Thoracic electrical impedance tomography: an adaptive monitor for dynamic organs
Electrical impedance tomography (EIT) is increasingly used in intensive care patients to monitor pulmonary and cardiac function non-invasively at the bedside. Impedance variations measured by EIT due to a change in thoracic air content represent the largest and most studied signal, allowing the measurement of end-expiratory lung volume (EELV), global and regional ventilation and spatial and temporal heterogeneity of ventilation distribution. This technique gives a dynamic, repeatable, global and regional description of the respiratory system mechanics, providing at the same time a dynamic picture of lung function. A smaller but detectable change in electrical impedance is due to the increase/decrease of blood content into lung vascular tree during the cardiac cycle. This signal is used to measure stroke volume and global and regional lung perfusion. The integration of the ventilation and perfusion components of the EIT signal can be used to measure ventilation-perfusion matching. The aim of this review is to describe the current clinical applications of EIT, providing a guide to the most relevant physiologic variables derived from EIT and their clinical meaning. We will focus on respiratory and cardiac-related variables and their application in both the anesthetized and awake patients. A special attention will be given to two greatly expanding fields of use: pediatric patients and the perioperative (adult and pediatric) setting. Finally, we will describe potential future applications which we expect will soon become a reality, with the potential to change our everyday practice in the intensive care unit and the operating theatre.