Evaluation of a fluidic ventilator: a new approach to mechanical ventilation.

T he emerging technology of fluid amplifiers and control devices has provided a new approach to the design and construction of mechanical ventilators. Fluidics, as this technology has come to be known, has not yet found wide application in medicine, and the fundamentals of fluid control devices are not familiar to many in that field. These are basically simple devices that employ fluids (gases and liquids) to perform operations such as amplification and on and off switching that one might normally associate with electrical devices.lJ As in any amplifier, a large signal is modulated by a small signal. In this instance the power jet of gas is modulated by smaller control jets or switched on and off by being directed from one channel to another by a control jet. The channels through which gas travels are cut into a solid block of suitable material. The fluidic circuit itself contains no moving parts, and potential for mechanical difficulty is reduced accordingly. This technology has found wide and sophisticated application in industrial and space equipment where stability is vital. With mass production and miniaturization techniques, fluidic circuits have become relatively inexpensive. Quite a number of ventilators of various capabilities and description are available here and abroad.3 They are popularly classified as pressure cycled, volume cycled, or time cycled, although they might more accurately be called pressure preset or volume preset machines.' The new device under considemtion (Fig 1) may be either pressure, volume, or time cyded. Inspiratory flow in pressure cycled machines continues until a preset pressure is reached and then terminates. In some machines such as the Bennett PR-1 or 2, inspiration terminates when flow falls to a low tenninal point at the preset pressure. Changes in pulmonary resistance or compliance will alter the volume of gas delivered from pressure cycled ventilators. Therefore, volume cycled ventilators are generally preferred in circumstances requiring continuous mechanical ventilation.