Abstract
Role of inspiratory muscle function in the genesis of dyspnea in COPD patients has yet to be fully studied. The present study investigated the possible relationship between respiratory muscle function and the sensation of dyspnea (modified Borg Scale) during exercise in eight patients with severe COPD (FEV1 0.61L±0.15L). The electrical activity of the diaphragm (EMGdi) was recorded with esophageal electrodes, and that of sternomastoid muscle (EMGsm) was recorded from the surface electrodes. The ratio of high frequency (150 to 350Hz) to low frequency (20 to 47Hz) power (H/L) of EMGdi and EMGsm was analyzed to assess inspiratory muscle fatigue, which was determined by a 20% fall of H/L ratio from the control value. Flow, volume, esophageal (Pes) and transdiaphragmatic pressure (Pdi) were measured. Tension time index (TTdi) was calculated from Pdi and the ratio of inspiratory time to total time for one cycle (T1/TTOT). At rest, we measured maximal esophageal pressure (Pesmax), maximal transdiaphragmatic pressure (Pdimax), maximal EMGdi (EMGdimax) and EMGsm (EMGsmmax). Progressive treadmill exercise test was performed, stating with 3 minutes' walk at a speed of 0.75mph at 0% grade, subsequently increasing the velocity at a rate of 0.25mph and the elevation at a rate of 4% per stage. Exercise was discontinued at maximum respiratory effort sensation. Six of the eight patients showed diaphragmatic fatigue at their maximal exercise.
With diaphragmatic fatigue, these patients were extremely dyspneic (Borg scale 9 or 10), and terminated the exercise. There were high correlations between the Borg scale and VE/MVV, and Pes/Pesmax and EMGsm/EMGsmmax, however, TTdi and EMGdi/EMGdimax showed less correlation with the Borg scale. These findings suggest that during treadmill exercise in COPD patients, the activity of the extradiaphragmatic ventilatory muscles is directly related to the severity of dyspnea.
