EFFECTS OF THE PATTERN OF VENTILATION AND OF AN INCREASE IN CARDIAC OUTPUT ON THE DISTRIBUTION OF BLOOD FLOW TO A HYPOXIC LUNG LOBE

Severe arterial hypoxaemia resulting from acute lung disease is frequently treated with positive end-expiratory pressure (PEEP) or an increase in inspiratory: expiratory (I:E) time ratio, the latter being produced either by a reduction in in-spiratory flow rate or by the addition of an inspiratory pressure plateau. The reduction in shunt associated with PEEP or an increase in I:E ratio is probably a result of the recruitment of previously collapsed alveoli secondary to an increase in end-expiratory lung volume [1]. How-ever, the improved oxygenation appears to be related most closely to the increase in mean airway pressure [2], and there is evidence that the addition of an inspiratory pressure plateau can decrease shunt even though end-expiratory volume is little changed [3]. In some patients normal values of PEEP produce little improvement in oxygenation, whilst higher values may increase alveolar deadspace and intrapulmonary shunt [4—6]. The increase in shunt with PEEP has been noted most frequently in patients with predominantly unilateral lung disease and there is evidence that, in these circumstances, PEEP may redistribute blood flow from the well-ventilated to the diseased area of lung [7-9]. The application of PEEP may also decrease cardiac output and mixed venous Po2. If shunt is unchanged, this will result in a reduction in arterial Po2. The reduction in cardiac output may be minimized by fluid loading or the use o