Daniel A. Baur, Katherine G. Baur, Abaigeal G. Doody, Beverley K. Buchanan, Miles J. Ortiz. Virginia Military Institute, Lexington, VA.

BACKGROUND: Soldiers are often required to carry heavy loads in high altitude environments, which necessitate compensatory increases in cardiac output. However, it is unknown to what degree load carriage influences hemodynamics nor how responses are affected by hypoxia. METHODS: Healthy male subjects (n=10) performed 3 exercise tests on a treadmill consisting of the following conditions: 1) unloaded normoxic (UL: FiO2=20.93%), 2) loaded (~30 kg) normoxic (LN), and 3) loaded hypoxic simulating ~3,650 m (LH: FiO2=~13%). Exercise consisted of 2 x 10 min walking (separated by 5 min rest) with stages matched with the UL condition for absolute VO2 (1.7 L/min) and walking velocity (1.45±0.15 m/s). Cardiovascular responses (i.e., stroke volume [SV], heart rate [HR], cardiac output [Q], end diastolic volume [EDV], and system vascular resistance [SVR]) were assessed via impedance cardiography. Data were analyzed via two-way repeated measures ANOVA (α=0.05). RESULTS: At rest, Q was increased with LH (7.1±1.0 L/min) relative to the other conditions (UL: 5.9±1.0 L/min; LN:6.0±0.7 L/min; p<0.01) owing to an increase in HR (LH: 72±9; UL: 63±9; LN: 62±8; p<0.01). When matched for absolute intensity, EDV (p=0.002) and SV (p=0.028) were reduced with LN (177.7±22.0 ml; 127.3±16.2 ml, respectively) relative to UL (188.8±25.2 ml; 138.0±20.2 ml). Q was increased with LH (141.7±10.1 L/min) relative to UL (111.5±8.6 L/min; p<0.01) via an increase in HR (LH: 141±10; UL:111±9; p<0.001), and versus LN (115.3±9.0 L/min) due to concomitant increases in HR (LN: 115±9; p<0.001) and SV (LH: 136.6±14.6 ml; LN: 127.3±16.2; p=0.032). At the same walking velocity, Q was increased with LN (147.7±14.8 L/min) versus UL (113.1±8.3 L/min; p<0.001) via an increase in HR (148±15 vs. 113±8; p<0.001). Q was further elevated in LH (168.8±10.2 L/min) versus the other conditions (p<0.01) owing to larger magnitude increases in HR (169±10; p<0.01). SVR was reduced with LH (~100-250 dyn·s·cm−5) relative to the other conditions at all timepoints (p<0.01) and with LN (331.0±49.0 dyn·s·cm−5) versus UL (444.6±76.5 dyn·s·cm−5) when matched for walking velocity (p<0.001).CONCLUSIONS: Load carriage reduces stroke volume when matched for absolute intensity. However, this effect is reversed in hypoxia suggesting increased cardiovascular strain to compensate for increased blood flow demands.GRANTS OR FUNDING INFORMATION: This study was funded by the Jackson Hope New Directions in Research grant.

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