation pressure relief when ventilation pressure exceeds a set maximum pressure. Description of Related Art : Medical ventilators are generally designed to ventilate a patient's lungs with breathing gas to assist a patient in breathing when the patient is somehow unable to adequately breath without assistance. Pressure assistance can be instituted, for example, when the patient has already begun an inspiratory effort. With such a system, it is desirable to immediately increase the pressure after a breath is initiated in order to reach a target pressure. This rise in pressure causes flow to be initiated in the patient airway which supplies breathing gas to the patient's lungs. Conventional pressure controlled ventilator systems typically implement a gas flow control strategy of terminating or reducing breathing gas flow when the target pressure is reached, to limit patient airway pressure. However, such a control strategy can result in over-pressurization of the patient's lungs, particularly when high pressure gasses are used for blending the breathing gas mixture. When the over-pressurization is sustained, the patient's lungs can be subjected to excessive pressure for an entire inspiration portion of a breath cycle. When this occurs, the possibility exists that the patient will be harmed by a higher than desirable pressure in the lungs. Such overpressure. can, for example, rupture sutures or blood vessels of a patient that has recently undergone thoracic or abdominal surgery. Similarly, frail or infirm patients, such asthmatic or emphysemic patients, can also be harmed if airway pressure is excessive. Conventional ventilators have also typically limited ventilator pressure to a maximum by a pressure relief valve or safety valve with a fixed maximum rated pressure. For example, FR-A-2,573,658 describes an electronic positive expiration pressure controller in which a membrane maintains a valve normally open but an attached magnetic core within a winding can exert a force against the valve seat restricting the flow of air. A circuit controlling the electromagnet comprises a comparator in which a pressure sensor output is compared with a reference voltage, and a switching circuit. When pressure falls below a threshold, the valve is closed. In operation the valve oscillates to maintain a precise value of expiration pressure. However, such systems do not provide for a setting of the desired maximum pressure, which may be considerably lowered for infants or intensive care patients. Patient airway pressure can also be actively controlled by an exhalation valve, although in some ventilators such exhalation valves can be simply closed during inspiration in order to achieve a set maximum ventilator pressure, so that it is possible for excessive pressure buildup to occur during inspiration pressure assistance. Ventilator system malfunctions can also result in overpressurization in the event of failure of a high pressure gas delivery valve controlling introduction of one of the breathing gas components into a high. pressure blender. For example, one known piston based lung ventilator utilizes a rolling-seal piston of low inertia and low frictional resistance for delivery of breathing gas, which is mixed in the piston chamber. For mixing of the breathing gas in the piston chamber, the piston chamber has an inlet connected to the airway of the patient, and a one-way valve allows air to enter the piston chamber during the exhalation phase of the respiratory cycle. Another gas inlet to the piston chamber, controlled by a solenoid valve, allows introduction of a desired gas mixture into the piston chamber. A valve for introducing the gas mixture is opened during expiration as the piston reciprocates to a baseline position. The oxygen content of the inspired gas can also be enriched by admitting a continuous flow of oxygen into the piston chamber through another inlet. However, the gases mixed in the breathing gas are supplied at high pressure. If a valve controlling the introduction of high pressure oxygen or air fails, breathing gas can be provided to a patient at an excessive pressure. It would be therefore be desirable to provide a programmable lung ventilator safety circuit useful with virtually any type of ventilator delivering breathing gas under pressure, to limit the ventilator airway pressure to a maximum pressure appropriate for each particular patient, so that even if the ventilator should fail for some reason to provide ventilator pressure below an acceptable maximum, breathing gas will not be provided to a patient at an excessive pressure. SUMMARY OF THE INVENTION According to a first aspect of the present invention there is provided a lung ventilator safety valve apparatus for relieving pressure in a patient airway of a ventilator system, comprising: a safety valve connected to the patient airway, said safety valve having a normally open position venting pressure from said patient airway and a closed position; pressure sensor means connected to said patient airway for monitoring pressure in said patient airway and for generating a pressure signal indicative of sensed pressure in said patient airway; valve control means connected to said safety valve for controlling operation of said safety valve, and connected to said pressure sensor means for receiving said pressure signal, said valve control means including means for setting a maximum airway pressure threshold, means for comparing said sensed pressure in said patient airway with said maximum airway pressure threshold, and means for generating a valve open signal when said sensed pressure in said patient airway is equal to or greater than said maximum airway pressure; and valve switching means connected to said valve control means for receiving said valve open signal and for switching said safety valve to said open position in response to said valve open signal, characterised in that said safety valve comprises a solenoid valve biased to the open position such that said valve opens in the event of a power failure; said means for setting a maximum airway pressure threshold is such as to allow a threshold value to be selected from a plurality of such values; and said valve switching means comprises a pulse power source for providing cyclical pulses of power to said solenoid valve for controlling the closing force of the solenoid valve independently of the pressure in the patient airway such that said safety valve additionally opens when the pressure in the patient airway is greater than said closing force. Embodiments of the present invention provide a lung ventilator safety valve apparatus that can be programmed to open at any desired pressure. In one embodiment the safety valve apparatus includes a pulse width modulated solenoid valve that permits low power consumption and a low operating temperature for the valve. In a preferred embodiment of the-invention, the valve switching means comprises pulse width modulation means for modulating the duration of the pulses for moving the solenoid valve between the closed position and the open position. These and other aspects and advantages of the invention will become apparent from the following detailed description, and the accompanying drawing, which illustrates by way of example the features of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram of a lung ventilator safety valve apparatus embodying the invention. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Over-pressurization of a patient's lungs can be particularly harmful to pediatric or adult critical care patients. While conventional ventilators typically limit patient airway pressure by a pressure relief valve or safety valve with a fixed maximum rated pressure, such systems do not provide for a setting of a desired maximum pressure, which may be considerably lowered for infants or intensive care patients. It is possible for excessive pressure buildup to occur during inspiration pressure assistance in many types of ventilator systems delivering pressurized breathing gas, such as bellows, piston, and pressure solenoid type ventilators, for example, and the like. As is illustrated in the drawings, the invention is embodied in a lung ventilator safety valve circuit 10 for relieving pressure in a patient airway of a ventilator system, such as a ventilator system providing pressure supported breaths, and having a limited air supply. With reference to Fig. 1, illustrating a preferred embodiment of the invention in combination with an exemplary piston type ventilator, the lung ventilator safety valve apparatus includes a safety valve 12 connected to a patient airway 14 of a ventilator system, such as the inspiratory line 16 connected through a check valve 18 to a piston cylinder 20 of the ventilator. The safety valve is a two position spring loaded solenoid valve, in which the spring 22 of the solenoid valve urges the valve to a normally open position 24 venting pressure from the patient airway. When activated, the coil 26 of the solenoid urges the valve to a closed position 28, allowing the inspiratory line to deliver pressure support without diminution from the piston cylinder of the ventilator. The safety valve is preferably a pulse width modulated type of spring biased DC solenoid valve actuated by a pulse width modulated signal from a pulse width modulator 30, which varies the duration of the power signal provided by a power source 32, such as a DC power supply. Alternatively, the safety valve can be controlled by amplitude modulation of an AC type solenoid, or by analog control of the base of a power transistor which would control the current available to the solenoid. With the DC operated solenoid, in order to gain power consumption control, which in turn provides force control, the solenoid is provided with a pulse width modulated signal with a high enough frequency to ensure that the solenoid is not given enough time to retract. Since the power consumption of the solenoid is controlled, the safety valve can be utilized as a relief valve. When the solenoid is closed, with a given power consumption, the solenoid will generate a specific force. Once this relationship is determined for the specific solenoid, by varying the power consumption of the solenoid, it is possible to program the valve to open like a relief valve at a given pressure. The safety valve is also used in conjunction with a pressure transducer which is used to determine if airway pressure exceeds the high pressure limit set by the operator. If the airway pressure exceeds the high pressure threshold, power can be cut to the safety valve, causing the valve to open. While pulse width modulation is not necessary for this strategy, pulse width modulation can reduce power consumption of the solenoid. Solenoid valves normally require a higher power consumption to close than to remain closed, as the closing force is normally greater than the holding force. By pulse width modulation control, the power required by the solenoid can be reduced once the solenoid is closed, without affecting the sealing properties of the solenoid valve. A pressure transducer 34 is connected through a conduit 36 and a valve 38 to the inspiratory line of the patient airway for monitoring pressure in the patient airway. The pressure transducer generates a pressure signal indicative of sensed pressure in the patient airway that is received by a control unit 40 via line 42. The control unit is connected to the pulse width modulator via line 44 for controlling the duration of the power signal pulses to the solenoid valve. The control unit also includes means. such as, for example, a dial or keyboard, for setting a desired maximum airway pressure threshold appropriate for a patient receiving breath assistance from the ventilator. The control unit includes means for comparing the sensed pressure in the patient airway, indicated by the pressure signal, with the maximum airway pressure threshold, which, for example, can be set by an operator, or which can be under program control set by an operator. In one preferred embodiment, the means for setting a maximum airway pressure threshold is programmable, so that the safety valve apparatus can be made to be compatible with various types of ventilation strategies, such as APRV (Airway Pressure Relief Ventilation) for example, in which two baseline pressure levels are cycled at a fixed rate in time with a constant flow rate, and a programmed cycle of patient airway pressure is superimposed over the preset baseline pressures. When the sensed pressure in the patient airway is equal to or exceeds the maximum airway pressure, the control unit reduces the duration of the pulsed power signal, to switch the solenoid valve to its normally open position, to vent excess pressure in the patient airway to the atmosphere. In operation of the safety valve apparatus, pressure is monitored in the patient airway by the pressure transducer, to provide a pressure signal indicative of the sensed pressure in the patient airway. A maximum airway pressure threshold is set in the control unit by an operator. The control unit receives the pressure signal from the pressure transducer and compares the sensed pressure in the patient airway with the maximum airway pressure threshold. The control unit generates a valve open signal when the sensed pressure in the patient airway is equal or greater to the maximum airway pressure, and the safety valve is then switched to the open position in response to the valve open signal. In one preferred embodiment of the method, the step of switching the solenoid valve preferably comprises providing cyclical pulses of power for operating the solenoid valve, and modulating the duration of the pulses for moving the solenoid valve to the closed position. It has thus been demonstrated that the described embodiment provides a lung ventilator safety valve apparatus and a method for operating the apparatus, in which the safety valve can be set to open at a pressure appropriate for the patient receiving breath support, to provide ventilation pressure relief when airway pressure reaches the set maximum pressure. The normally open, pulse width modulated solenoid safety valve opens in the event of a power failure, and the pulse width modulation of the valve permits low valve power consumption and a low operating temperature.

Claims: A lung ventilator safety valve apparatus (10) for relieving pressure in a patient airway (14) of a ventilator system, comprising: a safety valve (12) connected to the patient airway (14), said safety valve (12) having a normally open position (24) venting pressure from said patient airway (14) and a closed position (28); pressure sensor means (34) connected to said patient airway (14) for monitoring pressure in said patient airway (14) and for generating a pressure signal indicative of sensed pressure in said patient airway (14); valve control means (40) connected to said safety valve (12) for controlling operation of said safety valve (12), and connected to said pressure sensor means (34) for receiving said pressure signal, said valve control means (40) including means for setting a maximum airway pressure threshold, means for comparing said sensed pressure in said patient airway (14) with said maximum airway pressure threshold, and means for generating a valve open signal when said sensed pressure in said patient airway (14) is equal to or greater than said maximum airway pressure; and valve switching means (30,32) connected to said valve control means (40) for receiving said valve open signal and for switching said safety valve (12) to said open position (24) in response to said valve open signal, said safety valve (12) comprising a solenoid valve biased to the open position (24) such that said valve opens in the event of a power failure; characterised in that said means for setting a maximum airway pressure threshold is such as to allow a threshold value to be selected from a plurality of such values; and said valve switching means (30,32) comprises a pulse power source (32) for providing cyclical pulses of power to said solenoid valve (12) for controlling the closing force of the solenoid valve (12) independently of the pressure in the patient airway (14) such that said safety valve (12) additionally opens when the pressure in the patient airway (14) is greater than said closing force. The lung ventilator safety valve apparatus (10) of claim 1, wherein said valve switching means (30,32) further comprises pulse width modulation means (30) for modulating the duration of said pulses for moving said solenoid valve (12) to said closed position (28). The lung ventilator safety valve apparatus (10) of claim 1 or claim 2, wherein the solenoid valve (12) is a DC operated solenoid valve and is provided with a pulse width modulated signal to provide control of the closing force. The lung ventilator safety valve apparatus (10) of claim 1, wherein said means for setting a maximum airway pressure threshold is programmable.

Inventors: FARNHAM III FLOYD R, O'MAHONY JOHN J, FARNHAM III, FLOYD R., O'MAHONY, JOHN J.
Applicants: PURITAN BENNETT CORP, PURITAN-BENNETT CORPORATION
Application year: 1995
Main classification: A61M16
IPC codes: A61M16