Background: Respiratory muscle unloading during exercise could improve locomotor muscle oxygenation due to increased systemic oxygen delivery (higher cardiac output and/or CaO2) in patients with chronic obstructive pulmonary disease (COPD).
Methods: Sixteen non-hypoxaemic males (FEV1= 42.2 ± 13.9 % pred) 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 deoxy-hemoglobyn [HHb], oxi-Hb [O2Hb], tissue oxygenation index (TOI) and total Hb [Hbtot] in the vastus lateralis were measured by near-infrared spectroscopy. In order to estimate systemic oxygen delivery (DO2est, L/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 to sham. 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, i.e., DO2est remained unchanged with PAV (p>0.05). Interestingly, however, leg muscle oxygenation was significantly enhanced with PAV as the exercise-related decrease in Δ[O2Hb]% was lessened and TOI was improved; moreover, Δ[Hbtot]%, an index of local blood volume, was increased compared to sham ventilation (p<0.01).
Conclusions: Respiratory muscle unloading during high-intensity exercise can improve peripheral muscle oxygenation despite an unaltered systemic DO2 in patients with advanced COPD. These findings might indicate that a fraction of the available cardiac output had been redirected from the ventilatory to the appendicular muscles as a consequence of the respiratory muscle unloading.