Effects of body size on orthostatic tolerance
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Although physiological mechanisms have been considered to be associated with gender difference in orthostatic tolerance, physical factors could affect orthostatic tolerance potentially. This study is designed to explore effects of body size on orthostatic tolerance with and without consideration of gender. Methods: Forty-six (22 men and 24 women) healthy subjects participated in this study. After a 12 min baseline, subjects were exposed a graded lower body negative pressure (LBNP) from 0 [Difference symbol] -100 mmHg (or presyncope), with a reduction of 10 mmHg every 4 mins. Some physical factors and cardiovascular measurements were determined in the test. Least square linear regression analysis was used to assess the relationship between LBNP tolerance index (LTI) and physical and cardiovascular measurements. Results: No physical factors enter the final predict model with and without accounting for gender. The resting total peripheral conductance (TPC), pulse pressure (PP) and maximal change in heart rate (cHR) are positively related to LTI while the early change in HR (cHRe) has a negative coefficient. Two regression equations are LTI = 137.40 +36.13 gender + 746.90 TPC - 2.87 cHRe + 2.67 cHR and LTI = 82.68 + 1.35PP + 697.74 TPC -3.10 cHRe + 2.78cHR. The variety explained by gender in first model was replaced partly by variety explained by PP in second model. Conclusion: Physical factors have no effect on LBNP tolerance. Besides gender, LBNP tolerance is also associated with individual HR reserve and the releasing rate of HR reserve as well as resting vascular resistance. The gender difference in LBNP tolerance may be due to lower PP and other physiological factors rather than physical factors in women.