Orthostatic Function During a Stand Test Before and After Head‐Up or Head‐Down Bedrest

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Abstract
Astronauts may exhibit orthostatic dysfunction upon returning to 1 g after space flight Understanding cardiovascular changes at 0 G will provide insights into the mechanisms of the loss of orthostatic tolerance, whether due to space flight or bedrest. Bedrest is one model used to produce cardiovascular changes that are associated with space flight. In the current study, young male adults were placed at −5°, +10, +20, or +42° bedrest (0, 1/6, 1/3, and 2/3g, respectively) for 6 hours on 4 different days. This was preceded and followed by a stand test: 5 minutes in the supine position, and then 5 minutes in the standing position, with the feet 9 inches apart and 6 inches from the wall. Cardiovascular values were measured at 1‐minute intervals. Systolic and diastolic pressures were measured using an automated blood pressure device; mean arterial pressure (MAP; mm Hg) was calculated. Heart rate (bpm) and cardiac parameters were measured with a thoracic impedance device. Minute 3, 4, and 5 values were used to determine whether there were time or angle effects. Of six subjects, one reported nausea upon 3 minutes of standing after 6 hours of bedrest at −5°. The same subject was lightheaded in the first minute of standing after 6 hours of bedrest at +10°. Mean heart rate pre‐bedrest in the supine position was 63 and increased by 24 bpm on standing. Heart rate post‐bedrest in the supine position was 65 and increased by 35 bpm on standing; standing heart rate increased 11 bpm after −5° bedrest. The increases after +10°, +20°, and +42° tilts were 4,3, and 4 bpm, respectively. Changes in the mean arterial blood pressure were minimal. Results from the stand test pre‐ and post‐ 6 hours of bedrest at −5° but not at +10°, +20°, or +42° are similar to those after space flight.