Respiratory muscle unloading improves leg muscle oxygenation during exercise in patients with COPD

Abstract

Background: Respiratory muscle unloading during exercise could improve locomotor muscle oxygenation by increasing oxygen delivery (higher cardiac output and/or arterial oxygen content) in patients with chronic obstructive pulmonary disease (COPD). Methods: Sixteen non-hypoxaemic men (forced expiratory volume in 1 s 42.2 (13.9)% predicted) undertook, on different days, two constant work rate (70-80% peak) exercise tests receiving proportional assisted ventilation (PAV) or sham ventilation. Relative changes (Δ%) in deoxyhaemoglobin (HHb), oxyhaemoglobin (O2Hb), tissue oxygenation index (TOI) and total haemoglobin (Hbtot) in the vastus lateralis muscle were measured by nearinfrared spectroscopy. In order to estimate oxygen delivery (Do2est, 1/min), cardiac output and oxygen saturation (Spo2) were continuously monitored by impedance cardiography and pulse oximetry, respectively. Results: Exercise tolerance (Tlim) and oxygen uptake were increased with PAV compared with sham ventilation. In contrast, end-exercise blood lactate/Tlim and leg effort/Tlim ratios were lower with PAV (p<0.05). There were no between-treatment differences in cardiac output and Spo2 either at submaximal exercise or at Tlim (ie, Do2est remained unchanged with PAV; p>0.05). Leg muscle oxygenation, however, was significantly enhanced with PAV as the exercise-related decrease in Δ(02Hb)% was lessened and TOI was improved; moreover, Δ(Hbtot)%, an index of local blood volume, was increased compared with sham ventilation (p<0.01). Conclusions: Respiratory muscle unloading during highintensity exercise can improve peripheral muscle oxygenation despite unaltered systemic Do2 in patients with advanced COPD. These findings might indicate that a fraction of the available cardiac output had been redirected from ventilatory to appendicular muscles as a consequence of respiratory muscle unloading.