Fig. 11.
(A) Electrical impedance tomography (EIT) determines the distribution of intrathoracic impedance (Z) by applying a known alternating current (I) to an initial pair of electrodes and measuring the resulting surface potentials (voltage [V]) at each of the remaining 13 pairs of electrodes. Next, the current is applied to the adjacent electrode pair of electrodes, and the V is recorded at the other electrodes; this cycle is repeated for one cycle of current applications, resulting in one set of EIT raw data expressed as inspiratory cyclic changes in impedance (ΔZ; adapted from Drager brochure). The cycle takes 0.02 s; it is repeated continuously in each of the circuits in sequence, and the impedance is continuously measured. Because of the multiple circuits around the chest, ΔZ can be localized approximately to each of the quadrants. Because an increase in circuit impedance reflects an inspiratory increase in aeration, ΔZ reflects ventilation of the region in question. (B) Distribution of ventilation using EIT and the corresponding aeration in computed tomography (CT) images in a pig with lung injury. At positive end-expiratory pressure (PEEP) of 12 cm H2O, distribution of ventilation is homogeneous (left upper). The white dots identify the midline bisecting the thorax. The center of ventilation is calculated as [(ΔZ in dorsal half of lung) × 100]/[ΔZ in whole lung]187 ; if the midline is positioned, the percentage of ventilation that is dorsal is shown on the EIT display (and reflects the center of ventilation) so that off-line calculation is not necessary. In this example (left upper), with PEEP 12 cm H2O, the center of ventilation is 44%, and the corresponding CT shows no lung collapse (left lower). In contrast, when the PEEP is reduced to 4 cm H2O, ventilation is shifted to the nondependent lung, and the center of ventilation is now 25% (right upper), and the corresponding CT confirmed the presence of dorsal atelectasis in the same region (right lower).