Abstract
To adequately assess the viability of any particular causal theory of orthostatic intolerance, physical and physiological parameters thought to be associated with orthostasis must be evaluatedsimultaneously within the same individual. Time, cost, and complexity of instrumentation generally limit studies of orthostatic i tolerance to a single independent effect for any given sample of subjects. We, therefore, measured 6 key physical and physiological variables associated with various theories of orthostatic intolerance in 14 males to test the hypothesis that physical factors rather than physiological reflex mechanisms were dominant in contributing to orthostatic tolerance. Measurements included height, plasma volume (expressed as percent of total weight), variation inR-R interval, leg compliance, and carotid-cardiac (high pressure), and cardiepulmonary (low pressure) berereflex sensitivity. Subjects' orthostatic intolerante was quantified by time to syncope during progressive supine lower body decompression. Correlations, regression coefficients, and indices of replicability were calculated using 500 to 1000 bootstrap rasamplings of the original 14 observations. Although all six measurements correlated with time to syncope when evaluated individually, only height (negative), percent plasma volume weight (positive), and, to a lesser extent, carotid-cardiac barareflex sensitivity yielded consistent (reproducible) results when all measures were tested simultaneously. These results suggest that while orthostatic intolerance may be dependent upon a variety of physiological reflexes, physical factots such as height and plasma volume tend to dominate the prediction of time to syncope during lower body negative pressure(LBNP). Physiological reflexes, which act to maintain adequate profusion to the brain during normal terrestrial pasture in humans, may be overwhelmed by the additional orthostatic challenge imposed by progressive lower body decompression.