Abolition of the Postural Autonomic Requirements for Cardiovascular Control during Water Immersion

The cardiovascular status and its autonomic regulation were studied in 10 healthy men (25.5 ± 1.2 years; mean ± SEM) in the supine and upright positions in air (AIR) and in water (WI).Changing from supine to upright in AIRposture decreased stroke volume (SV, from 89.9 ± 5.3 to 81.1 ± 5.0 mL; p<0.05) and significantly increased heart rate (HR, from 56.5 ± 5.6 to 81.2 ± 2.7 bpm), systolic (SBP, from 132.4 ± 5.2 to 140.8 ± 4.7 mmHg) and diastolic (DBP, from 72.1 ± 3.0 to 89.8 ± 2.8 mmHg) blood pressure. Also, plasma catecholamines and the sympathetic indicators derived from spectral analysis of HR and BP variability increased (p<0.05), while parasympathetic indicators of cardiac control decreased (p<0.05). During supine WI, SBP (112.8 ± 4.2 mmHg) and DBP (61.1 ± 1.9 mmHg) were lower (p<0.05) than during supine AIR with higher HR (67.0 ± 2.9 bpm; p<0.05).Contrary to AIR, no or only minor cardiovascular and autonomic changes were observed with posture change. Thus, compared to upright AIR, SV was higher whereas HR and BP were lower (p<0.05)during upright WI. Also, parasympathetic indexes were higher and sympathetic indexeswere lower (p<0.05).It was concluded that th