ABC | Volume 115, Nº1, Suplement, July 2020

Case Report Guimarães & Bocchi Left Bundle Branch Block Arq Bras Cardiol 2020; 115(1Suppl.1):10-13 Exercise provides a useful tool for the indirect assessment of cardiac functional reserve, from left ventricular performance, the change from rest to peak exercise, and can be limited in the presence of disease. In this context, a drop in the cardiovascular pattern of VO 2 responses may represent impaired cardiac output (CO) or impaired peripheral oxygen extraction as is observed in heart failure. 10 On the other hand, in healthy individuals, the level to which oxygen consumption and peripheral oxygen extraction increase in response to exercise is much greater compared to changes in stroke volume and similar to the increase observed in HR. The decrease in VO 2 and O 2 pulse, in spite of the increase in HR, observed in this case, may indicate a possible cardiac abnormality. We suggest that the asynchronous motion of the left ventricle associated with delayed wall contraction can reduce the left ventricular workload, resulting in a lower stroke volume and indicating a decrease in cardiac output during maximal exercise, despite the increase in HR. 8-10 This reduction in left ventricular work by an apparent septal perfusion defect during CPX can lead to energy loss and waste of myocardial work, which may represent the hemodynamic impact of asynchronous electrical activation of the myocardium during LBBB, 8-10 and partly explain the drop of VO 2 observed during Figure 2 – Electrocardiogram demonstrating left bundle branch block: a resting rate of 75 bpm (A), a heart rate of 175 bpm during maximal exercise tolerance test (B) and a heart rate 153 bpm in the first minute of the recovery period (C). 12