Pathients with rthostatic intolerance have difficulty during the transition from supine to upright. Even healthy people may exhibit nausea, dizziness, or headache during long periods of standing. Lower body negative pressure (LBNP) is often used to simulate orthostatic stress to elucidate orthostatic reflexes. Near-infrared spectroscopy (NIRS) is used in various research and clinical fields. NIRS monitors tissue oxygenation changes by measuring oxygenated and deoxygenated hemoglobin (Hb) and it might be possible to apply NIRS to assess vascular responses during LBNP to blood volume change. The purpose of this thesis was to examine vasoconstrictor responses in the lower limbs during graded LBNP to determine the efficacy of using NIRS to investigate orthostatic mechanisms, and to investigate the physiological differences between subjects with high and low tolerance and between males and females during graded orthostatic stress. A series of five orthostatic studies were conducted: 1) Changes in (superficial) blood distribution in thigh during LBNP; 2) Assessments of tissue vascular responses between forearm and selective deep calf and calf with superficial portion during graded LBNP; 3) Comparisons of oxygenated Hb with blood flow measured by plethysmography and muscle sympathetic nerve activity during graded LBNP (reliability of calf blood flow assessment by selective deep oxygenated Hb); 4) Differential vasoconstrictor responses between subjects with low and high tolerance to graded LBNP; 5) Comparisons in peripheral vascular responses between genders (with high and low tolerant individuals) during graded LBNP. It was concluded that: 1) NIRS measurement of oxygenated and deoxygenated Hb was able to distinguish the tendency of blood distribution between the arterial and venous compartments during graded LBNP. 2) Selective deep calf oxygenated Hb responses assessed by a two-detector NIRS model may reflect (sympathetically mediated) blood flow changes in muscle vasculature. 3) Selective deep NIRS may be an useful tool to evaluate calf vasoconstrictor responses based on comparisons between selective deep oxygenated Hb responses (arbitrary units) and blood flow changes evaluated by mercury strain gauge plethysmography, and muscle sympathetic nerve activity assessed by microneurography. 4) The delayed reductions in the selective deep oxygenated Hb and heart rate (HR) increments in subjects with high compared to low tolerance may indicate that cardiovascular responses to LBNP in the High group were shifted toward more severe negative pressure levels during graded LBNP. 5) The greater reductions in oxygenated Hb with either negative pressure levels, or with blood pooling, in men compared to women suggest that men had greater vasoconstrictor responses during graded LBNP.
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