Ventilatory response to sustained isocapnic hypoxia in adult humans and other mammals is characterized by a biphasic pattern, with attenuation of neuromotor output to the diaphragm. However, there is no a priori reason that hypoxia-mediated attenuation of respiratory drive would be a common event among other respiratory muscles. At present, little is known about the function of the chest wall muscles during sustained hypoxia. As an obligatory inspiratory muscle with potential to act as a surrogate for neural drive to the relatively inaccessible costal diaphragm, parasternal intercostal has gained interest clinically: its function during a sustained hypoxic insult, as may occur in respiratory failure, warrants investigation. Therefore, in 11 chronically instrumented awake canines, we simultaneously recorded muscle length and shortening and electromyogram (EMG) activity of the parasternal chest wall inspiratory muscle, along with breathing pattern, during moderate levels of sustained isocapnic hypoxia lasting 20-25 min (mean 80 ± 2% oximeter oxygen saturation). Phasic inspiratory shortening and EMG activity of the parasternal intercostal were observed throughout room air and hypoxic ventilation in all animals. Temporal changes in parasternal intercostal shortening tracked the biphasic changes in ventilation during sustained hypoxia. Mean shortening and EMG activity of parasternal intercostal muscle increased significantly with initial hypoxia (P < 0.01) and then markedly declined with constant hypoxia (P < 0.05). We conclude that attenuation of central neural respiratory drive extends to the primary chest wall inspiratory muscle, the parasternal intercostal, during sustained hypoxia, thus directly contributing to biphasic changes in ventilation.NEW & NOTEWORTHY With the potential to act as a surrogate for the generally inaccessible costal diaphragm, parasternal intercostal has gained great interest clinically as a muscle to monitor neural drive and function in respiratory disease. This study demonstrates for the first time the impact of sustained hypoxia on neural activation and mechanical contraction of the parasternal intercostals. Parasternal intercostals reveal a biphasic action during the time-dependent hypoxic response, with a transient increase in shortening and EMG activity with acute hypoxia followed by a progressive decline when hypoxia is sustained.
Keywords: EMG activity; hypoxia; hypoxic ventilatory response; parasternal intercostal; respiratory muscle.