Contractile properties of the diaphragm in some mammals

Abstract

The mechanical properties of the diaphragm of dogs, cats, rabbits and rats have been evaluated stimulating the phrenic nerves and measuring the intratracheal pressure at closed airways. The contraction time measured in the four species indicates that the diaphragm has intermediate characteristics between “fast” and “slow” muscles. This value is higher in the dog (64.7 msec) than in the rabbit (32.0 msec), the cat (38.9 msec) and the rat (18.3 msec). Full tetanic fusion is attained at lower frequency of stimulation (100 stim/sec) in the dog than in the rabbit and in the cat. The rate of rise of tension is less in the dog than in cats and rabbits. The tetanus/twitch ratio is greater in the rabbit (4.87) than in the cat (3.91) and in the dog (3.81). These data suggest that the compliance of the series elastic elements and that the duration of the active state decreases from dogs to cats and from cats to rabbits. The diaphragm has a constrictory action in the rib cage which becomes greater at high lung volume. The pressure exerted by the diaphragm tetanically stimulated decreases with increasing lung volume. The slope of this pressure-volume diagram is dissimilar from the force-length diagram of muscles due to the constrictory action of the diaphragm on the rib cage, and to its progressive flattening which becomes more evident at large lung volume. When only one hemidiaphragm is stimulated at closed airways the pressure developed is 1 2 of that obtained during bilateral stimulation. On the other hand, at open airways, the volume introduced following the stimulation of one hemidiaphragm is appreciably higher than 1 2 of the volume introduced during the stimulation of the whole diaphragm. This is probably due to the unlinearity of the pressure-volume curve of the relaxed respiratory system.