This invention relates to the field of medical ventilators, and more particularly, to a ventilator exhalation valve for regulating the expiration of respiratory gases from a patient to the atmosphere.
Medical ventilators provide artificial respiration to patients whose breathing ability is impaired. Typically, the ventilator employs an inhalation valve to deliver a breath to the patient from a pressurized source of gas and an exhalation valve for permitting the breath to pass from the lungs to the atmosphere. The flow of the breath during inspiration is governed by the inhalation flow-control valve. When the flow-control valve opens, the pressurized gas forming the breath is introduced into the lungs of the patient. Upon passage of a predetermined volume of gas, the flow-control valve closes to end the inspiration phase of the breath. After the inspiration phase, the respiratory gases are vented from the patient, through the exhalation valve to the atmosphere. The respiratory gases pass through the exhalation valve which provides flow control after inspiration is completed and before the next inspiration cycle begins.
Prior ventilators have been capable of operating in several modes so that the degree of support the ventilator provides to the natural breathing pattern of the patient can be varied over a broad spectrum. At one end of the spectrum, the ventilator can provide fully controlled ventilation in which the ventilator has complete control over when the breath is delivered, the volume of gases delivered to the patient during each breath and the timing and pressure of the respiratory gases. In the "volume controlled" mode, all of the flow parameters are preset by an operator in accordance with the particular needs of the patient.
At the other end, the ventilator can be programed to permit "spontaneous" breathing by the patient. During the spontaneous breathing mode, the breath rate, the volume of gas inhaled during each breath and other flow parameters are not predetermined, but rather reflect the actual usage of the patient.
Intermediate of the volume controlled and the spontaneous modes, various degrees of ventilator supported respiration are available. One of the parameters which can be controlled by the ventilator during all modes of ventilation is the pressure in the lungs after the expiration phase is complete. Therapists have found that in some patients, it is beneficial to maintain a slight positive pressure within the lungs after expiration, so as to avoid a possible collapse of lungs. The pressure of the gases in or near the lungs and airway is called the "proximal pressure." Previous ventilators have included a "positive end expiration pressure" (PEEP) feature which enables the operator to determine and regulate the minimum proximal pressure after each expiration cycle is completed.
Previous ventilators have included micro-computer controllers which "servo" the position of the exhalation valve so as to regulate the proximal pressure to the desired level during the expiration phase of each breath, that is, the controller positions the valve based on feedback from a pressure sensor, and causes the movement of the valve as needed to maintain the desired proximal pressure. This is commonly referred to as a closed loop or "servo" control system.
Typical exhalation valves in closed loop ventilators generally fall into one of two configurations. The first configuration is the pneumatic balloon valve. In the pneumatic balloon valve a flexible balloon valve or diaphragm selectively engages a rigid seat in response to an externally generated pilot pressure. The pilot pressure is adjusted as required to close the valve during inspiration and open the valve during expiration so as to achieve the desired proximal pressure.
However, the pneumatic balloon valve is subject to the disadvantage that flow turbulence across the seat area generates a substantial audible noise in the form of "honk" or "squeal." In addition, the inherent delays in the transmission of the pneumatic pilot signal render the systems sluggish and difficult to control in a closed loop system. Further, the pilot pressure system required to drive the exhalation valve is a sensitive, mechanically complex and therefore expensive system.
The second configuration employed in exhalation valves is the electromechanical linear actuator. In the typical exhalation valve employing the linear actuator, the linear actuator takes the place of the pilot pressure. The linear actuator controls the valve by regulating the motion of a diaphragm relative to a valve seat. The linear actuator can be driven by electronics in a closed loop system to perform the various tasks of a ventilator including regulation of the proximal pressure.
However, the electromechanical linear actuator closed loop systems are subject to the disadvantage that the actuator slides in a bearing which experiences static and dynamic friction. The dynamic sliding friction creates discontinuities in the motion thereby making the system difficult to control. In addition, steady state error and instabilities may be generated as result of the friction. Further, due to a lack of positional or velocity feedback of the actuator, the system tends to an unstable configuration.
Therefore, a need exists for a ventilator exhalation valve for which may be used in a closed loop system without generating audible noise resulting from flow turbulence. A need also exists for a ventilator exhalation valve which provides for a dampening of turbulence in the flow. In addition, the need exists for a exhalation valve in a closed loop system which is readily responsive to control signals. A further need exists for an exhalation valve which does not require mechanically complex, expensive mechanisms to provide for the regulation of the proximal pressure. A further need exists for a ventilator exhalation valve which exhibits reduced instabilities so that a feedback system may be employed in a stable configuration.