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Respiratory muscle oxygenation and myoelectrical manifestations during normoxic and hypoxic inspiratory threshold loading

University of British Columbia

Objectives: To examine the acute effect of hypoxia and inspiratory threshold loading (ITL) on (1) oxygenation of inspiratory muscles (sternocleidomastoid [SCM], scalene [SA] and parasternal [PS]) and, (2) electromyography (EMG) of the SA and SCM in healthy adults using near-infrared spectroscopy (NIRS) and linear array EMG respectively. Methods: Twenty healthy adults (12M/8F) were randomly assigned to perform two ITL tests while breathing a normoxic (Norm) or hypoxic (HYP, FIO₂=15%) gas mixture. NIRS devices were placed over the SCM, PS, SA, and a control muscle, tibialis anterior (TA), to monitor oxygenated (O2Hb), deoxygenated (HHb), and total hemoglobin (tHb) as well as tissue saturation index (TSI). SA and SCM activation were evaluated based on a normalized EMG amplitude, whereas fatigue was expressed as a decline in EMG median frequency during maximal inspiratory pressure maneuvers after ITL. Subjects breathed the randomly-assigned gas mixture through the ITL device where the load was increased every two minutes until task failure. Maximal inspiratory pressure (MIP) and dyspnea were recorded before and after ITL. Also, arterial oxygen saturation (SpO₂), electrocardiogram (EKG) and ventilatory measures were monitored throughout the test. Result: Subjects were aged 31±12 years. At task failure, the maximum load, ventilatory parameters and dyspnea perception did not differ between the two ITL tests. At HYP-ITL task failure, SpO₂ was significantly lower, and HHb increased more extensively in SA, SCM and PS than Norm-ITL. SCM TSI decreased more during HYP-ITL compared with Norm-ITL. tHb in the inspiratory muscles increased significantly compared to the decrease in TA during both tests. A subgroup analysis of 13 subjects (9M/4F) revealed that SA and SCM were progressively recruited during Norm-ITL and to a higher extent during HYP-ITL. However, EMG median frequency and MIP did not decline after both ITL tests. Conclusion: The SCM was the most vulnerable to deoxygenation during incremental loading. The increase in SA and SCM EMG is reflective of increased ventilatory loads. Taken together, the deoxygenation and the activation of inspiratory muscles were accentuated by acute hypoxia. These findings suggest that task failure occurred because of other factors such as hypoventilation and dyspnea rather than peripheral muscle fatigue.

Text

2018-01-31

Attribution-NoDerivs 2.5 Canada

http://hdl.handle.net/2429/56672

Rehabilitation Sciences

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nd/2.5/ca/