The mechanisms underlying increased venous distensibility during exposure to microgravity are not well known yet. However, there seems to be evidence indicating that skeletal muscle changes resulting from exposure to microgravity play a very important role. The purpose of this experiment was to test the hypothesis that leg muscles could play an important role in the changes of leg venous distensibility observed in simulated microgravity. Twelve subjects were submitted for 28 days to a -6 degrees head-down bedrest. Changes in leg vein hemodynamics (filling and emptying) have been measured by mercury strain gauge plethysmography with venous occlusion. Six of these subjects trained their lower limbs with isometric and isokinetic exercises during bedrest (group CM), while the other 6 subjects (control group, C) had no training.
Cardiovascular hemodynamics were assessed by ultrasound echography and Doppler during a 28-d head-down tilt "CNES HDT: 87-88," and during the 25-d French-Soviet spaceflight "Aragatz 88." For both studies we used the same ultrasound methodology. The main hemodynamic parameters of the left heart function and of the peripheral arterial system (cerebral, renal, femoral arteries) were measured four times during the HDT (day 7, 14, 21, 28) and twice post-HDT. The same measurements were performed six times during the flight (day 4, 5, 15, 18, 20, 24) and five times postflight. During the HDT, two groups were studied: six subjects no countermeasures and six subjects with repeated lower body negative pressure (LBNP). In the first group the cardiac volumes and the cardiac output were significantly decreased, whereas in the group with LBNP these parameters were superior to the basal value. In the group without LBNP the cerebral flow was maintained because of a decrease of the brain vascular resistance. In this group the renal vascular resistance was decreased as inflight. In the lower limbs we observed a loss of the vasomotor control. The vascular resistance was decreased after the end of the HDT and the subjects suffered orthostatic intolerance. In the population with LBNP, we did not observe the same decrease of vascular resistance during the HDT, and after the HDT no sign of orthostatic intolerance was observed. During the flight, the left ventricular volume was significantly decreased. The carotid flow was maintained owing to a decrease of the cerebral vascular resistance.(ABSTRACT TRUNCATED AT 250 WORDS)
The objective was to check, during a lower body negative pressure (LBNP) test, new vascular parameters for the detection of orthostatic intolerance induced by head-down-tilt (HDT) and spaceflight.The lower-limb volume flow and vascular resistance were evaluated by Doppler ultrasound. The HDT population consisted of two groups: control [6 subjects resting in a HDT (-6 degrees) position for 28 d] and countermeasure (6 subjects also in HDT for 28 d, but with repeated LBNP and exercise). The LBNP orthostatic test (four steps: -20, -30, -40, -50 mm Hg, of 3 min each) was performed before, during, and after the HDT. For the 14-d spaceflight (Antares) the cosmonaut underwent the LBNP test (10 min at -25 mm Hg and 10 min at -45 mm Hg), at preflight (3 times), inflight (day 11), and postflight (twice).HDT--As the LBNP pressure decreased, the femoral blood flow decreased and the lower-limb vascular resistances increased in both HDT groups. In the control group the femoral flow was less reduced, at each of the 4 levels of depressure (p < 0.01). The amplitude of the leg vascular resistances was reduced at -40 mm Hg, and at -50 mm Hg, on HDT day 15 in both groups (before LBNP, after 1 week's exercise for the countermeasure group), and on post-HDT day 1 (p < 0.01) only in the control group. The femoral vascular resistance response had completely recovered in the countermeasure group on post-HDT day 1. During the post-HDT tilt table test, all 6 controls had a drop in blood pressure of 20 mm Hg (4 subjects) or 10 mm Hg (2 subjects); 3 had pre-syncopal symptoms. The HDT countermeasure subjects had neither any clinical signs of orthostatic intolerance nor any blood pressure drop. 14d-Spaceflight--During the flight, the cosmonaut did not use any countermeasures (exercise, LBNP). On flight day +11, and on postflight day 3, the femoral vascular resistance response to LBNP was decreased as observed in the control group on HDT day +15 and post HDT. On postflight day 7, the femoral response had completely recovered. The middle cerebral flow response to the various pre-, in-, and postflight LBNP tests consisted of a slight decrease of the cerebral flow together with resistances of comparable amplitude (-10 to -20%) to those measured during the same LBNP test in the HDT control group.The femoral hemodynamics are much more disturbed than the cerebral ones in vascular deconditioning. The assessment of the lower limb vascular reactivity will be of interest in predicting orthostatic intolerance, and checking the efficiency of counter-measures.
A dopaminergic agonist, piribedil, was administered by intravenous infusion (0.1 mg/kg over 30 minutes) to 20 patients known to suffer from migraine and 20 subject free from any kind of headache. Neither changes in cerebral blood flow (CBF), as measured by the 33Xenon inhalation technique, nor peripheral side-effects were noted in the control group. Patients with migraine exhibited an 18 p. cent increase in CBF and a 32 p. cent decrease in mean arterial pressure and rapidly developed nausea severe enough to discontinue the infusion in most cases. Prior administration of domperidone, a neuroleptic drug which does not cross the blood-brain barrier, suppressed the nausea and fall in blood pressure but had no effect on the increase in CBF. This study confirms the existence of central and peripheral hypersensitivity to dopaminergic agents in patients with migraine. The piribedil test could be used to distinguish genuine migraine from ordinary cephalalgia in patients prone to headache.
Exposure to actual or simulated microgravity is known to result in changes in lower limb venous compliance or distensibility which may play a role in post-bedrest or postflight orthostatic intolerance. Venous deconditioning has only been described in terms of changes in vascular compliance or distensibility. But a complete understanding of changes in venous hemodynamics and cardiovascular regulation occurring under these conditions has to take into account changes in emptying capacities of the veins which influence venous return, cardiac filling, and cardiac output regulation. Moreover, few data are available about the course of changes in venous hemodynamics for periods of simulated microgravity longer than 4 weeks. The purpose of this investigation was to measure parameters of venous compliance and venous emptying before, during, and after a 42-day period of bedrest at -6 degrees head-down tilt for a better understanding of long term venous physiological adaptation to microgravity.
This study was performed to determine the effects of 28 d of head-down tilt (HDT) (simulated weightlessness) on cardiovascular responses to orthostatic stress induced by lower body negative pressure (LBNP) (before, during, and after HDT) and +60 degrees head-up tilt (before and after HDT) in 12 subjects. Half of them underwent countermeasures (CM) of regular muscular exercise (isometric and isokinetic training) and LBNP sessions (-30 mm Hg) as generally performed during spaceflight; the other six were a control group (C). The countermeasure effect on the orthostatic responses to LBNP and tilt test was assessed by studying the changes after HDT in the two groups. Essentially, blood pressure was better maintained in group CM in the tilt test after HDT (MBP at the end of the tilt vs. baseline value: +16% (CM); -19% (C)). LBNP and muscular exercise may have contributed to this improvement. One of the probable contributing factors is the relative conservation of plasma volume, at the end of HDT, in group CM (-2.2%), compared to group C (-11.2%). Transcranial Doppler (TCD) recordings of middle cerebral artery (MCA) velocities permitted indirect evaluation of cerebral blood flow changes during the orthostatic tests. MCA velocities decreased significantly although slightly (-7 to -12%) during LBNP sessions without changes along the HDT showing that the cerebral circulation was well preserved in each group. On the other hand, subjects undergoing presyncopal symptoms presented a drop in MCA velocities, suggesting a decrease in cerebral blood flow.
