EFFECT OF 4-HOURS OF 6 DEGREES HEAD DOWN TILT BED REST ON CARDIAC FUNCTION DURING AN ORTHOSTATIC CHALLENGE TEST

摘要

We investigated the hypothesis that even brief exposure to elevated central venous blood volume with head down tilt bed rest (HDT) might be associated with a change in the central venous pressure-volume relationship that can impact on the normal cardiac output (CO) response. The cardiovascular responses to a 26-minute random lower body negative pressure (LBNP) orthostatic challenge test were observed in six healthy volunteers prior to and following 4 hours of 6° (HDT). Images of the inferior vena cava (IVC), obtained by Doppler ultra-sound imaging, cardiac output (CO), measured by pulsed wave Doppler ultrasound, and central venous pressure (CVP), obtained by catheter in the antecubital vein, were obtained during LBNP. There was no change in central venous compliance following HDT, but there was a trend at all levels of LBNP to lower values of CVP and CO. The CO drop to CVP ratio (%/cmH_2O) was significantly enhanced at each level of LBNP following HDT. Thus, even though there was no change in the central venous pressure-volume relationship, we did observe a rapid adaptation of the cardiovascular system to 4-h HDT resulting in reduced cardiac stroke volume for any CVP.