EFFECTS OF INSPIRATORY FLOW WAVE-FORMS ON ARTERIAL BLOOD-GASES AND RESPIRATORY MECHANICS AFTER OPEN-HEART-SURGERY

The clinical usefulness of inspirator flow pattern manipulation during mechanical ventilation remains unclear, The aim of this study was to investigate the effects of different inspiratory flow waveforms, i.e. constant, sinusoidal and decelerating, on arterial blood gases and res piratory mechanics, in mechanically ventilated patients. Eight patient s recovering after open heart surgery for valvular replacement and/or coronary bypass were studied, The ventilator inspiratory flow waveform was changed according to a randomized sequence, keeping constant the other variables of the ventilator settings, We measured arterial blood gases, flow, volume and pressure at the proximal (airway opening pres sure (Pao)) and distal (Ptr) ends of the endotracheal tubes before and after 30 min of mechanical ventilation with each inspiratory flow wav eform, We computed breathing pattern, respiratory mechanics (pressures and dynamic elastance) and inspirator) work, which was then partition ed into its elastic and resistive components. We found that: 1) arteri al oxygen tension (Pa,O-2) and arterial carbon dioxide tension (Pa,CO2 ) were not affected by changes in the inspiratory flow waveform; and 2 ) peak Pao and Ptr were highest with sinusoidal inspiratory flow, whil st mean Pro and Ptr and total work of breathing were least with consta nt inspiratory flow, mainly because of a concomitant decrease in resis tive work during constant flow inflation, The effects of the inspirato ry flow profile on Pao. Ptr and total inspiratory work performed by th e ventilator were mainly due to the resistive properties of the endotr acheal tubes. We conclude that the ventilator inspiratory flow wavefor m can influence patients' respiratory mechanics, but has no impact on arterial oxygen and arterial carbon dioxide tension.