Increased postflight carotid artery stiffness and inflight insulin resistance resulting from 6-mo spaceflight in male and female astronauts.

Removal of the normal head-to-foot gravity vector and chronic weightlessness during spaceflight might induce cardiovascular and metabolic adaptations related to changes in arterial pressure and reduction in physical activity. We tested hypotheses that stiffness of arteries located above the heart would be increased postflight, and that blood biomarkers inflight would be consistent with changes in vascular function. Possible sex differences in responses were explored in four male and four female astronauts who lived on the International Space Station for 6 mo. Carotid artery distensibility coefficient (P = 0.005) and ß-stiffness index (P = 0.006) reflected 17-30% increases in arterial stiffness when measured within 38 h of return to Earth compared with preflight. Spaceflight-by-sex interaction effects were found with greater changes in ß-stiffness index in women (P = 0.017), but greater changes in pulse wave transit time in men (P = 0.006). Several blood biomarkers were changed from preflight to inflight, including an increase in an index of insulin resistance (P < 0.001) with a spaceflight-by-sex term suggesting greater change in men (P = 0.034). Spaceflight-by-sex interactions for renin (P = 0.016) and aldosterone (P = 0.010) indicated greater increases in women than men. Six-month spaceflight caused increased arterial stiffness. Altered hydrostatic arterial pressure gradients as well as changes in insulin resistance and other biomarkers might have contributed to alterations in arterial properties, including sex differences between male and female astronauts.

Prior head-down tilt does not impair the cerebrovascular response to head-up tilt.

The hypothesis that cerebrovascular autoregulation was not impaired during head-up tilt (HUT) that followed brief exposures to varying degrees of prior head-down tilt (HDT) was tested in 10 healthy young men and women. Cerebral mean flow velocity (MFV) and cardiovascular responses were measured in transitions to a 60-s period of 75° HUT that followed supine rest (control) or 15 s HDT at -10°, -25°, and -55°. During HDT, heart rate (HR) was reduced for -25° and -55°, and cardiac output was lower at -55° HDT. MFV increased during -10° HDT, but not in the other conditions even though blood pressure at the middle cerebral artery (BPMCA) increased. On the transition to HUT, HR increased only for -55° condition, but stroke volume and cardiac output transiently increased for -25° and -55°. Total peripheral resistance index decreased in proportion to the magnitude of HDT and recovered over the first 20 s of HUT. MFV was significantly less in all HDT conditions compared with the control in the first 5-s period of HUT, but it recovered quickly. An autoregulation correction index derived from MFV recovery relative to BPMCA decline revealed a delay in the first 5 s for prior HDT compared with control but then a rapid increase to briefly exceed control after -55° HDT. This study showed that cerebrovascular autoregulation is modified by but not impaired by brief HDT prior to HUT and that cerebral MFV recovered quickly and more rapidly than arterial blood pressure to protect against cerebral hypoperfusion and potential syncope.

Cerebral hypoperfusion is exaggerated with an upright posture in heart failure: impact of depressed cardiac output.

OBJECTIVES: The purpose of this study was to examine cerebral blood flow (CBF) supine and during upright sitting in HF patients and control subjects to test the hypothesis that patients with HF will have a greater reduction in CBF from supine to seated compared with the control group. BACKGROUND: Reduced CBF has been reported in patients with heart failure (HF). However, previous work has only examined CBF while supine, although an upright posture common to daily living may lead to further reductions. METHODS: In 22 HF patients and 22 age- and sex-matched control subjects, continuous heart rate, mean arterial pressure, and end-tidal carbon dioxide readings were collected while supine and seated upright. Cardiac output was estimated from pulse contour analysis and was corrected for body size (cardiac index). The right internal carotid artery was imaged by using ultrasound to estimate CBF. RESULTS: Heart rate increased less in response to the upright posture in HF patients versus control subjects (p = 0.006). Mean arterial pressure was unchanged, whereas end-tidal carbon dioxide decreased in response to position (p = 0.004) but did not differ between groups. Cardiac index was lower in patients with HF (p < 0.001) and decreased in both groups in response to the upright posture (p = 0.025), with a trend for a greater decrease in the HF group (p = 0.065). CBF decreased more in response to the upright posture in the HF group than in the control group (p = 0.007). CONCLUSIONS: The reduction in CBF was exaggerated in the upright posture in HF patients and may increase the risk for subsequent cognitive impairment.

Effects of an artificial gravity countermeasure on orthostatic tolerance, blood volumes and aerobic power after short-term bed rest (BR-AG1).

Exposure to artificial gravity (AG) in a short-arm centrifuge has potential benefits for maintaining human performance during long-term space missions. Eleven subjects were investigated during three campaigns of 5 days head-down bed rest: 1) bed rest without countermeasures (control), 2) bed rest and 30 min of AG (AG1) daily, and 3) bed rest and six periods of 5 min AG (AG2) daily. During centrifugation, the supine subjects were exposed to AG in the head-to-feet direction with 1 G at the center of mass. Subjects participated in the three campaigns in random order. The cardiovascular effects of bed rest and countermeasures were determined from changes in tolerance to a head-up tilt test with superimposed lower body negative pressure (HUT), from changes in plasma volume (PV) and from changes in maximum aerobic power (V̇o2 peak) during upright work on a cycle ergometer. Complete data sets were obtained in eight subjects. After bed rest, HUT tolerance times were 36, 64, and 78% of pre-bed rest baseline during control, AG1 and AG2, respectively, with a significant difference between AG2 and control. PV and V̇o2 peak decreased to 85 and 95% of pre-bed rest baseline, respectively, with no differences between the treatments. It was concluded that the AG2 countermeasure should be further investigated during future long-term bed rest studies, especially as it was better tolerated than AG1. The superior effect of AG2 on orthostatic tolerance could not be related to concomitant changes in PV or aerobic power.

Heart rate and daily physical activity with long-duration habitation of the International Space Station.

INTRODUCTION: We investigated the pattern of activity and heart rate (HR) during daily living on the International Space Station (ISS) compared to on Earth in 7 long-duration astronauts to test the hypotheses that the HR responses on the ISS would be similar to preflight values, although the pattern of activity would shift to a dominance of arm activity, and postflight HR would be elevated compared to preflight during similar levels of activity. METHODS: HR and ankle and wrist activity collected for 24-h periods before, during, and after spaceflight were divided into night, morning, afternoon, and evening segments. Exercise was excluded and analyzed separately. RESULTS: Consistent with the hypotheses, HR during daily activities on the ISS was unchanged compared to preflight; activity patterns shifted to predominantly arm in space. Contrary to the hypothesis, only night time HR was elevated postflight, although this was very small (+4 +/- 3 bpm compared to preflight). A trend was found for higher postflight HR in the afternoon (+10 +/- 10 bpm) while ankle activity level was not changed (99 +/- 48, 106 +/- 52 counts pre- to postflight, respectively). Astronauts engaged in aerobic exercise 4-8 times/week, 30-50 min/session, on a cycle ergometer and treadmill. Resistance exercise sessions were completed 4-6 times/week for 58 +/- 14 min/session. DISCUSSION: Astronauts on ISS maintained their HR during daily activities; on return to Earth there were only very small increases in HR, suggesting that cardiovascular fitness was maintained to meet the demands of normal daily activities.