Respiration Physiology (1986) 64, 267-276 267 Elsevier A COMPARATIVE ANALYSIS OF CONTRACTILE CHARACTERISTICS OF THE DIAPHRAGM AND OF RESPIRATORY SYSTEM MECHANICS 1 JAMES C. LEITER 2, JACOPO P. MORTOLA 3 and S.M. TENNEY Department of Physiology, Dartmouth Medical School, Hanover, NH 03756, U.S.A. Abstract. The mean inspiratory flow rate (VT/TI) is used as an index of central respiratory 'drive', and, at rest, it varies interspeeifically in proportion to body weight (BW) raised to the 0.74 power (Boggs and Tenney, 1984). VT/TI is determined by the level of central neural respiratory output, the velocity of contraction of respiratory muscles, and the mechanical characteristics of the respiratory system. We have examined the last two factors in 13 species ranging in weight from 0.025 to 515 kg. We determined the 'effective' inspiratory mechanical characteristics of the respiratory system (time constant, resistance, and compliance) and the time course of diaphragmatic contraction during bilateral supramaximal phrenic nerve stimulation in anesthetized animals. We also measured passive expiratory mechanical variables and made morphometric measurements of the diaphragm. We found that VT/TI during phrenic nerve stimulation was proportional to BW°'s2. The 'effective' respiratory time constant (gs) and passive expiratory time constant (rvs) scaled in proportion to body weight with nearly similar exponents: z~soc BW°'26 and "Crs OC BW°'21. In addition, the time constant of diaphragmatic contraction (r~¢) was proportional to BW°2°. Inspiratory time is proportional to ~s and zinc, and tidal volume during stimulation was almost directly proportional to body weight. Thus, interspecific changes in VTfrI during stimulation were related to interspeeific changes in the mechanical characteristics of the respiratory system and the velocity of muscular contraetion. We conclude that interspecific changes in VTfrl need not reflect interspecific variation in central respiratory drive under resting conditions. We found that diaphragm weight and volume and diaphragm muscle thickness were geometrically similar in all species studied. Inspiratory pressure is an interspecific constant; therefore, by the Law of Laplace, smaller animals must develop greater tension per unit of muscle mass. Diaphragm Muscle contraction Muscle force Respiratory mechanics Time constant Velocity of shortening Accepted for publication 1 March 1986 This work was supported by grant HL02888 of the National Heart, Lung, and Blood Institute of the National Institutes of Health, Bethesda, MD, U.S.A. 2 Supported by a postdoctoral fellowship on Training Grant No. HL07449 of the National Heart, Lung, and Blood Institute of the National Institutes of Health, Bethesda, MD, U.S.A. 3 Present address: Department of Physiology, Faculty of Medicine, McGill University, Montreal, P.Q., H3G 1Y6 Canada. 0034-5687/86/$03.50 © 1986 Elsevier Science Publishers B.V. (Biomedical Division)