Human Cardiovascular Adaptation to Space Flight

Experiment 294 had two primary objectives:

1) to investigate the circulatory adaptation to microgravity by examining specific aspects of central and peripheral cardiovascular function and neurohumoral control mechanisms, and, 2) to test the validity of a 24 hour head-down tilt period as a model of microgravity. Multiple procedures were performed before, during and after the SLS-1 flight. Inflight procedures included central venous pressure (CVP), cardiovascular measurements at rest, maximal exercise (in collaboration with experiment 066), leg blood flow and compliance, static leg volume, and heart rate/blood pressure monitoring during reentry. These studies were supported by pre- and postflight sessions, including a stand test, lower body negative pressure (LBNP) test and an autonomic function test documenting the responses to graded infusions of alpha- and beta-adrenergic agonists. A 24-hour head-down tilt study was performed a year before launch and included most of the sessions listed above.

The experiment was designed to test the following principal hypotheses:

1. Cardiovascular adaptation to microgravity is rapid and effective. The principal stimulus is a headward shift of intravascular and interstitial fluids. The degree of cardiovascular dysfunction in space is minor, and major regulatory mechanisms remain intact.

2. Postflight cardiovascular dysfunction, which includes central hypovolemia with decreased cardiac filling, is a direct consequence of an effective adaptation to microgravity suddenly rendered inappropriate by the return to normal gravity.

3. Bed rest with head-down tilt provides a satisfactory model of microgravity for short-term studies of cardiovascular function. Essential features of early cardiovascular adaptation can be reproduced during a 24-hour period of bed rest with head-down tilt at -6 degrees.

1. To investigate in detail the cardiovascular adaptation to microgravity, including the overall circulatory responses to the headward fluid shift and the impact on cardiovascular control mechanisms.

2. To test the validity of 24-hour head-down tilt as a model of microgravity by comparing data obtained in preflight simulation studies and during actual flight in the same group of crewmembers.

Preflight

Head-Down Tilt Simulation Study, Autonomic Function Test, Lower Body Negative Pressure (LBNP) Test, Supine-Standing Measurements, SVOP Flow and Compliance, SVOP Flow, Compliance and Hyperemic Blood Flow, Resting CV with upright 30%, 60% and 100% Maximal exercise

Inflight

CVP Measurements, Leg Volume Measurement, Echocardiography, Resting CV, Resting CV with upright 30% and 60% maximal exercise, Resting CV with upright 30%, 60% and 100% maximal exercise, SVOP flow and compliance, Descent/Reentry HR, BP and ECG

Postflight

Autonomic Function Test, Lower Body Negative Pressure (LBNP) Test, Supine-Standing Measurements, SVOP Flow and Compliance, SVOP Flow, Compliance and Hyperemic Blood Flow, Resting CV with Upright 30%, 60% and 100% Maximal Exercise

Central venous pressure measurements

Gravity has important effects on cardiac filling pressure and intravascular fluid distribution. Without gravity, a major central fluid shift occurs. Data from ground-based models suggest this should raise central venous pressure (CVP), but indirect measurements in space do not support this. To resolve this controversy, we directly measured CVP in one subject aboard the Spacelab Life Sciences-One Shuttle flight. Continuous CVP measurements, using a 4 fr. catheter began 4 hours before launch and continued into zero-G. Pre-launch, the astronaut lay supine with legs elevated.

CVP dropped promptly below preflight values in space. This refutes the hypothesis that CVP always increases due to the initial zero-G induced fluid shift. In zero-G the relationship between pressure and volume in different fluid compartments may be more complex than initially thought.

Stand test results

Orthostatic intolerance occurs commonly after spaceflight but the hemodynamics and underlying mechanisms remain unclear. We studied six individuals before and after the nine day Spacelab Life Sciences-1 Space Shuttle flight. Heart rate, blood pressure, cardiac output and leg volume were measured supine and standing. After spaceflight supine hemodynamics were unchanged from preflight values, while standing measurements showed significantly increased heart rates (82 bpm preflight vs. 98 bpm postflight) and significantly lower stroke volumes (52 ml preflight vs. 42 ml postflight). Standing leg volume was not increased compared to preflight. Three individuals had near-syncope. All three had different patterns of pre-syncope (one vaso-vagal, one gradual hypotension, one idiopathic). We conclude that the primary physiological effect of spaceflight was an excessive postural fall in stroke volume. Acutely, reflex responses maintained mean blood pressure. Ultimately, orthostatic intolerance may ensue via diverse mechanisms.

Results from the head-down tilt, lower body negative pressure, autonomic function and SVOP sessions need the data from SLS-2 to be completed.

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