EFFECT OF RESPIRATORY MUSCLE FATIGUE ON BREATHING PATTERN DURING INCREMENTAL EXERCISE

We examined the breathing pattern during incremental exercise before and after induction of inspiratory muscle fatigue. Our aim was to determine whether induction of fatigue alters the ventilatory response to exercise and in particular whether such changes are most apparent at high levels of exercise when minute ventilation and thus inspiratory load are greatest. A group of 10 healthy subjects was studied on a cycle ergometer. Fatigue was achieved by having the subject breathe against an inspiratory threshold load that required the subject to generate 80% of the predetermined maximal mouth pressure to initiate airflow. Breathing pattern, oxygen consumption (VO2), mouth occlusion pressure (P0.1), and a visual analog scale (VAS) for respiratory effort were obtained for 3 min at rest and at 25, 50, 75, and 100% of the subject's maximal work load (Wmax) as determined by preliminary testing. Exercise was performed on two separate occasions, once immediately after induction of fatigue and the other as a control. Induction of fatigue had no effect on resting breathing and only minimal effects at the lower work loads (25 and 50% Wmax). At the higher work loads (75 and 100% Wmax) induction of fatigue significantly altered the pattern of breathing during exercise. At 75% of Wmax the respiratory frequency (f) increased from 22.5 +/- 4.4 (SD) during control to 27.0 +/- 6.7 breaths/min (p < 0.02) following induction of fatigue; tidal volume was not significantly altered, 2.15 +/- 0.65 versus 2.24 +/- 0.74 L during control. The increase in f was due to reductions in both inspiratory and expiratory time because fractional inspiratory time remained unchanged. Mouth occlusion pressure and the VAS for respiratory effort also were increased following induction of fatigue, 12.9 +/- 4.0 versus 10.7 +/- 4.4 cm H2O (p < 0.02) and 54 +/- 16 versus 41 +/- 17 mm (p < 0.05). In contrast, induction of fatigue had no effect on VO2, CO2 production, or heart rate. These results indicate that induction of respiratory muscle fatigue can alter the response to high-intensity exercise, producing rapid but not shallow breathing and increasing neuromuscular output (P0.1) and respiratory sensation (VAS) for a given work load.