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Evaluation of Electrical Impedance Tomography in the Measurement of PEEP-Induced Changes in Lung Volume^*

^* From the Department of Pulmonary Medicine (Drs. Kunst, de Vries, and Postmus), Institute for Cardiovascular Research, Academic Hospital Vrije Universiteit, Amsterdam, The Netherlands; and the Department of Intensive Care Medicine (Dr. Bakker), Hospital Centre Apeldoorn, Apeldoorn, The Netherlands.

Correspondence to: P.W.A. Kunst, MD, Academic Hospital Vrije Universiteit, Department of Pulmonary Medicine, P.O. Box 7057, 1007 MB, Amsterdam, The Netherlands

Study objectives: A new noninvasive practical technique called electrical impedance tomography (EIT) was examined for the measurement of alveolar recruitment.

Design: Prospective clinical study.

Setting: ICU of a general hospital.

Patients: Acute respiratory failure (ARF) patients.

Measurements: The ventilation-induced impedance changes (VICs) of the nondependent and the dependent part of the lung were determined by EIT as a measure of tidal volume distribution. By the use of an impedance ratio (IR), defined as the VIC of the nondependent part of the lung divided by the VIC of the dependent part of the lung, the ventilation performances in both parts of the lung were compared to each other.

Results: Between patients, the VIC of the nondependent part of the lung was significantly lower in the patients with a level of positive end-expiratory pressure (PEEP) of > 10 cm H₂O than in patients with a PEEP of < 5 cm H₂O (p < 0.05). A significantly lower IR (± SD) was found in the group with PEEP of > 10 cm H₂O than in the group with PEEP between 0 and 5 cm H₂O (1.28 ± 0.58 vs 2.99 ± 1.24, respectively; p < 0.01). In individual patients, the VIC of the whole lung increased when the PEEP level was increased. The VICs of the nondependent part of the lung and of the dependent part of the lung showed significant increases at a PEEP of 10 cm H₂O compared to a PEEP of 0 cm H₂O (p < 0.05). Also the IR decreased in individual patients when the PEEP was increased; a significant decrease was found at 10 cm H₂O compared to 0 cm H₂O (1.67 ± 1.24 vs 2.23 ± 1.47, respectively; p < 0.05).

Conclusions: The decrease in IR indicates an increase in VIC in the dependent part of the lung above the nondependent part of the lung. The increase in VIC can be regarded as an increase in lung volume, implying alveolar recruitment in the dependent part of the lung. The same results also have been shown in earlier reports by CT scan. Since EIT is far more practical than CT scanning and also is a bedside method, EIT might help in the adjustment of ventilator settings in ARF patients.

Key Words: acute lung injury • electrical impedance • positive end-expiratory pressure • recruitment • tomography

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