Intrapulmonary shunt is not increased by 100% oxygen ventilation in acute respiratory failure.

Increase of shunt has been demonstrated during short periods of pure oxygen breathing, mostly in patients with mild acute respiratory failure (ARF). Twenty patients with a large range of venous admixture (12 to 63%) were studied when FIO2 was increased from maintenance to one. Intrapulmonary shunt was measured with both the conventional oxygen method [QS/QT (O2)] and the multiple inert gas elimination technique [QS/QT (IG)]. Mean venous admixture decreased from 29 to 24% when FIO2 was increased and QS/QT (IG) remained unaltered. The pattern of blood flow distribution remained similar in both conditions, even in the eleven patients with ARF secondary to bacterial pneumonia and who had a low V/Q mode, highly liable to collapse with 100% oxygen. As no increase in shunt was documented in these patients, reluctance to measure shunt during 100% oxygen breathing should be re-evaluated.