Adaptation of Systemic and Pulmonary Circulation to Acute Changes in Gravity and Body Position.

Changes in gravity or body position provoke changes in hydrostatic pressure in the arterial system and in venous return. Potential asymmetries between left (Q) and right ventricular (Q) cardiac output during transient gravity changes were investigated. It was hypothesized that blood volume is temporarily stored in the pulmonary vessels, with amount and duration depending on the level and directions of gravity. Eight healthy, male subjects (32 ± 3 yr, 182 ± 7 cm, 82 ± 6 kg) were tested on a tilt seat (TS), in a long arm human centrifuge (laHC), and during parabolic flights (PF). The gravitational changes during PF were reconstructed by changing gravity in a laHC and different body positions on a TS. All participants were tested in the seated, resting position. Heart rate and blood pressure were recorded continuously and Q was calculated, applying the Modelflow Algorithm. Gas exchange was measured breath-by-breath. Q was calculated from these data according to the Fick Principle. Four sequences were superimposed and analyzed by ANOVA with the factors Time, Ventricle (Q, Q), and Mode (TS, PF, laHC). After reductions in gravity Q and Q were transiently desynchronized. ANOVA showed no main effect for Mode, but significant changes were found for Time and Ventricle and all interactions. Phases of reduced gravity seem to lead to transiently increased storage of blood volume inside the pulmonary vascular system. A more detailed understanding of these mechanisms might help to describe the compliance of the pulmonary vascular system in diseases of the pulmonary circulation.