Effects of positive end-expiratory pressure on cardiac output distribution in the pig

Cardiac output (CO) and the blood flow to the heart, cerebellum, kidney, pancreas, spleen and skeletal muscle were studied in 20 pigs during spontaneous breathing (SB) and intermittent positive pressure ventilation (IPPV) with a positive end-expiratory pressure (PEEP) of 0, 8, 16 or 24 cmH2O. Microspheres (15 micrometers) labelled with either 85-sr or 141-Ce were used. Injection of microspheres labelled with one of the isotopes was given during SB (all pigs) and with the other isotope during IPPV with PEEP of 0, 8, 16 or 24 cmH2O (five pigs at each level). CO decreased by 11% during IPPV with PEEP of 0 and 31%, 53% and 66% during PEEP of 8, 16 or 24 cmH2O, respectively. Mean arterial blood pressure was fairly well maintained in all groups except the group with PEEP of 24 cmH2O. The perfusion of the six organs deteriorated, but when taken as fractions of CO measured at the same time, the blood flow to the heart, cerebellum and kidney increased with increasing airway pressure, while that to the pancreas, spleen and skeletal muscle decreased. The vascular resistance of the three former organs did not change, while in the latter it increased markedly. It is concluded that when CO decreases as a result of positive pressure ventilation, a redistribution takes place, mainly due to vascular constriction in skeletal muscle, which acts to preserve the blood flow to vital organs.