Continuous positive airway pressure during mechanical and spontaneous ventilation. Effects on central haemodynamics and oxygen transport.

The effect of continuous positive airway pressure during continuous mechanical (CMV + PEEP) and spontaneous (CPAP) ventilation on central haemodynamics and systemic oxygen transport was studied in 10 male patients who had undergone aortocoronary bypass graft operation 18 h earlier. With the change from CMV + PEEP 5 cmH2O to CPAP 5 cmH2O, cardiac index was found to increase from 2.58 +/- 0.44 (s.e. mean) to 2.88 +/- 0.19 l/min/m2 (P less than 0.005), and systemic oxygen transport improved from 8.5 +/- 0.6 to 9.5 +/- 1.0 ml/min/kg (P less than 0.05). Arterial oxygen tension and content did not change, but mixed venous blood oxygen tension increased from 3.5 +/- 0.2 to 4.2 +/- 0.2 kPa (P less than 0.005), reflecting the increase in cardiac output. Arteriovenous oxygen content difference decreased from 4.6 +/- 0.5 (CMV + PEEP) to 3.6 +/- 0.2 (CPAP) ml/100 ml (P less than 0.05), while total oxygen consumption remained unchanged. Mean systemic arterial pressure was found to increase from 10.8 +/- 0.4 to 11.6 +/- 0.4 kPa (P less than 0.05) and mean pulmonary arterial pressure changed from 2.2 +/- 0.1 to 2.4 +/- 0.1 kPa (P less than 0.05). Right atrial and pulmonary capillary wedge pressures did not change. Our observations suggest that, in terms of central haemodynamics and tissue oxygen supply, CPAP offers a noteworthy alternative weaning method and an alternative to CMV + PEEP in cases where therapy is prolonged and the patient is able to breathe spontaneously.