Mechanics of the canine diaphragm

Abstract

Diaphragmatic force-length behavior was assessed in anesthetized dogs using two techniques. One employed measurements in a diaphragm strip with intact nerve and blood supply; the second related transdiaphragmatic pressure (Pdi) to direct estimates of diaphragmatic tension from strain gauge arches during bilateral, tetanic, supramaximal phrenic nerve stimulation. In strip preparations the diaphragm exerted active force at lengths as short as 40% of resting in situ length (Lo), and maximal force was registered at a length averaging 25% greater than Lo. This suggested a broader effective length range for the diaphragm as compared to other skeletal muscles. In the intact preparation both Pdi and directly measured diaphragmatic tension were inversely related to lung volume and when they were used to calculate the diaphragm's effective radius of curvature (r), r was found to change little or decrease at large lung volumes rather than increase as theory would predict. These findings suggest that length tension characteristics outweigh geometric considerations in explaining the diaphragm's function in normal dogs and probably normal men.

The diaphragm: contractile properties and fatigue.

“…Data from human and dog studies (15,20) show a similar, slightly curvilinear relationship of diaphragm length to lung volume, with AL/AV decreasing as lung volume increases. It is probable that there is a relatively constant relationship between diaphragm length, configuration, and lung volume under the conditions of those studies, although under other circumstances the diaphragm can be made to assume various other configurations at a given lung volume (34)(35)(36).…”

“…The forcelength curve constructed from these values of Pdi and muscle length fits in vitro force-length curves quite well, and L" seems to be at a lung volume below the resting volume (functional residual capacity, FRG). In the open-chest dog, the costal diaphragm contractile force and length responses to tetanic phrenic nerve stimulation have been measured directly (20). The resultant force-length curve also has the same characteristics as curves from in vitro muscle, and again L" seems to be at lung volume below FRC.…”

“…In hamsters, P0 is not altered by exercise training, emphysema induced by intratracheal instillation of elastase, or simple semistarvation (26)(27)(28). The maximal force produced by tetanic stimulation of the in situ canine diaphragm is 1.5-2 kg/cm2 (20). We used the shape and dimensions of the human diaphragm and body cavity to calculate its contractile force during a static inspiratory effort in which Pdi was 120 cm H20 (29).…”

Effects of Lung Volume on Parasternal Pressure‐Generating Capacity in Dogs

“…Optimum resting length for force generation for the diaphragm and external intercostal muscles lies near FRC (Kim et al 1976;Farkas et al 1985;Road et al 1986). It is generally thought that with hyperinflation, a common occurrence in patients with obstructive lung disease, the inspiratory muscles become foreshortened reducing their capacity to generate force (Macklem, 1984;Rochester & Braun, 1986;Sharp, 1986).…”

“…Based upon these findings alone, one would predict that the pressuregenerating capacity of the parasternal muscles would be better preserved at high lung volumes compared to the other inspiratory muscles. While the relationship between diaphragm pressure-generating capacity and lung volume can be easily assessed with phrenic nerve stimulation (Kim et al 1976;Road et al 1986), comparable assessment of the parasternal muscles has not been performed due to the difficulty in achieving reproducible synchronous contraction of this muscle group. In the present investigation, we employed spinal cord stimulation (DiMarco et al 1987(DiMarco et al , 1989a to activate the parasternal muscles as a group, at a constant level of motor drive and in the absence of other inspiratory muscle contraction.…”