Using Electric Impedance Tomography to Assess Regional Ventilation at the Bedside

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Abstract
Mechanical ventilation is a cornerstone in the treatment of acute respiratory failure, but like a double-edged sword, it can both save lives and kill. It ensures adequate oxygenation if ventilation is distributed to most of the lung (or, ideally, is distributed in proportion to regional lung blood flow), but it may, and often will, cause harm to the lung itself. Unfortunately, we have no bedside technique for monitoring regional ventilation and perfu- sion or pending lung damage. How then can we know whether our ventilator settings are optimal? Arterial blood gas analysis and the recording of airway pressure-volume curves can guide us in the ventilator setting. Even if both techniques are available at the bedside, however, they reflect only overall lung function, and regional abnormalities may pass undetected. Regional ven- tilation and perfusion can be studied by isotope and magnetic resonance techniques, and aeration of the lung by computed tomography. None of these are bedside tools. In this issue of the Journal (pp. 791-800), Victorino and coworkers (1) report on measurements of regional ventilation by a truly bedside tech- nique, electric impedance tomography, in mechanically venti- lated patients suffering from acute respiratory failure. Electric impedance tomography was presented 20 years ago (2). By injecting an alternating current between sequential pairs of adjacent electrodes around the chest, voltage differences be- tween other pairs of non-injecting electrodes can be recorded and translated into regional impedance values. Air is a poor con- ductor of electric current and causes high impedance, whereas water or blood is a good conductor. The difference makes it pos- sible to detect changes in air and tissue content, enabling the assessment of ventilation distribution (3). Good agreement with ventilation measurements by fast electron-beam computed to- mography (4) and ventilation scintigraphy (5) has been shown in animal experiments. For the first time, Victorino and coworkers present results in severely ill patients with marked inhomogene- ity of ventilation distribution (1). There was excellent reproduc- ibility of the measurement of ventilation distribution when parti- tioning the lungs into four zones, with a variation of only 4 to 7%. This is equal to, or better than, the reproducibility of most physiological measurements. Bias was minor and the difference between the impedance tomography technique and computed tomography was less than 10% for detecting imbalances between the right and left lung. This is good enough to make the technique an interesting bedside tool. The reader is encouraged to look and listen to the narrated video that can be found in the online supplement (1). The asyn- chronous pattern of opening and expansion of different lung re- gions is fascinating and educational. The video sequences suggest that the non-aerated regions are collapsed, not fluid-filled, and need a threshold airway pressure to pop open. Additional studies are required, however, to distinguish between possible mecha- nisms of airlessness (6). An interesting finding was that dependent lung regions did not inspire until some time had elapsed during the slow inflation, with air going initially to upper regions. This

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