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The Effect of Height on Cardiovascular Responses to Orthostasis.  L.L. Bergeron1, K.D. Cohen2, C.A. Rickards3, and V.A. Convertino3    1US Air Force Academy, CO;  2The Bionetics Corporation, Kennedy Space Center, FL;  3US Army Institute of Surgical Research, TX

   Previous studies have demonstrated that orthostatic tolerance is inversely related to height, i.e., a taller person has less orthostatic tolerance. This relationship may be explained by observations that individuals with longer legs result in greater pooling of blood in the lower extremities or a greater hydrostatic column from the heart to the head leads to less cerebral blood flow. We hypothesized that a shorter hydrostatic column from the heart to the head would be related to a smaller reduction in mean cerebral blood flow velocity (Vm) upon standing and subsequently improved rating of perceived orthostatic stress. Twenty three healthy subjects (13 males and 10 females) participated in the study after giving voluntary consent. We used a squat-stand test (SST) protocol consisting of moving to a standing posture after a 4-min period in a deep knee squat. Vm (transcranial Doppler), arterial blood pressure (infrared photoplethysmography), and heart rate were measured during the final 15 sec of squatting and the initial 10 sec of standing. Overall height, heart-to-head distance, and leg length were measured on each subject. Correlation coefficients were calculated between relative changes (%∆) in Vm during the transient phase of the SST and body heights. %∆Vm was poorly correlated with overall height (r2 = 0.063), heart-to-head distance (r2 = 0.048), and leg length (r2 = 0.014). Against expectations, the transient reduction cerebral blood flow during the SST was poorly related with body height dimensions. Our data do not support the hypothesis that cerebral blood flow is significantly influenced by body height, heart-to-head distance, or leg length.