Basic mechanisms and clinical consequences of cyclic changes in pulmonary blood flow and blood volume during mechanical ventilation.

One of the main functions of the pulmonary circulation is its capacity function, including its function as a reservoir of blood. Changes in pulmonary blood volume occur if the input (right ventricular output) and the output (left ventricular output) of the pulmonary circulation are different. Transient differences between the input and the output occur during each ventilatory cycle of mechanical ventilation due to changes in intrathoracic pressure and consequent changes in central venous pressure. The haemodynamic effects of these transient changes in input and output and their dependence on the volaemic condition have been reduced to basic physiological mechanisms. The consequences have been extrapolated to clinical conditions.