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.

In anesthetized dogs studied both supine and prone, the electromyogram (EMG) of the diaphragm recorded directly from three portions of the diaphragm (crural, anterior, and costal) was compared with simultaneous recordings of the diaphragmatic EMG recorded from 10 sites in the esophagus and stomach. Effects upon the EMG of lung volume change and esophageal electrode position change were determined during bilateral supramaximal tetanic phrenic stimulation with airway occluded. Lung volume change had little effect upon the directly recorded EMG. The effect of lung volume change upon the EMG recorded from the esophagus was somewhat greater and marked change was noted as the esophageal recording site was varied. In supine dogs two sites of maximal signal were observed, one 1 cm above the cardia and the other 4--6 cm below the cardia. In prone dogs a single site for maximal signal was observed 3 cm above the cardia. An electrode site as close to the cardia as possible appears to be optimal from the point of view of variation in signal due to lung volume change and due to body position change. Gastric balloon stabilization is recommended. Proximity of the electrode and posterior gastric wall to the diaphragmatic crura may explain the maximal EMG signal recorded below the cardia.