Evaluation of electrical impedance tomography in the measurement of PEEP-induced changes in lung volume

Study objectives: A new noninvasive practical technique called electrical i mpedance tomography (EIT) was examined for the measurement of alveolar recr uitment. 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 nonde pendent and the dependent part of the lung were determined by EIT as a meas ure of tidal volume distribution, By the use of an impedance ratio (IR), de fined as the VIC of the nondependent part of the lung divided by the VIC of the dependent parr 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-expirator y 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 (1.2 8 +/- 0.58 vs 2.99 +/- 1.24, respectively; p < 0.01), In individual patient s, the VIC of the whole lung increased when the PEEP level was increased. T he VICs of the nondependent part of the lung and of the dependent part of t he lung showed significant increases at a PEEP of 10 em H2O compared to a P EEP of 0 cm H2O (p < 0.05). Also the IR decreased in individual patients wh en the PEEP aas increased; a significant decrease was found at: 10 cm H2O c ompared 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 depende nt part of the lung above the nondependent part of the lung, The increase i n VIC can be regarded as an increase in lung volume, implying alveolar recr uitment ill 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.