IJCS | Volume 32, Nº1, January/ February 2019

62 Figure 1 - Schematic illustration of impedance cardiography application, four-pole technique. A1 and A2 correspond to the current- applying electrodes; R1 and R2 to the current-receptor electrodes. Source: adapted from Cybulski et al. 14 Leão and Silva Impedance cardiography and hypertension Int J Cardiovasc Sci. 2019;32(1)61-69 Review Article independent and low-cost hemodynamic monitoring tool that allows defining the patients’ hemodynamic profiles, leading to a more adequate selection of the antihypertensive therapy. 10 Impedance cardiography Biological tissues are complex anisotropic conductors with reactive and resistive components. Thebioimpedance value depends on the type of tissue analyzed and can be altered by translocation of organs or tissues, by changes in shape or structure, by the volume or location of intracellular fluids, or by the frequency of the current used. The ICG consists in the evaluation of the electrical properties of the biological tissues of the chest. 11 The bioimpedance measures the way the tissues conduct the alternating electric current and varies according to the amount of body fluids. Thus, the chest impedance increases or decreases, depending on the changes in intrathoracic fluid with each heartbeat. 12,13 The most common technique uses four electrodes, two of which are the current electrodes and the two that detect voltage changes. Since the current amplitude is constant, the detected voltage is proportional to the tissue impedance. 14 Figure 1 represents the four-pole impedance measurement scheme. The effective evaluation of the chest impedance during a cardiac cycle is hindered by several factors, such as chest size and shape, obesity, body weight, position and posture, thoracic circulation and respiratory rate. For this reason, although this method was published in 1940 by Nyboer et al., 15 it took several years and many studies to reach a system that would correct them. 14,16-19 Current technology, with data processing and modeling techniques, has demonstrated that ICG has a high correlation, reproducibility and precision when compared to invasive hemodynamic monitoring techniques and echocardiography – which was considered more time-consuming, operator- dependent and technically demanding. Therefore, the ICGallows safe, non-invasive and low-cost hemodynamic and cardiac cycle monitoring. 20-25 The ICG detects, analyzes, and records hemodynamic changes by measuring electrical resistance changes in the thorax, graphically translating them as impedance and electrocardiography waves (Figure 2). It allows the calculation of several hemodynamic parameters such as systolic volume (SV), cardiac output (CO), systemic vascular resistance (SVR), velocity and acceleration indexes, thoracic fluid content (TFC), pre-ejection period, left ventricular ejection time, systolic time ratio, left cardiac work, heart rate and mean BP. 26 The assessed parameters and respective formulas are shown in table 1. The first derivative of the waveform (ΔZ) describes fluid velocity and is a smooth wave, which corresponds to the systole, called S wave. The initial slope of the S