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. Furthermore, the magnitude of the increase of plasma catecholamines during -40 mmHg LBNP was significantly higher after 24 h of BR with HDT, suggesting that the -40 mmHg LBNP-induced increased activity of the sympathetic nervous system was reinforced after 24 h of BR with HDT. Thus deficient vascular resistances during -40 mmHg LBNP are facing an apparent increased activity of the sympathetic system.