Effects of the level of CPAP on central haemodynamics and oxygen transport

The effects of different levels of continuous positive airway pressure (CPAP) on central haemodynamics and oxygen transport were studied in ten spontaneously breathing male patients who had undergone aortocoronary bypass graft operation 18 h earlier. With increasing CPAP levels ranging from 5 cmH2O(0.49kPa)(CPAP 5) to 15 cmH2O(1.47kPa)(CPAP 15), the cardiac index was found to decrease significantly, while the intraluminal pulmonary capillary wedge and right atrial pressures increased simultaneously. The mean systemic arterial pressure remained unaltered, while the mean pulmonary arterial pressure increased with increasing CPAP. Systemic oxygen transport changed concomitantly with the changes in cardiac output, since arterial blood oxygen content was not altered. The mixed venous blood oxygen tension decreased with increasing CPAP, as did the cardiac output. No changes in the total oxygen consumption or in the arteriovenous oxygen content difference were found at the various CPAP levels. Pulmonary vascular resistance was significantly higher during hypoxia caused by closure of the small airways as a consequence of a reduction in the functional residual capacity during CPAP 0. Thus, low level CPAP might be beneficial in maintaining proper lung volume in an intubated patient after aortocoronary bypass surgery. The observations also suggest that, in these patients, CPAP levels exceeding 10 cmH2O bring about cardiac depression leading to an undesirable reduction in systemic oxygen transport. Mixed venous blood oxygen tension may offer information useful in the adjustment of the level of CPAP.