Twenty-five patients in acute respiratory failure were randomised to receive either 5 cm of positive end-expiratory pressure (PEEP) or no-PEEP while weaning from controlled ventilation. The use of PEEP resulted in a significant reduction in the increase in alveolar-arterial oxygen tension gradient (AaDO21) which occurred in the group of patients who were converted from controlled ventilation to spontaneous ventilation without PEEP. Patients who weaned without PEEP had a mean increase in AaDO21 of 102 +/- 35 mmHg (SE) while those who weaned with PEEP had a mean increase of only 10 +/- 22 mmHg (p < 0.03). The use of PEEP was also associated with a significant improvement in the vital capacity and the maximum inspiratory force. Patients who weaned with PEEP had an increase in vital capacity of 258 +/- 108 ml (p < 0.05) and an increase in inspiratory force of -15 +/- 5 cmH2O (p < 0.01), while patients who weaned without PEEP did not have significant changes in these measurements. The use of PEEP during weaning may be helpful in patients who fail to wean because of the development of hypoxaemia due to rapid alveolar collapse, since PEEP appears to minimise the increase in intrapulmonary right-to-left shunt which normally occurs during weaning from controlled ventilation.

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