Determination of local phase changes in lung ventilation by EIT during continuous postural changes

Abstract

Regional changes in lung ventilation depend mainly on the body position. Changes in the direction of the gravitational force causes a fluid shift between upper and lower parts of the lung and also changes in the local dynamic behavior. In this study we compared state-differential images measured by electrical impedance tomography (EIT) which has been divided into four regions of interest (ROI): left anterior, left posterior, right anterior and right posterior. For the detection of local changes a mean signal for each region of interest and a reference signal as the median over all pixels inside the lung was calculated. The measurements were taken from healthy males lying in a rotating bed. All measurements started in supine position, then the body was rotated to the left by -62° at a speed of approximately 30° per minute. Then a continuous slow rotation was performed to the right until reaching 62°. The maneuver was accomplished by rotating back to the supine position and leafing the subject in rest for five minutes. Then we analysed the magnitude and phase of relative impeditivity changes in each ROI. Whereby the reference phase was taken as the average over the whole lung. The phase information was calculated using the Hilbert transform. The phase difference seems to be a very sensitive indicator for changes in the regional ventilation during postural changes. This might give important information about lung mechanics during a kinetic therapy used in intensive care units for patients with lung injury. © Springer-Verlag 2007.