Effects of spontaneous breathing during airway pressure release ventilation on respiratory work and muscle blood flow in experimental lung injury.

STUDY OBJECTIVES To evaluate the effects of spontaneous breathing at ambient airway pressure (Paw) and during airway pressure release ventilation (APRV) on respiratory work and respiratory muscle blood flow (RMBF) in experimental lung injury. DESIGN Prospective experimental study. SETTING Research laboratory of a university hospital. SUBJECTS Twelve hemodynamically stable, analgosedated, and tracheotomized domestic pigs. MEASUREMENTS Respiratory work was estimated by the inspiratory pressure time product (PTPinsp) of esophageal pressure, and RMBF was measured with colored microspheres. Lung injury was induced with IV boli of oleic acid. The first set of measurements was performed before induction of lung injury while pigs were breathing spontaneously at ambient Paw, the second after induction of lung injury while breathing spontaneously at ambient Paw, and the third with lung injury and spontaneous breathing with APRV. RESULTS After induction of lung injury PTPinsp increased from 138 +/- 14 to 214 +/- 32 cm H2O s/min when pigs breathed spontaneously at ambient Paw (p < 0.05) and returned to 128 +/- 27 cm H2O s/min during APRV. While systemic hemodynamics and blood flow to the psoatic and intercostal muscles did not change, diaphragmatic blood flow increased from 0.34 +/- 0.05 before to 0.54 +/- 0.08 mL/g/min after induction of lung injury and spontaneous breathing at ambient Paw (p < 0.05) and returned to 0.32 +/- 0.05 mL/g/min during APRV (p < 0.05 vs spontaneous breathing at ambient Paw [lung injury]). CONCLUSION Respiratory work and RMBF are increased in acute lung injury when subjects breathe spontaneously at ambient Paw. Supporting spontaneous breathing with APRV decreases respiratory work and RMBF to physiologic values.