Microgravity and orthostatic intolerance: carotid hemodynamics and peripheral responses.

A ground-based model [24 h of bed rest (BR) with head-down tilt (HDT)] was used to investigate the cardiovascular deconditioning responsible for orthostatic intolerance, frequently observed after weightlessness flights. This experimental deconditioning is shown to be distinguished by an increase of mean blood pressure (P < 0.05), with increased total peripheral resistances (TPRs). Systolic tangential tension of the carotid arterial wall, cardiac output and frequency (spectral analysis), and plasma norepinephrine and epinephrine were not significantly altered, while plasma dopamine was increased (P < 0.05). Cardiovascular homeostasis was challenged before and after 24 h of BR with HDT through -40 mmHg lower body negative pressure (LBNP). Systolic tangential tension of the carotid wall was decreased, with a decrease of systolic pressure and cardiac output; increased heart rate was likely due to an increase of sympathetic drive with a decrease of vagal braking. The overall picture was not changed after 24 h of BR with HDT, except for a lack of increase of TPRs: their increase (+13.7%, P < 0.05) before was no longer observed after (-2.6%) 24 h of BR with HDT. This apparent deficiency cannot be explained. However, a heterogeneity in the response of TPR should be considered because the magnitude of the increase of blood pressure to cold pressor test was the same after 24 h of BR with HDT as it was before.(ABSTRACT TRUNCATED AT 250 WORDS)

Twenty-four hours of bed rest with head-down tilt: venous and arteriolar changes of limbs.

The hemodynamic changes after 24 h of bed rest with -5 degree head-down tilt were studied in six normal subjects as a condition causing an intrathoracic shift of blood volume with a resulting urinary sodium and water loss and a subsequent reduction in total intravascular volume. Vascular resistance and venous tone of the upper and the lower limbs were investigated as were systemic hemodynamics throughout the procedure. Whereas systemic hemodynamic parameters did not change significantly, vascular resistance and venous tone of the upper and lower limbs decreased significantly up to the sixth h and then returned toward baseline values at 24 h. Systemic and forearm vasoconstriction responses to lower body negative pressure (LBNP) were studied just before and at the end of the study period. With LBNP, -5, -10, and -15 mmHg pressure levels were used to investigate mechanoreceptors in the low-pressure system, whereas the -40 mmHg pressure level was used to explore baroreceptors both in low and high pressure systems. Changes in vascular resistance in response to LBNP did not differ at the beginning and at the end of the head-down tilt, whereas an exaggerated heart rate response was observed at -40 mmHg at the end of 24 h of bed rest. The study showed that after 24 h of bed rest with -5 degree head-down tilt, the adaptative changes in venous tone and vascular resistance to blood volume reduction were altered. A dissociation between cardiac and vascular baroreflex response was observed in situations simulating tilting.