A variable waveform ventilator.

There have been a number of studies of the physiological effects of different tracheal pressure wave patterns in patients requiring artificial ventilation with intermittent positive pressure (IPPV) (Werko, 1947; Cournand et al., 1948; Maloney et al., 1953; Watson, 1962a, b; Bergman, 1963, 1967; Grenvik, 1966; Auchinloss and Gilbert, 1967; Robinson, 1967; Herzog and Norlander, 1968; Adams et al., 1970). These studies have in some instances used specially designed ventilators which could vary the pressure wave pattern (Clutton-Brock, 1957; Watson, Spalding and Smith, 1962; Adams, 1970). The tracheal pressure waveform that is produced by IPPV is only partially controlled by the ventilator itself, as the final waveform is dependent very largely upon the patient's respiratory mechanics. Also in any given situation the pressure waveform alters in shape as it is observed at different points in the airway downstream from the ventilator, and these changes in turn may vary at different flow rates. However, the tracheal flow pattern does not suffer these variations as at any given instant the flow will be the same at any two points from the ventilator output to the bifurcation of the trachea, assuming no leaks, except for very minimal changes due to gas compression and heating. Flow measurement is, therefore, a better measure of the pattern of ventilation than pressure measurement. This is especially so if the ventilator can be made powerful enough to be a flow generator (Mapleson, 1962) and so independent of patient characteristics.