Diaphragm fatigue after submaximal exercise with chest wall restriction.

PURPOSE: We asked whether submaximal exercise with chest wall restriction (CWR), as a model of restrictive pulmonary disease, would result in fatigue of the diaphragm in healthy humans.

METHODS

To address this question, we used cervical magnetic stimulation of the phrenic nerves along with measures of transdiaphragmatic pressure. Seven healthy young men (30 ± 7 yr) completed pulmonary function tests and a maximal cycle exercise test. On a separate day, baseline measures of diaphragm contractility were obtained followed by cycle exercise at 45% of maximum intensity for 10 min with no restriction (NCWR). Diaphragm contractility was assessed again at 10 and 30 min after exercise. One hour later, inelastic straps were applied to reduce forced vital capacity by 40% followed by exercise at the same intensity. Diaphragm contractility was reassessed at 10 and 30 min after exercise. During exercise, the work of breathing, respiratory pressures, ventilatory parameters, and perceptions of respiratory and leg discomfort were recorded.

RESULTS

The work of breathing and dyspnea ratings were greater during CWR exercise compared with NCWR (P < 0.05). The CWR condition had reductions in diaphragm contractility 10 min after exercise using nonpotentiated (-20.2% ± 15.3%) or potentiated twitches (-23.3% ± 12.4%, P < 0.05). There were no differences after exercise for NCWR (P > 0.05). The reduction in diaphragm contractility was correlated with the inspiratory elastic work of breathing (r2 = 0.74, P < 0.05).

CONCLUSIONS

We conclude that fatigue of the diaphragm occurs under restricted conditions and likely contributes to poor exercise tolerance in patients with restrictive disease.