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Imbalances in Regional Lung Ventilation

A Validation Study on Electrical Impedance Tomography

Respiratory ICU, Hospital das Clínicas, Pulmonary Department; General ICU, Hospital das Clínicas, Emergency Clinics Division; Radiology Department, Hospital das Clínicas, University of São Paulo, São Paulo, Brazil; and Department of Intensive Care, Fundación Jiménez Díaz, Madrid, Spain

Correspondence and requests for reprints should be addressed to Marcelo Amato, M.D., Laboratório de Pneumologia, LIM09, Faculdade de Medicina da USP, Av Dr Arnaldo, 455 sala 2206 (2nd floor), CEP: 01246–903, São Paulo, SP, Brazil. E-mail: amato{at}unisys.com.br

Imbalances in regional lung ventilation, with gravity-dependent collapse and overdistention of nondependent zones, are likely associated to ventilator-induced lung injury. Electric impedance tomography is a new imaging technique that is potentially capable of monitoring those imbalances. The aim of this study was to validate electrical impedance tomography measurements of ventilation distribution, by comparison with dynamic computerized tomography in a heterogeneous population of critically ill patients under mechanical ventilation. Multiple scans with both devices were collected during slow-inflation breaths. Six repeated breaths were monitored by impedance tomography, showing acceptable reproducibility. We observed acceptable agreement between both technologies in detecting right–left ventilation imbalances (bias = 0% and limits of agreement = -10 to +10%). Relative distribution of ventilation into regions or layers representing one-fourth of the thoracic section could also be assessed with good precision. Depending on electrode positioning, impedance tomography slightly overestimated ventilation imbalances along gravitational axis. Ventilation was gravitationally dependent in all patients, with some transient blockages in dependent regions synchronously detected by both scanning techniques. Among variables derived from computerized tomography, changes in absolute air content best explained the integral of impedance changes inside regions of interest (r² 0.92). Impedance tomography can reliably assess ventilation distribution during mechanical ventilation.

Key Words: artificial respiration • physiologic monitoring • validation studies • adult respiratory distress syndrome • respiratory insufficiency

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