Diaphragmatic fatigue in unanesthetized adult sheep

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Abstract
Diaphragmatic muscle function during inspiratory flow resistive (IFR) loaded breathing was studied in unanesthetized sheep. The change in transdiaphragmatic pressure (dPdi) and arterial blood gas tensions were measured and the diaphragmatic electromyogram (EMG) was recorded from electrodes implanted on the muscle. With IFR loads < 150 cmH2O .cntdot. 1-1 .cntdot. s, dPdi and the integrated EMG [iEMG] increased, reached a plateau and were maintained at high levels. The centroid frequency (fc) of the EMG power spectrum did not consistently change. With IFR loads > 150 cmH2O .cntdot. l-1 .cntdot. s, O2 was administered to prevent hypoxia, cyanosis and agitation. With these loads, dPdi increased severalfold above base line, reached a plateau and then started to decrease. Arterial PCO2 increased sharply at the time when dPdi decreased. The iEMG and fc started to decrease gradually 10-20 min before dPdi started to decrease. The diaphragm is capable of generating large pressures for prolonged periods with no evidence of fatigue; with very high IFR loads, mechanical failure of the diaphragm can occur in the unanesthetized awake sheep; diaphragmatic fatigue is associated with acute hypercapnia and therefore failure of the entire respiratory pump; and a decrease in iEMG and a concommitant shift in the power spectrum density towards lower frequencies precede the mechanical failure of the diaphragm.