Heliox Delays Exercise Dynamic Hyperinflation and Leg Muscle Fatigue in Ventilatory Limited Patients with COPD

Abstract

PURPOSE: Patients with COPD exhibit exercise intolerance that is due to dynamic hyperinflation or leg muscle fatigue, but it is unknown how these exercise limitations interact. We hypothesized that, by delaying dynamic hyperinflation with heliox, exercise tolerance and leg muscle fatigue would increase in patients limited by ventilation, and not in those limited by leg muscle fatigue. METHODS: Eleven patients with COPD (FEV1 52 ± 17% predicted, VO2max 14 ± 4.6 ml/kg/min) completed two randomized exercise bouts at 80% of peak power output on a cycle ergometer breathing either room air or heliox (21% O2, 79% He) and one exercise bout breathing heliox but stopping at isotime on the room air test. Exercising end-expired lung volume was determined from inspiratory capacity (IC). Contractile leg muscle fatigue was determined by direct motor point magnetic stimulation to obtain vastus lateralis potentiated twitch prior to exercise and at 5, 10, and 20 minutes post exercise. Patients were grouped into fatiguers (F; 5 minute potentiated twitch (5PT) ≤ 85% of pre exercise value on the room air test) and non fatiguers (NF). RESULTS: Seven of the 11 patients were NF (5PT 93 ± 4.9%) and increased their exercise endurance time while breathing heliox from 355 ± 203 to 610 ± 317s (P < 0.001) while exercise endurance time did not change in the four F (5PT 74 ± 6.9%). At end exercise, the NF had a lower IC on both the room air and heliox tests (1.47 ± 0.38 L and 1.57 ± 0.50 L vs. 2.23 ± 0.41 L and 2.58 ± 0.30 L, respectively; P < 0.01) and reached a higher end inspiratory lung volume (EILV; 96.6 ± 2.2 and 96.6 ± 2.2% vs. 92.6 ± 4.2 and 88.7 ± 5.9%, respectively, P < 0.05). The increased exercise time with heliox in the NF resulted in a greater level of muscle fatigue (5PT decreased to 73.6 ± 6.4%; P < 0.001). At isotime with heliox, the NF group demonstrated an increased IC (1.89 ± 0.59 L), a lower EILV (93.3 ± 3.7%) and a similar level of muscle fatigue (89.8 ± 5.5%)(P < 0.05). The change in exercise time was most highly correlated with room air 5PT (r = 0.79, P <.01) and EILV (r = 0.68, P < 0.05) and there was a significant association between room air 5PT and EILV (r = −0.77, P < 0.01) indicating that patients with the least amount of muscle fatigue were more ventilatory limited and had a greater response to breathing heliox. CONCLUSIONS: The ability to decrease exercise hyperinflation with heliox in patients with COPD depends on the relative contributions of ventilatory and leg muscle limitations during cycling exercise. Heliox was less effective in improving exercise tolerance in patients limited by leg fatigue, suggesting that targeted leg muscle rehabilitation may be more beneficial in these patients.