Most modern mechanical ventilators have program-controlled gas delivery systems with either one inspiratory valve or a pair of inspiratory valves. These valves are almost always operated by an electromagnetic motor (Fig 2). Ventilators with one electromagnetic valve are supplied with gas from a reservoir which contains compressed gas at a specified oxygen concentration. Ventilators with a pair of inspiratory valves proportion compressed air and compressed oxygen independently to produce a selected concentration of oxygen. The function of the inspiratory valve(s) relative to the type and degree of ventilatory support selected is discussed later.
Fig. 2 Schematic illustration of an inspiratory valve of a mechanical ventilator. A microprocessor directs electric current into the coil, via a digital-to-analog converter (D/A), to produce an electromagnetic field with the same polarity as the permanent magnet. Variation in the magnitude of the field causes the armature and ball to be positioned so as to produce the desired rate of gas flow. (Modified with permission from product literature for the Evita, Dragerwerk AG, Lubeck, Germany.)
The function of the expiratory valve is to seal the breathing circuit during the mechanical inspiratory phase and to vent gas to ambient during the mechanical exhalation phase (Fig..3). In most ventilators, the expiratory valves are program controlled. The functional operation of the expiratory valve relative to the type of ventilatory support selected is described later.
Fig. 3 Schematic illustration of an expiratory valve of a mechanical ventilator. During mechanical insufflation the microprocessor directs electric current into the coil, creating an electromagnetic field with the same polarity as the permanent magnet. Thus the membrane is 'seated' by the downward motion of the actuating rod, which also compresses a metal spring. When the mechanical inspiratory phase is terminated, the electric current is stopped, eliminating the electromagnetic field. The weight of the actuator rod is matched by the tension of the metal spring, thus causing the membrane to unseat and allowing gas to be vented to ambient. Clinician-defined PEEP is produced when the electronic logic directs an electric current through the coil and the membrane is seated with the appropriate pressure. (Modified with permission from product literature for the Veolar, Hamilton Medical Corporation, Reno, NV, USA.)
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