Central Haemodynamics and Oxygen Transport With and Without Continuous Positive Pressure Ventilation After Open-heart Surgery

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Abstract
Twelve patients subjected 20 h earlier to coronary artery bypass surgery were studied on discontinuation of the postoperative mechanical ventilation employing PEEP [positive end-expiratory pressure] + 5 cm H2O (CMV [continuous mandatory ventilation] + PEEP). Compared to the values obtained during CMV + PEEP, cardiac index and mixed venous blood O2 tension increased with the change to spontaneous ventilation at ambient pressure, employing a 28% O2 Ventimask for the intubated patient. There was a slight decrease in the arterial O2 content and O2 tension, but the increase in cardiac output compensated for the lowering in the arterial O2 content and the systemic O2 transport was statistically unaltered. O2 consumption, pulmonary shunt fraction and arteriovenous O2 content difference also remained unaltered. After open-heart surgery, CMV using a PEEP as low as + 5 cm H2O may exert, compared to controlled O2 therapy during spontaneous breathing, a significant lowering effect on the already compromised cardiac performance. This necessitates continuous weighing of the beneficial effects of postoperative CMV + PEEP against the adverse hemodynamic effects. The alterations in cardiac output may partly ensue from the changes in metabolism, muscular effort and O2 consumption during the 2 modes of ventilation, although there was no significant increase in O2 consumption.