Global and local inhomogeneity indices of lung ventilation based on electrical impedance tomography

Abstract

ObjectivesElectrical impedance tomography (EIT) is a new, noninvasive, radiation-free imaging technique that is potentially capable of monitoring the regional lung aeration and tidal volume distribution. The aim of this study was to develop both global and local inhomogeneity indices of lung ventilation during mechanical ventilation in order to support an improved ventilation strategy.Subjects10 anesthetized mechanically ventilated patients with healthy lungs. The patients were ventilated under volume-controlled ventilation mode for surgical intervention and monitored by EIT.Methods and Main ResultsImpedance sequences were recorded using the EIT Evaluation Kit 2 (Dräger medical, Lübeck, Germany). Standardized PEEP-wave maneuver (positive end-expiratory pressure from 0 to 28 mbar, 2 mbar per step) was conducted and mechanical data recorded with the E4Lab system (Dräger medical, Lübeck, Germany).The variation of the tidal image, i.e. the difference in impedances between end-inspiration and end-expiration, was used as an indicator of the inhomogeneous ventilation of the lung. A global and a local inhomogeneity index were developed to show the degree of inhomogeneity of the intrapulmonary air distribution. Since the indices are already normalized, they are inter-patient comparable.There were minimum values of the global and the local inhomogeneity index for every patient during a PEEP-Wave maneuver. Since the minimum values of these two indices are not always at the same pressure level, a trade-off is made by multiplying both indices. The minimum of this trade-off indicated that the most homogeneous distribution of ventilation occurred when the total compliance -derived from respiratory signal analysis- began to decrease.ConclusionsThe inhomogeneity indices may provide new insights into the relation between lung mechanics and tidal volume distribution and may be used to guide ventilator settings.