Maintained cardiac output during positive end-expiratory pressure ventilation in open-chest pigs

Background: Does ventilation with positive end-expiratory pressure (PEEP) act to reduce cardiac output (CO) not only by impeding venous return but also by inducing myocardial depression? The present study was aimed to demonstrate the possible existence of this latter mechanism. Methods: Eight pigs of Swedish native breed weighing 20–25 kg and 10–12 weeks old were anaesthetized, tracheotomized and connected to a volume-controlled ventilator. To prevent intra-thoracic pressure from interfering with venous return, the heart and juxtacardiac vessels were exposed to atmospheric pressure by opening and retracting the chest and pericardium. Heart rate (HR), CO, stroke volume (SV), mean arterial (MAP), mean right (MRAP) and left (MLAP) atrial pressures were recorded before and after retransfusion of 500 ml of autologous blood. This procedure was carried out twice in each animal - during ventilation with zero and with 15 cm H₂O of PEEP. Results: Comparison of the two ventilation modes before volume load revealed negligible differences in HR, CO, SV, MAP, MRAP and MLAP. Moreover, the changes evoked by volume load were practically identical. Conclusions: Addition of PEEP to regular positive pressure ventilation does not induce any haemodynamically detectable myocardial depression in the piglet.