EXHAUSTIVE TREADMILL EXERCISE DOES NOT REDUCE TWITCH TRANSDIAPHRAGMATIC PRESSURE IN PATIENTS WITH COPD

Reduced diaphragm contractility has been described in normal subjects after whole body endurance exercise, and it indicates low frequency fa tigue (LFF); it is unknown whether LFF is of clinical importance. We t herefore studied the effect of treadmill exercise to exhaustion on dia phragm contractility in six patients with severe chronic obstructive p ulmonary disease (COPD) (mean FEV(1), 0.71, 27% predicted). The subjec ts first performed a short (control), treadmill walk and then, after r esting, a second walk to a state of severe dyspnea. Cervical magnetic stimulation of the phrenic nerve roots was performed at the start of t he study and 20 and 30 min after each walk. The twitch transdiaphragma tic pressure (Tw Pdi) was reproducible (mean coefficient of variation, 5.3%; range, 2 to 72.5%). Mean Twi Pdis were 18.4 cm H2O at baseline and 19.6 cm H2O and 19.2 cm H2O 20 and 30 min after the control walk. At the same times after the exhaustive walk, mean Tw Pdis were 19.6 an d 20.4 cm H2O. Tw Pdi was not reduced by exhaustive treadmill walking (p > 0.9), and a power calculation showed that the study had a 95% cha nce of detecting a 10% fall at the 5% significance level. We conclude that Tw Pdi is not reduced when patients with severe COPD walk to a st ate of extreme breathlessness and that therefore low frequency fatigue of the diaphragm does not occur.