Beyond the lungs in fibrotic interstitial lung disease: does supplemental O2 improve skeletal muscle oxygenation and fatigue?

Abstract

Background: The severity of exertional hypoxaemia in fibrotic interstitial lung disease (f-ILD) finds no parallel in respiratory medicine. The prevailing view is that supplemental oxygen (O₂) may improve exercise tolerance in f-ILD by lessening the hypoxic ventilatory drive and dyspnoea. However, it is conceivable that part of the ergogenic effects of O₂ are secondary to improved muscle O₂ delivery and fatigue.

Methods: Sixteen f-ILD patients (13 males, 73±8 years) performed a constant-load (60% peak) cycle test to symptom limitation on normoxia (Tlim_norm). Fourteen controls cycled up to patients’ Tlim_norm. Later on, patients exercised up to Tlim_norm under supplemental O₂ (0.41±0.07). Changes in quadriceps oxyhaemoglobin concentration ([HbO₂]) were assessed by near-infrared spectroscopy. A >15% post-cycling fall in quadriceps force response to magnetic nerve stimulation (twitch, Tw_quad) indicated the presence of muscle fatigue.

Results: Patients showed severe exertional hypoxaemia (O₂ saturation= 80.7±7.4% after 17±4 min). Fatigue was found in 13/16 patients (Tw_quad fall= 21.5±8.0%) and was larger than in controls (13.5±9.1%; p<0.05). Supplemental O₂ improved quadriceps [HbO2] (+6.9±3.6 vs +2.4±2.6 μmol; p<0.05) leading to a decrease in muscle fatigue (12.7±8.5%; p<0.05): 7/13 patients were no longer considered as ‘fatiguers’. Leg symptoms dropped by at least one Borg unit in 9 of these 13 patients.

Conclusion: The beneficial effects of exertional O₂ supplementation in patients with f-ILD are not limited to “the lungs”: lessening peripheral muscle fatigue to improve exercise tolerance is a relevant therapeutic target that deserves clinical attention.