Objective: To determine the validity of functional electric impedance tomography to monitor regional ventilation distribution in experimental acute lung injury, and to develop a simple electric impedance tomography index detecting alveolar recruitment.
Design: Randomized prospective experimental study.
Setting: Academic research laboratory.
Subjects: Sixteen anesthetized, tracheotomized, and mechanically ventilated pigs.
Interventions: Acute lung injury was induced either by acid aspiration (direct acute lung injury) or by abdominal hypertension plus oleic acid injection (indirect acute lung injury) in ten pigs. Six pigs with normal lungs were studied as a control group and with endotracheal suction-related atelectasis. After 4 hrs of mechanical ventilation, a slow inflation was performed.
Measurements and main results: During slow inflation, simultaneous measurements of regional ventilation by electric impedance tomography and dynamic computed tomography were highly correlated in quadrants of a transversal thoracic plane (r2 = .63-.88, p < .0001, bias <5%) in both direct and indirect acute lung injury. Variability between methods was lower in direct than indirect acute lung injury (11 +/- 2% vs. 18 +/- 3%, respectively, p < .05). Electric impedance tomography indexes to detect alveolar recruitment were determined by mathematical curve analysis of regional impedance time curves. Empirical tests of different methods revealed that regional ventilation delay, that is, time delay of regional impedance time curve to reach a threshold, correlated well with recruited volume as measured by CT (r2 = .63). Correlation coefficients in subgroups were r2 = .71 and r2 = .48 in pigs with normal lungs with and without closed suction related atelectasis and r2 = .79 in pigs subject to indirect acute lung injury, respectively, whereas no significant correlation was found in pigs undergoing direct acute lung injury.
Conclusions: Electric impedance tomography allows assessment of regional ventilation distribution and recruitment in experimental models of direct and indirect acute lung injury as well as normal lungs. Except for pigs with direct acute lung injury, regional ventilation delay determined during a slow inflation from impedance time curves appears to be a simple index for clinical monitoring of alveolar recruitment.