Alterations in respiratory muscle activation in the ischemic fatigued canine diaphragm.

The purpose of the present study was to examine the respiratory motor response to diaphragm fatigue. Studies were performed using in situ diaphragm muscle strips dissected from the left costal diaphragm in anesthetized dogs. The left inferior phrenic artery was isolated, and diaphragmatic strip fatigue was elicited by occluding this vessel. Strip tension, strip electromyographic activity, parasternal electromyographic activity, and the electromyogram of the right hemidiaphragm were recorded during spontaneous breathing efforts before, during, and after periods of phrenic arterial occlusion. In separate trials, we examined the neuromuscular responses to phrenic arterial occlusion at arterial PCO2 (PaCO2) of 40, 55, and 75 Torr. No fatigue and no alteration in electromyographic activities were observed in trials at PaCO2 of 40 Torr. During trials at PaCO2 of 55 and 75 Torr, however, diaphragm tension fell, the peak height of the diaphragm strip electromyogram decreased, and the peak heights of the parasternal and right hemidiaphragm electromyograms increased. Relief of phrenic arterial occlusion resulted in a return of strip tension and all electromyograms toward base-line values. In additional experiments, the left phrenic nerve was sectioned in the chest after producing fatigue. Phrenic section was followed by an increase in the peak height of the left phrenic neurogram (recorded above the site of section). This latter finding suggests that diaphragm strip motor drive may be reflexly inhibited during the development of fatigue by neural traffic carried along phrenic afferents.