Airway pressure-time curve profile (stress index) detects tidal recruitment/hyperinflation in experimental acute lung injury

Objective: To evaluate whether the shape of the airway pressure-time (Paw-t) curve during constant flow inflation corresponds to radiologic evidence of tidal recruitment or tidal hyperinflation in an experimental model of acute lung injury. Design: Prospective randomized laboratory animal investigation. Setting: Department of Clinical Physiology, University of Uppsala, Sweden. Subjects: Anesthetized, paralyzed, and mechanically ventilated pigs. Interventions: Acute lung injury was induced by lung lavage. During constant inspiratory flow, the Paw-t curve was fitted to a power equation: airway pressure = a·timeb+ c, where coefficient b (stress index) describes the shape of the curve: b = 1, straight curve; b < 1, progressive increase in slope; and b > 1, progressive decrease in slope. Tidal volume (VT) was 6 mL/kg, and positive end-expiratory pressure was set to obtain a b value between 0.9 and 1.1 before (b = 1) and after (b = 1 after recruiting maneuver) application of a recruiting maneuver. Positive end-expiratory pressure was decreased and VT increased to obtain 0.9 > b > 0.8 and 0.8 > b > 0.6, whereas positive end-expiratory pressure and VT were both increased to obtain 1.3 > b > 1.1 and 1.5 > b > 1.3. Experimental conditions sequence was random. Measurements and Main Results: Pulmonary computed tomography was obtained during end-expiratory and end-inspiratory occlusions. Tidal recruitment was quantified as nonaerated (between -100 and +100 Hounsfield units) lung area at end-expiration minus end-inspiration. Tidal hyperinflation was quantified as hyperinflated (between -900 and -1000 Hounsfield units) lung area at end-inspiration minus end-expiration. Computed tomography images showed that tidal recruitment and tidal hyperinflation corresponded to b < 1 and b > 1, respectively. Stress index values and tidal recruitment and tidal hyperinflation values were significantly correlated (R2 = .917 and R2 = .911, p < .0001, respectively). Conclusions: Shape of the Paw-t curve detects tidal recruitment and tidal hyperinflation.