Respiratory and cardiovascular responses to PEEP in artificially ventilated patients after cardiopulmonary bypass surgery.

The respiratory and haemodynamic effects of incremental levels of positive and expiratory pressure (PEEP) to 9 cm H2O were studied in ten adult patients 3--6 hours after uneventful cardiopulmonary bypass surgery. Functional residual capacity was increased and deadspace-tidal volume ratio tended to fall, the latter approaching significance at +6 and +9 cm PEEP. Thus lung volume was increased and there was a tendency to improved gas distribution to the alveoli. However there was no significant change in PaO2, alveolar-arterial oxygen tension difference or venous admixture. Cardiac index, and left ventricular strokework index were marginally depressed at 6 cm PEEP and further at 9 cm, while right atrial pressure and pulmonary artery occlusion pressure were raised at 9 cm PEEP. It would appear that low levels (3--6 cm) of PEEP do not improve gas exchange in the lungs to any worthwhile degree, and levels (6--9 cm) may impair cardiac performance.