The cardiopulmonary exercise test (CPET) is a consolidated tool in the functional and prognostic assessment of patients with heart failure with reduced ejection fraction (HFrRF), being a cornerstone in the evaluation for the indication of advanced therapies in HFrEF. ^, Cardiac resynchronization therapy (CRT), in addition to reducing mortality, can improve cardiorespiratory fitness, leading to an increase in peak oxygen consumption (VO₂peak) and a reduction in the slope of the respiratory equivalent CO2 ratio (VE /VCO2 slope). ^– In recent years, with the evolution of treatments, general mortality and the risk of the sudden death of patients with HFpEF have been reduced. ^, In this context, reviewing the prognosis and the values associated with a higher risk among the CPET variables in patients undergoing CRT becomes important.

The measurement of end-tidal carbon dioxide pressure (PETCO2) during CPET, both at rest, and at the first ventilatory threshold or anaerobic threshold (P_ETCO2_LA), has a well-established prognostic value in heart failure. ^, The increase of dead space ventilation, caused by the impairment of the ventilation/perfusion (V/Q) ratio, for example, in patients with left ventricular dysfunction, leads to a reduction in alveolar CO2 and, consequently, in P_ETCO₂. It is expected that there will be an increase in its measurement up to the anaerobic threshold, which is correlated with an increase in cardiac output.

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