Abstract
The mean inspiratory flow rate ( Vt/ Ti) is used as an index of central respiratory ‘drive’, and, at rest, it varies interspecifically 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 anaesthetized 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 0.82. The ‘effective’ respiratory time constant ( τ′ rs) and passive expiratory time constant ( τ rs) scaled in proportion to body weight with nearly exponents: τ′ rs ∝ BW 0.26 and τ rs ∝ BW 0.21. In addition, the time constant of diaphragmatic contraction ( τ mc) was proportional to BW 0.20. Inspiratory time is proprotional to τ′ rs and τ mc, and tidal volume during stimulation was almost directly proportional to body weight. Thus, interspecific changes in Vt/ Ti during stimulation were related to interspecific changes in the mechanical characteristics of the respiratory system and the velocity of muscular contraction. We conclude that interspecific changes in Vt/ Ti 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.
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