IJCS | Volume 32, Nº1, January/ February 2019

63 Figure 2 - Electrocardiography and impedance waves. PEP: pre-ejection period; LVET: left ventricular ejection time; ECG: electrocardiogram. Source: adapted from Thompson et al. 27 Leão and Silva Impedance cardiography and hypertension Int J Cardiovasc Sci. 2019;32(1)61-69 Review Article PEP LVET wave correlates with cardiac contractility, and its height and width, with systolic volume. Several indices, such as pre-ejection period, left ventricular ejection time, velocity index, acceleration index, left cardiac work index, and so on, can be obtained through the waveform, allowing non-invasive monitoring of CO and contractility, for instance. The second derivative of the waveform (dZ/ dt), describes fluid acceleration and represents a more detailed wave, containing four reference points (A, B, C, and X) associatedwith both atrial and ventricular systole, and the point o, which is related to the onset of diastole. Point A coincides with the electrocardiogram (ECG) p-wave and marks the beginning of the end of diastolic filling. The A wave only exists in the presence of an atrial contraction, being small and round, with its end clearly separated from the beginning of the S wave. The basal impedance corresponds to point B. Point C defines the maximum acceleration of blood output from the ventricles. The slope corresponding to the rise from point B to point C is associated with cardiac contractility: the steeper the upward curve, the greater the cardiac contractility. After reaching point C, there is a rapid deceleration to point X, which represents the inversion point of the intrathoracic fluid and corresponds to the closure of the aortic valve. After point X, the wave returns to the baseline and starts to form an early diastolic wave, associated with the opening of the mitral valve, the O wave. Themoment of greatest opening of themitral valve is represented by the peak of the S wave – point O. This interval between points X and O corresponds to the time of isovolumetric relaxation. 28 This technology can be used, for instance, to evaluate postural cardiac rehabilitation, pacemaker optimization, sleep studies, hemodynamic monitoring in pregnant women and outpatients, and therapy and/or monitoring of hypertensive patients. 13,29-39 ICG is a technique that has evolved in recent years and has become an attractive and cost-effective method of improving patients’ clinical approach.