Pulmonary Blood Flow and Blood Volume During Positive Pressure Ventilation

Abstract

Flow and pressure in the systemic and pulmonary circulation are affected by two types of changes in airway pressure: a static type applied for a relatively long time like positive end-expiratory pressure (PEEP), and a dynamic type of rise and fall during each ventilatory cycle. In both types, the increase in airway pressure will increase intrathoracic and central venous pressure. Consequently, a decrease in venous return occurs [1–8]. However, for both types, the same rise in airway pressure will not have the same quantitative effect, because during PEEP two additional control mechanisms are involved. A lung stretch reflex is increasing the negative effects of central venous pressure on flow and arterial blood pressure [8–10], and control mechanisms elicited by baroreceptor activity are compensating for these negative effects [8, 11]. During a ventilatory cycle, the time of rise and fall in airway pressure is too short to elicit these neurogenic and humoral control mechanisms. Changes in circulation during a ventilatory cycle, therefore, will be mainly due to direct mechanical effects.