Cardiovascular responses to standing: effect of hydration

Many astronauts experience intolerance to orthostatic stress after space flight, despite the ingestion of salt tablets and water equivalent to 0.9% saline just before their return to Earth. Previous research indicates that the ingestion of 1.07% saline solution increased plasma volume more than did 0.9% saline. Therefore, the authors hypothesized that the 1.07% saline would be more effective in reducing orthostatic stress during standing. In this study, six men (22-47 years) performed a 5-minute "stand test" (5 minutes supine followed by 5 minutes standing) under four hydration conditions: 1) hypohydrated (HYPO, 20 mg intravenous [IV] Lasix), 2) euhydrated (EU), 3) rehydrated with 1 L 0.9% saline 2 hours after Lasix, or 4) rehydrated with 1 L 1.07% saline. Stand tests were done 4 5 hours after rehydration. Plasma volume was reduced 10% after Lasix, and was restored by both rehydration solutions. When subjects stood, their diastolic pressure, mean pressure, heart rate (HR), and peripheral resistance increased (P < .05), and their stroke volume (SV), cardiac output (CO), and thoracic fluid (TF, by impedance cardiography) decreased (P < .05). Systolic arterial pressure (SBP) increased when subjects stood after saline, but decreased if subjects were HYPO or EU (P < .05 for 1.07% versus HYPO and EU). Heart rate (HR), another indicator of orthostatic stress, did not differ among hydration states. During the last minute of the stand test, TF was greater if subjects had fluid countermeasures. Stroke volume, CO, and TF were significantly less during minute 5 of standing than during minute 3. Whether they would continue to fall in a longer stand test is not known. The results for SBP indicate that 1.07% saline may have advantages over 0.9% saline as a countermeasure to postspace-flight or postbedrest orthostatic intolerance.