Respiration Physiology, 76 (1989) 337-346 Elsevier 337 RSP 01537 Relationship between diaphragmatic activation and twitch tension to superimposed electrical stimulation in the cat Thomas E. Dick and Steven G. Kelsen* Pulmonary Division, Department of Medicine, Case Western Reserve University, Cleveland, OH 44106-5000. U.S.A. (Accepted for publication 6 February 1989) Abstract. The purpose of these experiments was to evaluate the validity of the 'twitch-occlusion' method as an index of the extent of diaphragmatic activation and to assess the extent of diaphragmatic activation during inspiration. Studies were performed in situ on innervated, perfused muscle strips from the costal region of the diaphragm in ten anesthetized cats. We measured isometric tension generated by the diaphragm during inspiration and following an interpolated electrical stimulation (2Hz, 100/Jsec, 3.0 x threshold) of the nerve. The extent of MU activation was assessed by comparing twitch amplitudes during electrical stimulation applied in expiration and in inspiration. Spontaneous inspiratory activity was induced by adding CO2 to the inspired oxygen. Within an animal, the relationship between diaphragmatic activation and twitch occlusion was linear (range of r values was from -0.88 to -0.94). The extent of spontaneous diaphragmatic activation was normalized by dividing tension at end inspiration by the average twitch tension caused by stimuli applied during expiration. Across animals, twitch amplitude was inversely related to diaphragmatic activation (y = - 0.36x + 1.13, r = - 0.94). At a respiratory drive with end-tidal Pco, approximately 1% above apneic threshold (end-tidal Pco, between 5 and 6 %), twitch occlusion was less than 5.0%. Increasing end-tidal Pco2 to at least 5% above apneic threshold (end-tidal Pco, between 9 a,d I I ,%), twitch occlusion was still less than 50%. These results from a preparation that allows direct me,~surement of isometric tension of the diaphragm show that the interpolated twitch is linearly related to the e~ent o~' muscle activation through a broad range of muscle activity. In addition, these data indicate that at higher respiratory drives there exists a large reserve in the phrenic motor pool. Control of respiration; Diaphragm; Phrenic motor pool The ability to assess the extent of respiratory muscle activation is important in deter- mining respiratory muscle strength and the intensity of the motor command during spontaneous breathing. An approach which assesses the extent of muscle activation has Correspondence address: Dr. T.E. Dick, Pulmonary Division, Department of Medicine, Case Western Reserve University, Cleveland, OH 44106-5000, U.S.A. * Present address: Division of Pulmonary Medicine, Parkinson Pavilion no. 931 Temple University Hospital, Broad and Tioga Streets, Philadelphia, PA 19140. 0034-5687/89/$03.50 © 1989 Elsevier Science Publishers B.V. (Biomedical Division)