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

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 (LFRC) and tidal diaphragmatic shortening (%LFRC), tidal volume (VT) and transdiaphragmatic pressure swings (delta Pdi) in supine dogs after vagotomy. We postulated that the reduced diaphragmatic shortening was mainly a result of the decrease in diaphragm LFRC 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 LFRC and tidal %LFRC, VT and delta Pdi. Tilting produced similar decreases in crural diaphragm LFRC (23% from control values) as the weight of the abdominal contents was removed, but less decrease in costal LFRC (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, delta Pdi decreased less during tilting (20%) than CPAP (47%). There was no significant change in the level of diaphragmatic EMG with either CPAP or tilting.(ABSTRACT TRUNCATED AT 250 WORDS)