Thoracic load carriage impairs the acute physiological response to hypoxia in healthy males

Abstract

To assess the impact of thoracic load carriage on the physiological response to exercise in hypoxia. Healthy males (n = 12) completed 3 trials consisting of 45 min walking in the following conditions: (1) unloaded normoxia (UN; F_IO₂:20.93%); (2) unloaded hypoxia (UH; F_IO₂:~13.0%); and (3) loaded hypoxia (LH; 29.5 kg; F_IO₂:~13.0%). Intensity was matched for absolute VO₂ (2.0 ± 0.2 L·min^-1) across conditions and relative VO₂ (64.0 ± 2.6 %VO_2max) across hypoxic conditions. With LH versus UH, there were increases in breathing frequency (5-11 breaths·min^-1; p < 0.05) and decreases in tidal volume (10%-18%; p < 0.05) throughout exercise due to reductions in end inspiratory lung volumes (p < 0.05). Consequently, deadspace (11%-23%; p < 0.05) and minute ventilation (7%-11%; p < 0.05) were increased starting at 20 and 30 min, respectively. In addition, LH increased perceived exertion/dyspnea and induced inspiratory (~12%; p < 0.05 vs. UN) and expiratory (~10%; p < 0.05 vs. pre-exercise) respiratory muscle fatigue. Expiratory flow limitation was present in 50% of subjects during LH. Cardiac output and muscle oxygenation were maintained during LH despite reduced stroke volume (6%-8%; p < 0.05). Finally, cerebral oxygenated/total hemoglobin were elevated in the LH condition versus UH starting at 15 min (p < 0.05). Thoracic load carriage increases physiological strain and interferes with the compensatory response to hypoxic exposure.

Keywords: altitude; expiratory flow limitation; hemodynamics; near‐infrared spectroscopy; respiratory muscle fatigue; ventilation.

FIGURE 1

Experimental design and protocol. MEP, maximal expiratory pressure; MIP, maximal inspiratory pressure; NIRs, near‐infrared spectroscopy.

FIGURE 2

(a) Minute ventilation; (b) tidal volume; (c) breathing frequency; and (d) deadspace ventilation during exercise. Data are presented as mean ± SE. LH, loaded hypoxic; UH, unloaded hypoxic; UN, unloaded normoxic. ^a,b,cDenotes different from prior time point within UN, UH, and LH, respectively (p < 0.05).

FIGURE 3

Operating lung volumes during exercise. Data are presented as mean ± SE. EELV, end expiratory lung volume; EILV, end inspiratory lung volume. ^a,b,cDenotes different from prior time point within UN, UH, and LH, respectively (p < 0.05).

FIGURE 4

(a) Cardiac output; (b) heart rate; (c) stroke volume during exercise. Data are presented as mean ± SE. LH. Loaded hypoxic; UH, unloaded hypoxic; UN, unloaded normoxic. ^a,b,cDenotes different from prior time point within UN, UH, and LH, respectively (p < 0.05).

FIGURE 5

(a) Muscle deoxygenated hemoglobin; (b) muscle oxygenated hemoglobin; (c) muscle total hemoglobin. Data are presented as mean ± SE. LH, loaded hypoxic; UH, unloaded hypoxic; UN, unloaded normoxic. ^a,b,cDenotes different from prior time point within UN, UH, and LH, respectively (p < 0.05).

FIGURE 6

(a) Cerebral deoxygenated hemoglobin; (b) cerebral oxygenated hemoglobin; (c) cerebral total hemoglobin. Data are presented as mean ± SE. LH. loaded hypoxic; UH, unloaded hypoxic; UN, unloaded normoxic. ^a,b,cDenotes different from prior time point within UN, UH, and LH, respectively (p < 0.05).

FIGURE 7

The relationship between dyspnea and minute ventilation. Data are presented as mean ± SE. LH, loaded hypoxic; UH, unloaded hypoxic; UN, unloaded normoxic; V_E, minute ventilation.