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INCREASED CEREBRAL OXYGENATION DESPITE REDUCED STROKE VOLUME WITH PROLONGED LOAD CARRIAGE IN HYPOXIA

Abstract

BACKGROUND: Thoracic load carriage reduces stroke volume in normoxia; however, it is unknown whether this compromises oxygen delivery to the muscle or brain during exercise in hypoxia. Therefore, the purpose of this study was to determine the effects of load carriage on hemodynamics and oxygen kinetics during prolonged exercise in hypoxia.  METHODS: In a single-blinded and randomized crossover design, healthy male subjects (n=12) performed 3 exercise tests on a treadmill consisting of the following conditions: 1) unloaded normoxic (UN: FiO2=20.93%), 2) unloaded hypoxic stimulating 3,650 m (UH: FiO2=~13%), and 3) loaded (~65 lb) hypoxic (LH). Exercise consisted of 45 min uphill (8%) walking at speeds individually customized to elicit an equivalent relative intensity in hypoxia (64.0±2.6%VO2max) and absolute VO2 across conditions (2.0±0.2 L/min). Near-infrared spectroscopy, impedance cardiography, sphygmomanometry, and pulse oximetry were used to assess oxygen kinetics (i.e., muscle and cerebral oxygenated [O2HHb], deoxygenated [HHb], and total hemoglobin [tHHb]), hemodynamics (i.e., cardiac output [Q], stroke volume [SV], heart rate [HR], arterial blood pressure [SBP and DBP]), and oxygen saturation (SpO2), respectively. Two-way repeated measures ANOVAs were employed with post hoc one-way ANOVAs (i.e., given interactions effects) to analyze differences between conditions (α=0.05). RESULTS: In both hypoxic conditions, SpO2 and Q were decreased and increased throughout exercise relative to UN (p<0.05). However, SV and HR were reduced and increased, respectively, with LH versus UH (p<0.05). Additionally, DBP was increased at 5 min with LH relative to the other conditions, but there were no other differences in arterial blood pressures. In muscle, hypoxia reduced O2HHb throughout exercise relative to UN (p<0.05). Muscle HHb was increased until 20 min and throughout exercise with UH and LH vs. UN, respectively (p<0.05), with no differences between hypoxic conditions. Finally, cerebral HHb and O2HHb were increased and decreased with UH versus UN throughout exercise (p<0.05). With LH, cerebral HHb was similarly elevated versus UN (p<0.05), but O2HHb was similar and increased versus UN and LH, respectively, starting at 15 min (p<0.05). Moreover, tHHb was increased with LH relative to UN and UH starting at 15 min (p<0.05). CONCLUSIONS: Load carriage reduces cardiovascular efficiency in hypoxia. Nevertheless, muscle oxygenation is maintained while cerebral oxygenation and regional blood flow are increased relative to unloaded hypoxic exercise. This potential increase in cerebral perfusion may increase the risk for altitude illnesses. Grant or funding information: The Jackson Hope New Directions in Research grant funded this study.

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