Date of Award

Spring 5-2019

Degree Type

Honors College Thesis

Department

Human Performance and Recreation

First Advisor

Daniel Credeur

Advisor Department

Human Performance and Recreation

Abstract

Inter-individual responses to orthostasis (i.e., ability to maintain consciousness in the upright posture) exist. However, few studies provide insight into the potential mechanisms for this variation. The purpose of this thesis project was to explore individual differences (i.e., sex, fitness, and body composition) on the central hemodynamic response to a modified head-up tilt table test (HUT). Fourteen volunteers with an average age of 22±1 years and an average body mass index (BMI) of 8±1 kg/m2 underwent assessments of pulse wave analysis, heart rate variability, and perfusion determination via near-infrared spectroscopy over the gastrocnemius muscle while supine, followed by a 5-min HUT (torso; 70°). Aerobic fitness (VO2 peak; 3-min step test) and body composition (body fat percentage; skinfolds) were estimated. During HUT, heart rate (+5±1 bpm; p<0.001), reflection magnitude (+4±2%; p=0.017), and gastrocnemius perfusion (total hemoglobin—tHB) increased (+4±1 μM), with no change occurring in augmentation index (AIx: p=0.31) and mean arterial pressure (p=0.95). The low-high frequency component ratio increased during HUT (LF/HF: +2.8±1.5 AU), but was only significant at p=0.08. Females exhibited an increase in AIx to HUT (females=+7±2 vs. males=-1.9±3%; p=0.38). Independent of sex, there was a relationship between VO2 peak and LF/HF change to HUT (r=0.68; p=0.02). No interactions were noted for body fat percentage and HUT. These preliminary findings indicate that individual differences (i.e., sex and fitness) influence the cardiovascular response to orthostasis.

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