Effect of Lung Volume and Respiratory Impedance on Transdiaphragmatic Pressure and Muscle Tension in Dogs

Abstract

The mechanism for the change in transdiaphragmatic pressure (Pdi) with lung volume and respiratory impedance was studied in supine anesthetized dogs. We recorded Pdi during bilateral stimulation of the phrenic nerves (20 Hz, 50 Hz) at 4 lung volumes between functional residual capacity (FRC) and total lung capacity (TLC) before and after the infusion of 60-100 ml/kg of Ringer's lactate into the abdominal cavity. The in situ length of the diaphragm was measured from the x-ray projection images of radiopaque markers previously sewn onto muscle bundles of the abdominal diaphragm surface. All length values were normalized by the length of the passive diaphragm at FRC before liquid infusion (LFRC[E]). Passive L/LFRC(E) changed between 0.80 and 1.11 with volume and liquid load. However, lung volume had little effect on the active diaphragm length, which remained near constant between 0.47 at TLC and 0.54 at FRC (50 Hz stimulations). In contrast, active Pdi rose from 8 cm H2O at TLC to 51 cm H2O at FRC. With abdominal liquid, active length increased at all volumes, but Pdi rose less than predicted from the baseline Pdi/length relationship. Thus, there was not a single Pdi/length curve representative of both baseline and liquid loaded state.