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

Abstract

In artificial ventilation with alteration in inspiratory flow waveforms and time, there were statistically significant differences in a number of physiological variables. Tidal volume and respiratory frequency were kept constant and inspiratory time and waveform both varied independently. With shortening of inspiratory time physiological deadspace to tidal volume ratio (VD/VT) increased, total dynamic and lung compliance decreased, PaO2, decreased, PaCO2 increased and mean airway and oesophageal pressures decreased. There was no change in alveolar to arterial PO2 difference (PAO2–PaO2), in cardiac output, or in chest wall compliance. The most favourable flow waveform for most variables was the reversed ramp waveform which resulted in decreased VD/VT and PaCO2, and increased PaO2, and total dynamic and lung compliance, when compared with the ramp waveform. The effects of sine and top-hat waveforms could not be separated statistically but were significantly different from ramp and reversed ramp waveforms, having effects between these two. The reversed ramp waveform produced the greatest mean airway and oesophageal pressures, and the ramp waveform the least. No differences were found for chest wall compliance and (PAO2–PaO2) and the cardiac output changes were equivocal (but with the suggestion of waveform effects with ramp waveform allowing the best output and reversed ramp and top-hat waveforms the poorest). Pulmonary venous admixture increased with shortening of inspiratory time, and ramp flow waveform, and decreased with the reversed ramp flow waveform.