Tidal variation of pulmonary blood flow and blood volume in piglets during mechanical ventilation during hyper-, normo- and hypovolaemia.

Effects of changes in blood volume on changes in pulmonary blood flow and pulmonary blood volume during the ventilatory cycle during mechanical ventilation with a positive end-expiratory pressure of 2 cm H2O were determined in six pentobarbital anaesthetized, curarized pigs weighing about 10 kg. Haemodynamic variables were analysed for each cardiac cycle in eight ventilatory cycles in four consecutive series under hyper-, normo- and hypovolaemic conditions. Cardiac output was highest in hypervolaemia. Compared with normo- and hypovolaemia, it decreased less during inflation, due to a smaller rise in central venous pressure and presumably a larger filling state of the venous system. The smaller decrease in right ventricular output in hypervolaemia coincided with a larger fall in transmural central venous pressure (right ventricular filling pressure), due to right ventricular action at a higher, less steep part of its function curve. The difference between right ventricular-output (electromagnetic flow measurement) and left ventricular-output (pulse contour) indicated changes in pulmonary blood volume. In hypervolaemia less blood shifted from the pulmonary circulation into the systemic system during inflation than in normo- and hypovolaemia. This difference can be explained by two mechanisms namely, the smaller fall in input into the pulmonary vascular beds and a smaller pulmonary vascular volume decrease as a result of transmural pressure fall at a steeper part of the pressure-volume curve.