Effect of lung inflation and upright posture on diaphragmatic shortening in dogs

Abstract

In an earlier study (Road and Leevers (1988), J. Appl. Physiol. 65: 2283–2389), the application of continuous positive airway pressure (CPAP) produced a marked reduction in diaphragm initial length (L FRC) and tidal diaphragmatic shortening (%L FRC), tidal volume (V t) and transdiaphragmatic pressure swings (ΔPdi) in supine dogs after vagotomy. We postulated that the reduced diaphragmatic shortening was mainly a result of the decrease in diaphragm L FRC but an increase in afterload could not be excluded. In this study, we attempted to define the role of these two mechanisms during postural change. Eight, pentobarbital-anaesthetized, vagotomized dogs were studied in the supine position during CPAP and during postural change (tilting towards upright). As before, CPAP produced a prompt reduction in diaphragm L FRC and tidal %L FRC, V t and ΔPdi. Tilting produced similar decreases in crural diaphragm L FRC (23% from control values) as the weight of the abdominal contents was removed, but less decrease in costal L FRC (17% during CPAP compared to 10% during tilting). A given reduction in crural diaphragm initial length (15%) resulted in less tidal shortening during tilting compared to CPAP, whereas costal diaphragm shortening was similar at a given reduction in initial length (10%). Both CPAP and tilting reduced tidal volumes ( 47.8 ± 5.1 and 56.5 3.1% of control), however, ΔPdi decreased less during (20%) than CPAP (47%). There was no significant change in the level of diaphragmatic EMG with either CPAP or tilting. These results suggest that decreased initial length is the major factor limiting costa %L FRC during postural change to the upright position, whereas crural %L FRC is limited by the afterload, as well as initial length.