EFFECTS OF RESPIRATORY MECHANICAL UNLOADING ON THORACOABDOMINAL MOTION IN MECONIUM-INJURED PIGLETS AND RABBITS

Impaired pulmonary mechanics can cause chest wall distortion (CWD) so that work of breathing is dissipated in deforming the rib cage. We hyp othesized that respiratory mechanical unloading as a technique of assi sted mechanical ventilation would reduce CWD in animals with injured l ungs. We studied five piglets and five adult rabbits to test across di fferent ages and chest configurations. As a result of intratracheal me conium instillation, lung compliance decreased from 21 (median; range 17-35) to 9.5 (6.7-14) mL/kPa/kg in rabbits and from 26 (18-31) to 7.9 (4.9-11) in piglets. Airway resistance increased from 5.0 (4.6-6.1) t o 6.9 (5.8-7.9) kPa/L/s in rabbits only. Respiratory inductive plethys mography was used to measure the phase shift between the rib cage and abdominal compartment movements and the total compartmental displaceme nt ratio. We aimed at unloading at least three-fourths of lung elastan ce in all animals and 2.0 kPa/L/s of resistance in rabbits, Elastic un loading decreased the phase shift in all but one animal, It reduced th e total compartmental displacement ratio from 1.27 (1.14-3.73) to 1.16 (1.02-1.82) in piglets and from 1.77 (1.45-5.24) to 1.37 (1.11-4.78) in rabbits. The inspiratory rib cage expansion increased, whereas abdo minal expansion did not. The tidal esophageal pressure deflection decr eased. Tidal volume increased, whereas respiratory rate remained unaff ected so that the partial pressure of arterial CO, decreased. Resistiv e unloading as an adjunct to elastic unloading further reduced CWD and induced a more rapid, shallower breathing. We conclude that respirato ry unloading as a mechanical support to spontaneous breathing reduces CWD, We speculate that the decrease in CWD increases ventilatory effic iency for a given diaphragmatic effort.