Ventilation/perfusion mismatch measured by electrical impedance tomography at the bedside: potentialities and challenges

for stabilization and to estimate the apnea-related drift in impedance [4], then ~ 10 ml of contrast agent is pushed through a central line in < 1-2 s.Apnea normally lasts ≥ 8 s [6], but longer times have been used experimentally and may sometimes be advisable.Three components are typically combined in most thoracic perfusion traces, termed [4, 5] pre-lung-likely corresponding to the right heart and greater pulmonary vessels-lung and post-lung-encompassing an early left heart rebound, bronchial circulation and recirculation [5] (Fig. 1b).Approximate peak times for each component depend on hemodynamics [7] and have been reported in swine [5,7], while only inferred from physiology in humans.The low spatial resolution of EIT magnifies the partial volume effect (PVE), which must be addressed by post-processing.Several algorithms are in use to isolate and reconstruct lung perfusion images, greatly differing in how they deal with PVE.The best validated [4,5,7] fit multiple gamma variates to data, only to retain the lung signal.Others remove a cluster of cardiac pixels or manually select a time frame between the pre-lung and lung peaks.Irrespective of the algorithm, regional perfusion is assumed to be proportional to the maximum upslope of the indicator signal, disregarding wash-out (Fig. 1b, e) [5].To date, the need for stable apnea warrants caution when analyzing indicator-based V′/Q by EIT in spontaneously breathing patients [6,8].Alternatively, the pulmonary pulse can be isolated from EIT.However, while related to stroke volume passing from the heart to lung