Summary
Experiments were undertaken to determine the effects of hydration status on a) orthostatic responses, and on b), relative changes in intravascular volume and protein content, during 70‡ head-up tilt (HUT). Six men underwent 45 min of HUT, preceded by 45 min supine, first dehydrated, and again 105 min later after rehydration with water. Heart rate was consistently lower following rehydration (p<0.01), while supine diastolic pressure was higher (p<0.02). Systolic pressure fell during dehydrated HUT (p<0.01), but not during rehydrated HUT. Postural haemoconcentration, which was reduced after rehydration (p<0.001), was accompanied by a decrease in intravascular albumin content (p<0.05). Two subjects experienced severe presyncopal symptoms during dehydrated HUT, but not during rehydrated HUT. Thus, it appears that rehydration after fluid restriction improves orthostatic tolerance. Furthermore, extravascular hydration status may be more important than intravascular hydration status in determining orthostatic tolerance.
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Harrison, M.H., Hill, L.C., Spaul, W.A. et al. Effect of hydration on some orthostatic and haematological responses to head-up tilt. Europ. J. Appl. Physiol. 55, 187–194 (1986). https://doi.org/10.1007/BF00715003
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DOI: https://doi.org/10.1007/BF00715003