Most mechanical ventilators do not incorporate a mechanism dedicated to providing gas for spontaneous breathing, i.e. a 'demand valve'. Typically, a single gas delivery system is used for all modes of ventilatory support. The difference between the various modes is the algorithmic control of the inspiratory valve(s). Therefore performance of mechanical ventilators during spontaneous breathing is determined by factors similar to those discussed for patient-triggered pressure-controlled ventilation, except that the pressure is program controlled during both inspiration and expiration.
During spontaneous breathing, the opening pressure of the inspiratory valve is less than the reference pressure (e.g. continuous positive airway pressure (CPAP)) and the opening pressure of the exhalation valve is greater than the reference pressure ( Fig.4). End-expiratory pressure determines the starting point for the subsequent inspiration. The reduction in pressure necessary to initiate gas delivery is equal to the gradient between the closing pressure of the exhalation valve and the opening pressure of the inspiratory valve. Therefore the design and function of the exhalation valve are also important determinants of ventilator performance during spontaneous breathing modes.
Fig. 4 Block diagram of the operation of a mechanical ventilator during CPAP. To maintain CPAP, the electronic logic first acquires input from the control panel (i.e.
the reference value for CPAP) and directs the CPAP. The microprocessor acquires the level of CPAP from the control panel and directs the CPAP control valve to generate pressure on the expiratory valve that is approximately equal to the reference pressure. A transducer (P) in the patient circuit senses the pressure and the signal is transmitted to the electronic logic via an amplifier (E). If the pressure in the patient circuit is lower than the reference pressure, the electronic logic closes the exhalation valve and opens the inspiratory valves, which provides gas flow in order to bring the differential between the reference and patient circuit pressures to zero. If the pressure in the breathing circuit is higher than the reference pressure, the electronic logic closes the inspiratory valves and opens the exhalation valve in order to bring the differential between the reference and patient circuit to zero. (Modified with permission from product literature for the Evita, Dragerwerk AG, Lubeck, Germany.)
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