EFFECTS OF VARYING INSPIRATORY FLOW WAVEFORM AND TIME IN INTERMITTENT POSITIVE PRESSURE VENTILATION II: VARIOUS PHYSIOLOGICAL VARIABLES

In artificial ventilation with alteration in inspiratory flowwaveforms and time, there were statistically significant differencesin a number of physiological variables. Tidal volume and respiratoryfrequency were kept constant and inspiratory time and waveformboth varied independently. With shortening of inspiratory timephysiological deadspace to tidal volume ratio (VD/VT) increased,total dynamic and lung compliance decreased, Pa_O2, decreased,Pa_CO2 increased and mean airway and oesophageal pressures decreased.There was no change in alveolar to arterial P_O2 difference (PA_O2–Pa_O2),in cardiac output, or in chest wall compliance. The most favourableflow waveform for most variables was the reversed ramp waveformwhich resulted in decreased VD/VT and Pa_CO2, and increased Pa_O2,and total dynamic and lung compliance, when compared with theramp waveform. The effects of sine and top-hat waveforms couldnot be separated statistically but were significantly differentfrom ramp and reversed ramp waveforms, having effects betweenthese two. The reversed ramp waveform produced the greatestmean airway and oesophageal pressures, and the ramp waveformthe least. No differences were found for chest wall complianceand (PA_O2–Pa_O2) and the cardiac output changes were equivocal(but with the suggestion of waveform effects with ramp waveformallowing the best output and reversed ramp and top-hat waveformsthe poorest). Pulmonary venous admixture increased with shorteningof inspiratory time, and ramp flow waveform, and decreased withthe reversed ramp flow waveform. ^*Present addresses:Department of Anaesthesia, University ofOtago, New Zealand.Present addresses: St Vincent's Hospital, Melbourne, Australia.