PRESSURE‐CYCLED VENTILATORS

Mechanical ventilators are divided into three categories according to the principle on which the inspiration and expiration phases are cycled.’ Time-cycled ventilators are those in which the inspiration and expiration phases are determined by a fixed breathing rate which is set on the machine. The chief example of this type is the tank respirator. Volume-cycled ventilators are those in which both the tidal volume and the breathing rate are set in advance and are controlled by the machine. Pressure-cycled ventilators are those in which the inspiration phase is interrupted when the pressure in the lungs reaches a preset value. This paper deals mainly with the physiological effects of the pressure-cycled ventilator and its clinical use. It has the great advantage of adapting itself completely to the breathing rhythm of each individual patient. Pressure-cycled ventilators are not basically automatic since the patient initiates the inspiration, which is then supported and increased by the machine. All recent pressure-cycled ventilators incorporate a device which starts the next inspiration automatically if the signal for inspiration, a very slight negative pressure, fails to come from the patient himself. This time cycling, however, is cut out again immediately after spontaneous respiration is resumed. The requirements which must be made of a ventilator vary greatly according to the cause of the respiratory failure. Basically, there are two different situations in which mechanical respiration is necessary: cases in which the respiratory muscles are inactivated, but the lungs do not display any structural or functional changes; and cases in which the mechanical properties of the lungs are adversely affected, but the respiratory muscles are intact. As far back as 1947, Motley et aL2 worked out the basic requirements which should be fulfilled by the ideal pressure-cycled respirator: “The device should be simple, small, sturdy, and compact. Exhaled air should leave the patient’s mouth, not go through the respirator. A type 111 curve should be produced so as not to decrease the cardiac output. The cycling mechanism should be excessively sensitive for effortless, smooth action. High instantaneous flow rates should be produced, but in view of the wide range of pulmonary flow rates desirable in different patients, inspiratory flow rates should be adjustable. There should be humidification of breathing gas.” In cases where respiration fails for extrapulmonary reasons, artificial positivepressure respiration involves hardly any difficulties. The mechanical properties of the lungs are unchanged. Compliance and airway flow resistance are normal. Under these circumstances, practically all the ventilators available today are suitable, provided the positive pressure and the respiratory rate are selected in such a way as to avoid any diminution in cardiac output. If respiration fails for intrapulmonary reasons, the problem of artificial respiration becomes considerably more complicated. In the case of restrictive pulmonary changes, reduced compliance constitutes the main obstacle because this calls for the use of high inspiratory pressures. Airway resistance is not greatly increased as a rule and presents fewer difficulties. In obstructive lung disease, adequate ventilation is rendered difficult by the presence of both an increase in airway resistance and a decrease in compliance. In these conditions, it is essential that the ventilator should be capable of