General Principle of Operation
SenTec EIT is based on the principles of electrical impedance tomography (EIT), where weak alternating currents are applied and travel along the paths of least resistance through an object, and the electric potentials resulting at the object’s surface are continuously measured by an array of electrodes placed on the object. Typically, electrode arrangements are designed to sequentially pick up signals coming from different directions. In this way, the electrical “point of view” is constantly changing and images reflecting local impedance within the object and variations thereof can be created with image rates of typically 50 Hz.
Principle of Operations if used for lung function monitoring
When a belt with embedded electrodes is fastened around the chest of a subject - for example a human, a horse, dog, cat, pig or rat - it is possible to continuously monitor and visualize local impedance variations within a cross-section of the subject’s thorax. As impedance variations are primarily caused by lung function (i.e. air flowing in, distributing within, and flowing out of the lungs) and, to a less extent, perfusion and cardiac activity, it is possible to continuously monitor variations of regional air content within the subject’s lungs noninvasively, without radiation and at the bedside. In particular, various images (e.g. images related to distribution of tidal volume), waveforms as well as a wide variety of indices and parameters can be derived and trended, including but not limited to
- Plethysmogram representing variations of lung impedance, hence, being related to lung volume (air content) variations with breathing
- Impedance Respiratory Rate (RRi)
- End-Expiratory Lung Impedance (EELI) being related to end-expiratory lung volume, i.e. to the Functional Residual Capacity (FRC), when breathing normally, or to the Residual Volume (RV), in a forced expiration
- End-Inspiratory Lung Impedance (EILI) being related to end-inspiratory lung volume
- Tidal (Impedance) Variation (TVi), the difference between EILI and EELI, hence, being related to the volume inhaled in one breath, i.e. to the Tidal Volume, when breathing normally, or to the Inspiratory Capacity (IC), in a forced inspiration
- Aeration (AE) defined as the mean lung impedance over a preset analysis interval of a fixed duration of, for example, typically 15 seconds, hence, being related to mean lung volume
- Center of Ventilation (CoV) characterizing the ventilation distribution and represented, for example, in vertical and horizontal direction, with the CoV vertical component (CoV(v)) defining the position of the Horizon of Ventilation (HoV)
- Silent Spaces, reflecting the distribution and percentage of lung areas receiving little or no ventilation. Taking into account the influence of gravity on lung tissue and fluids within the lungs, Silent Spaces being localized above or below the HoV are further defined as Non-dependent Silent Spaces (NSS) and Dependent Silent Spaces (DSS), respectively. Silent Spaces may be helpful to identify conditions such as displacement of the endotracheal tube, pneumothorax, and pleural effusion as well as conditions influenced by gravity such as collapsed, fluid filled or distended lung areas, with DSS reflecting the first two conditions and NSS the latter.
- Functional Lung Spaces (FLS) reflecting the distribution and percentage of ventilated lung areas and, hence, being related to the Functional Lung Size, i.e. the percentage of the aerated remaining lung  and sometimes referred to as the size of the available lung volume.
For a more in depth description of the principles of operation of SenTec EIT, please refer to the respective Technical Bulletin. The EIT methodology has been an object of study for decades and there is plenty of literature on the subject; a summary description can be found for example in Costa et al. . For more information on its clinical applications, please also refer to reviews available in literature, e.g. Frerichs et al. , Putensen et al. , Lobo et al. .
Particularities of SenTec EIT
SenTec EIT is the only EIT system worldwide selecting the thorax and lung contours being best adapted to the individual patient from a set of predefined, CT-derived thorax and lung contours. It continuously evaluates the skin-contact quality of all 32 electrodes and its advanced, unique image reconstruction algorithms are able to compensate up to six electrodes having bad or no impedance coupling to the skin. If operated in neonatal mode and respective data is entered via the GUI by the operator, inadvertent belt displacement around the patient’s thorax is additionally compensated. SenTec EIT also features a patented position sensor continuously evaluating the patient’s position (rotation and inclination) and, hence, permitting the clinician to unambiguously assess the influence of gravity on lung mechanics and ventilation distribution in the lungs.
 Marcelo B.P. Amato, et al., “Driving Pressure and Survival in the Acute Respiratory Distress Syndrome”, N Engl J Med 2015; 372:747-55. DOI: 10.1056/NEJMsa1410639.
 E. L. Costa et al., “Electrical impedance tomography”, Curr. Opin. Crit. Care 15(1), 2009.
 I. Frerichs et al. “Chest electrical impedance tomography examination, data analysis, terminology, clinical use and recommendations: consensus statement of the TRanslational EIT developmeNt stuDy group” Thorax. 72(1), 2017.
 C. Putensen et al. “Electrical Impedance Tomography for Cardio-Pulmonary Monitoring”, J. Clin. Med. 8(8), 2019.
 B. Lobo et al. “Electrical impedance tomography”, Ann. Transl. Med. 6(2), 2018.