Vascular mechanisms of adaptation to head-down tilt body posture

Using occlusion plethysmography and rheography, the state of peripheral circulation of 18 test subjects during 25 exposures to head-down tilt was investigated. The test subjects were subdivided into two groups: nine subjects actively changed their body position by 90 degrees and nine others were passively transferred to the head-down position at an angle of -12 degrees. It was found that regular training facilitated the development of vascular mechanisms which assisted adaptation to the head-down tilt: it decreased the tone of resistance vessels of the arms and legs and that of capacitance vessels of the legs; it increased the tone of cerebral resistance and capacitance vessels. The tone of leg capacitance vessels varied, depending on the type of training: passive tilting increased their compliance and active tilting decreased it. The final result of the tone redistribution in the peripheral vascular bed during training was a smaller increase of head blood content and a smaller fluid outflow from the legs.     

Central circulation in the healthy human being during 7 days of head-down tilt and decompression of different body areas

The effect of 7-day head-down tilt (-15 degrees) and decompression of various body parts (lower body-LBNP, upper body in the area of the hydrostatically indifferent point-UBNP, and local negative pressure applied to both calves-LNP) on central circulation was investigated on eight healthy test subjects who, for 10 days, had catheters (Swan-Ganz) implanted into the pulmonary and radial arteries. It was shown that, when calculated by square centimeter of the decompression area, the effect of UBNP on central venous pressure (CVP) and pulmonary artery pressure (PAP) was 3-4 times greater than that of LBNP or LNP. This indicates a high sensitivity of this body part to the exposure. During the 7-day study, CVP and PAP showed the most distinct changes. By the 7th hour of the head-down tilt study, CVP remained unchanged and systolic PAP increased by 5.5 mm Hg (27%) (p less than 0.05). This was paralleled by a decrease of plasma aldosterone and renin. By the 2nd day of the study, CVP and PAP were close to the pretest level; on the 3rd day, they began to decline and remained about 3 mm Hg lower than the pretest values to the end of the study (p less than 0.05). During this same period, the contractility of the right heart (the mean rate of right ventricular pressure increment) decreased by 34% (p less than 0.05) and its work by 27% (p less than 0.05). By the 24th hour after the study (the recovery period), CVP and PAP were close to the pretest values, whereas heart rate, cardiac index and oxygen tension in the mixed venous blood were significantly higher than the pretest values (p less than 0.05). The factors responsible for these changes and the potential application of the catheterization technique in biomedical investigations during real space flight are discussed.     

Coronary circulation of the healthy man exposed to tilt tests, LBNP, and head-down tilt

The effect of tilt (head-up and head-down) tests, LBNP tests, and 7-d head-down tilt (at -15 degrees) on coronary circulation was investigated in healthy male volunteers. Catheters were implanted into the coronary sinus and brachial artery. The Ganz catheter in the coronary sinus was used to measure volume flow in the area (constant thermodilution), pressure, and to withdraw samples of outflowing blood for biochemical analysis (acid-base equilibrium and oxygenation). Transfer from supine to upright body position, lower body negative pressure (-30 mm Hg for 20 min), as well as 15 degrees head-down (by day 5-6) produced similar changes in the basic parameters of coronary circulation-reduction of blood flow and oxygen consumption, decrease of pressure in the coronary sinus, and increase of coronary resistance. Transfer from head-up to head-down position caused opposite changes of the above parameters. The changes in coronary circulation were adequate for myocardial metabolic requirements since the biochemical composition of the outflowing blood remained essentially constant during the gravitational exposures described.     

Effect of lower body negative pressure against orthostatic intolerance induced by 21 days head-down tilt bed rest

BACKGROUND: Exposure to actual or simulated weightlessness is known to induce orthostatic intolerance in humans. Many different methods have been suggested to counteract orthostatic hypotension. The repetitive or prolonged application of lower body negative pressure (LBNP) has shown beneficial effects to counter orthostatic intolerance, but devoting so much time to countermeasures is not compatible with space mission objectives or costs. The purpose of the present study was to assess the effects of brief LBNP sessions against orthostatic intolerance during a 21-d head-down tilt (HDT) bed rest. METHODS: There were 12 healthy male volunteers who were exposed to -6 degrees HDT bed rest for 21 d. Six subjects received -30 mm Hg LBNP sessions for 1 h x d(-1) from day 15 to day 21 of the HDT, and six others served as control. Orthostatic tolerance was assessed by means of standard tilt test. RESULTS: Before HDT, all the subjects in the two groups completed the tilt tests. After 21 d of HDT, five subjects of the control group and one subject of the LBNP group could not complete the tilt test due to presyncopal or syncopal symptoms. The mean upright time in the control group 13.0 +/- 4.0 min) was significantly shorter (p < 0.05) than that in the LBNP group (19.0 +/- 2.2 min). Body weight decreased significantly in the control group during HDT, while increasing significantly on day 21 of HDT in the LBNP group. Urine volume increased on days 15-21 of HDT in the control group, but remained unchanged throughout HDT in the LBNP group. A significant decrease in cardiac output and cardiac index, and a significant increase in total peripheral resistance, pre-ejection period, plasma renin activity, aldosterone, and prostaglandin 12 were observed during HDT in both groups. There were no significant differences in these parameters between the two groups. CONCLUSIONS: Brief daily LBNP sessions were effective in preventing orthostatic intolerance induced by 21 d HDT bed rest. However, it did not improve cardiac pump and systolic functions and did not preserve volume regulating hormones.     

Central blood circulation in the healthy human being during 7 days' head-down tilt hypokinesia

The effect of 7-day head-down tilt (-15 degrees) and lower body negative pressure on circulation and oxidative metabolism was investigated on 13 healthy male test subjects. For 7-10 days they had Swan-Ganz catheters implanted in the pulmonary artery and a special cannula in the radial artery. The most marked changes were seen in the pulmonary artery pressure (PAP) and central venous pressure (CVP) that varied in a phase-like manner. By the 7th hour of bed rest the PAP increased significantly; this was followed by increases in the total lung resistance and the right ventricle function, as well as by a slight decrease of renin and aldosterone. Beginning with bed rest days 2 or 3 the PAP and CVP declined and remained lowered, as compared to the pretest level, till the end of bed rest. The responses to LBNP tests changed by bed rest day 2. Possible mechanisms of the above changes at rest and during LBNP tests are discussed.     

Disruption of the circadian rhythm of the body temperature in monkeys during head-down tilt hypokinesia

A rhesus monkey was restrained in the prone position with the head kept at -6 degrees. The day: night cycle was 16:8 and ambient temperature 20-22 degrees C. Skin temperature (ST) in the ankle area and body temperature (BT) in the armpit were measured by thermistors every 16 min during 14 days. Mean daily values of both temperatures decreased continuously beginning with day 5 and reached a minimum on day 11. Between days 5 and 11 the BT rhythm was split into two components, i. e. morning and evening (with the 12-hour periodicity being predominant in the spectrum). The amplitude of the ST rhythm declined and reached a minimum on day 4. On that day the rhythm phase was sharply shifted (6-7 hour delay) and thereafter the amplitude and phase of the ST rhythm gradually restored. The above effects (dissociation of the BT rhythm, drop of the amplitude and delay of the phase of the BT rhythm) can be explained by a transient attenuation of the relationship (release) of circadian oscillators due to a greater load on the mechanisms of adaptation to simulated microgravity.     

Clinico-physiological aspects of the tissue oxygen supply of the human body during head-down tilt hypokinesia

In three antiorthostatic hypokinesia studies of varying duration gas exchange, central and regional hemodynamics were investigated. The hypokinetic effect on oxygen changes in the human body was evaluated by comparing the above results with biochemical data. It is concluded that bed rested subjects developed hypoxic changes of mixed circulatory-respiratory type. When oxygen supply changes drastically under the conditions that were simulated in our study, it is recommended to use oxygen therapy.     

Brain blood circulation and oxygen supply in the healthy human being during measured physical loading in a head-down tilt posture

Eleven healthy male test subjects performed exercises of 600 kgm/min (98 W) for 20 min in the head-down position (-15 degrees). A day before exercises they were catheterized, with catheters implanted into the internal jugular vein and brachial artery. It was shown that exercises in the head-down position led to an increase in cerebral circulation, a decrease in oxygen utilization and a decrease in jugular pressure. CO2 tension in arterial blood and blood outflowing from the brain remained comparatively stable, while base deficiency and buffer capacity decreased by a similar value.     

Human amino acid metabolism during combined short-term exposure to head-down tilt hypokinesia and ultraviolet irradiation

Changes in the amino acid pool in the plasma of test subjects exposed to short-term head-down tilt combined with ultraviolet irradiation were investigated. Exposure to head-down tilt alone (Group 1 of 3 test subjects) and combined with ultraviolet irradiation (10 sessions) (Groups 2 and 3 of 3 test subjects each) acted as a stress-agent that diminished the amino acid pool. Exposure to 2-hour head-down tilt in combination with 20 UV-irradiations (Groups 2 and 3) increased the amino acid pool as a result of inhibition of anabolic and stimulation of catabolic processes. The amino acid pool did not return to normal within the recovery period allowed.     

Hemodynamic and cerebral ventricular functions during head-down tilt at -15 degrees

Sixteen men were exposed to head-down tilt at -15 degrees for 6 h. Cerebral circulation was examined by bipolar rheoencephalography, ventricular function by one-dimensional echoventriculometry, and blood pressure by Korotkoff's sounds. Group 1 consisting of 10 test subjects tolerated the antiorthostatic exposure well enough (they reported only blood rush to the head and moderate facial puffiness). They showed stable parameters of cerebral circulation, ventricle size, and mean blood pressure, which is indicative of adequate compensatory capabilities of their organisms. Group 2 consisting of 6 test subjects exhibited polymorphic clinical symptoms, including the syndrome of liquor hypertension (burst-like headache, autonomic dysfunction, etc). This was accompanied by decreased pulse pressure of cerebral vessels, primarily in the vertebrobasilar area, instability of the vessel tone against the background of marked dilatation of the capacitance vessels and brain ventricles.     

Lipid metabolic indices in man during head-down tilt hypokinesia and their correction

Twenty one test subjects exposed to head-down tilt for 120 days were subdivided to four groups: Group 1--nine subjects used as controls, Group 2--three bed rested subjects who performed regular exercises, Group 3--four bed rested subjects who were given selected drugs, including Vitamin F-99 that influenced lipid metabolism, and Group 4--four bed rested subjects who performed regular exercises and received Vitamin F-99. At different stages of bed rest and recovery the content of lipoprotein fractions and lipids of different classes in serum was measured by thin-layer chromatography. The concentration of cholesterol in biliary lipids was determined. In Group 1 and 2 subjects bed rest led to a drastic and significant increase of cholesterol esters in blood, a decrease of phospholipids, variations of triglycerides and non-esterified fatty acids, and a lower percentage content of alpha-lipoproteins. The use of Vitamin F-99 produced positive changes in the above parameters of lipid metabolism (it normalized the level of cholesterol and phospholipids). In Group 4 subjects the effect of exercise combined with drugs was most distinct.     

Postexercise heat loss and hemodynamic responses during head-down tilt are similar between genders

PURPOSE: We evaluated the hypothesis that during recovery from dynamic exercise in the 15 degrees head-down tilt (HDT) position, the attenuation of the fall in mean arterial pressure (MAP), cutaneous vascular conductance (CVC), and sweat rate, and the augmentation of the rate of esophageal temperature (T(es)) decay relative to the upright seated (URS) posture, would be different between males and females. METHODS: Fourteen subjects (seven males, seven females) performed two experimental protocols: 1) 15 min of cycle ergometry at 75% VO2peak and then 60 min of recovery in the URS posture; or 2) 15 min of cycle ergometry at 75% VO2peak and then 60 min of recovery in the 15 degrees HDT position. Mean skin temperature, Tes, CVC, sweat rate, cardiac output (CO), stroke volume (SV), heart rate (HR), total peripheral resistance (TPR), and MAP were recorded at baseline; end of exercise; 2 min, 5 min, 8 min, 12 min, 15 min, and 20 min after exercise; and every 5 min until the end of recovery (60 min). RESULTS: During recovery from exercise, we observed significantly greater values for MAP, CVC, and sweat rate with HDT in comparison with the URS recovery posture (P 相似文献   

Stretch- and H-reflexes of the lower leg during whole body cooling and local warming

BACKGROUND: This study was undertaken to evaluate if possible changes in stretch- and H-reflexes could be related to the changes in the EMG activity of the cooled lower leg muscles observed during a stretch-shortening cycle exercise. METHODS: Eight subjects wearing shorts and jogging shoes were exposed once to 27 degrees C and twice to 10 degrees C for 60 min each. During the second exposure to 10 degrees C, the subject's lower legs were kept warm (10 degrees Clw) with electrical pillows. After the exposures Achilles tendon reflex (stretch reflex) was induced and the EMG activity of the triceps surae was measured. Immediately after reflex measurements the EMG activity of the triceps surae and tibialis anterior during a drop-jump (stretch-shortening cycle) was measured. After similar thermal exposures electrically induced H-reflex from the calf was measured. RESULTS: During the preactivity and stretch phases the EMG activity of the triceps surae increased after the exposure to 10 degrees C, whereas during the shortening phase it decreased. During the shortening phase cooling, on the contrary, increased the activity of tibialis surae anterior. These changes disappeared at 10 degrees Clw. At 10 degrees C the maximum EMG-amplitude of triceps surae during stretch reflex decreased (p<0.05), reflecting suppressed muscle spindle activity. Suppressed spindle activity causes the agonist to be unfacilitated and the antagonist muscle contraction to be uninhibited, which was seen in the present study as decreased agonist and increased antagonist EMG activity during the shortening phase at 10 degrees C. The Hmax/Mmax-ratio, H-reflex latency and amplitude increased at 10 degrees C (p<0.05), reflecting increased motoneuron pool excitability. This in part may explain the increased EMG activity during the preactivity and stretch phases. CONCLUSION: Cooling-induced increase in the excitability of the motoneuron pool and suppression of muscle spindle activity seem to be responsible of the EMG activity changes during the stretch-shortening cycle, consequently decreasing muscular performance.     

The coupling between upper and lower extremity synergies during whole body reaching

When performing whole body reaching movements, all four limbs participate in the task. We hypothesized that the synergies that characterize upper and lower extremity movement are flexible and become coupled into one functional unit to transport the body towards the target. To test this hypothesis, subjects reached to three targets, one within and two beyond arm's length. In addition, subjects reached at two speeds and either stopped at the target or returned to the original start position. To assess the coupling during the various whole body reaches, a principal component analysis was performed on the displacements of the five primary joints used to accomplish the task (ankle, knee, hip, shoulder and elbow). Analysis of the loadings from the principal component analysis indicated that the first component represented the reaching element of the task, while the second and third components represented the postural element. When reaching within arm's length the variance explained by the joint coupling was distributed between the first three principal components. However, as reach distance increased, the distribution shifted with most of the variance being explained by the first principal component. Neither movement velocity nor final joint configuration affected the coupling between the joints. Analysis of center of mass indicated that it shifted progressively forward as reached distance increased. We conclude that as target distance increased, the reach and postural synergies became coupled resulting in the arms, legs and trunk working together as one functional unit to move the whole body forward.     

Vital capacity and airflow measured from partial flow-volume curves during 5 degrees head-down tilt

Ten healthy young males were subjected to 7 min of 5 degrees head-down tilt, during which their forced vital capacity, peak flow rate (from complete flow-volume curves), and MEF 40% and 25% (airflows when 40% and 25% of the vital capacity remains in the lungs) from the partial curves were measured. The values of these variables and the heart rate and blood pressure were not significantly different from the values obtained in the supine position. In view of these findings it is concluded that the increase in the intra-thoracic blood volume, known to occur with 5 degrees head-down tilt used as a model for simulating weightlessness, does not embarrass respiratory mechanics.     

Trans-thoracic fluid shifts and endocrine responses to 6 degrees head-down tilt.

A tomographic method of measuring electrical impedance known as Applied Potential Tomography (APT) has been used to image the impedance changes within the thoraxes of 8 healthy volunteers (4 male, 4 female) during 4-h periods of 6 degrees head-down tilt (HDT). A large decrease in impedance, reflecting an increase in thoracic fluid, was apparent within 1 min of tilting, peaked after 45 min, and was maintained throughout, although during the 4 h there was an 8% return towards baseline resistivity. Resistivity changes were most obvious in the region of the lungs. Simultaneous measurements of the key fluid regulating hormones revealed a significant increase in atrial natriuretic peptide (ANP) and a significant decrease in angiotensin II (AII) and aldosterone. There was no significant difference in plasma antidiuretic hormone level. These results illustrate the dynamic nature of fluid shifts during HDT, the spatial distribution of the fluid within the thorax and the associated endocrine responses.     

Cardiovascular adjustments to exercise distributed between the upper and lower body

The present study examined the hemodynamic differences between upper- and lower-body exercise where the total power output (PO) was proportionally distributed between the upper and lower body. Six males completed five combinations of arm-leg exercise at maximal and three submaximal intensities. The ratio of arm PO to total PO for each exercise combination was 0, 25, 50, 75, and 100%. At each submaximal intensity, VO2 and cardiac output (Q) were not different (P greater than 0.05) across exercise combinations. Likewise, heart rate (HR) responses were not different for 0, 25, 50, and 75% at level 1 (mean = 102, 102, 106, 106 beats.min-1, respectively), level 2 (mean = 114, 110, 119, 118 beats.min-1, respectively), and level 3 (mean = 127, 124, 132, 131 beats.min-1, respectively). However, HR for 100% (arm-only exercise) tended to be higher than 0% at level 1 (delta HR = 10 beats.min-1; P less than 0.10), level 2 (delta HR = 12 beats.min-1, P less than 0.06) and level 3 (delta HR = 10 beats.min-1; P less than 0.06). At level 1, stroke volume (SV) remained essentially unchanged from 0-75%, while SV at 100% (108 ml) was slightly though not significantly lower (P less than 0.10) than 0% (125 ml). At exercise levels 2 and 3, SV remained unchanged for 0 and 25%; however, SV at 50, 75, and 100% were generally lower (P less than 0.05) compared with 0%. These results indicate that involving the leg musculature to varying degrees during arm-leg exercise attenuates the hemodynamic differences observed during strict upper body versus strict lower body exercise.