The effect of positive end-expiratory pressure on central haemodynamics in the pig.

The effect on central haemodynamics of a stepwise increase in airways pressure from spontaneous breathing (SB) to intermittent positive pressure ventilation with a positive end-expiratory pressure (PEEP) of 0, 8, 16 and 24 cmH2O was studied in eight pigs under ketamine anaesthesia. Compared with SB, cardiac output (CO) was reduced by 12, 36, 50 and 64% at the respective ventilator settings. The transmural pressures of the right and left atrium, measured as the difference between atrial and pleural pressure, both decreased with increments in airway pressure. At a PEEP level of 24, there was a threefold increase in pulmonary vascular resistance. This increase was secondary to the decrease in CO and no signs of CO deterioration due to the increased right ventricular afterload were found. When 250 ml of dextran 70 was administered at a PEEP level of 24 and the airway pressure was then released stepwise, the left ventricular function curve improved, disclosing a relative myocardial failure at the highest PEEP levels. It is concluded that the principal causative mechanisms in CO reduction due to increased intrathoracic pressure is a decrease in preload to the right ventricle. At high PEEP levels there are also signs of myocardial depression.