Central haemodynamics and oxygen transport with and without continuous positive pressure ventilation after open-heart...

Twelve patients, subjected 20 h earlier to coronary artery bypass surgery, were studied on discontinuation of the postoperative mechanical ventilation employing PEEP+5 cmH2O (CMV+PEEP). Compared to the values obtained during CMV+PEEP, cardiac index and mixed venous blood oxygen tension increased with change to spontaneous ventilation at ambient pressure, employing a 28% O2 Ventimask for the intubated patient. There was a slight decrease in both the arterial oxygen content and oxygen tension, but the increase in cardiac output compensated well for the lowering in the arterial oxygen content, and consequently the systemic oxygen transport was statistically unaltered. Oxygen consumption, pulmonary shunt fraction and arteriovenous oxygen content difference also remained unaltered. The observations suggest that after open-heart surgery, CMV using a PEEP as low as +5 cmH2O may exert, in comparison to controlled oxygen therapy ;during spontaneous breathing, a significant lowering effect on the already compromised cardiac performance. This necessitates continuous weighing of the beneficial effects obtained by employing postoperative CMV+PEEP, against the adverse haemodynamic effects, although the alterations in cardiac output may partly ensue from the changes in metabolism, muscular effort and oxygen consumption during the two modes of ventilation, although there was no significant increase in oxygen consumption.