Ventilation efficiency during exercise: the delicate balance behind carbon dioxide removal

Ventilation (VE) efficiency, a cardinal feature of cardiopulmonary diseases, is defined as the amount of VE needed for a given carbon dioxide production (VCO2). During exercise it is mainly assessed as the relationship between VE and VCO2 provided that arterial CO2 pressure is not increased. However, the current understanding of VE efficiency, is limited and interpretative challenges remain for the wider clinical workforce. This review emphasizes the importance of the VE/VCO2 relationship, by assessing its exercise physiological mechanisms in health and disease, delving into how different diseases, including heart failure (HF), chronic obstructive pulmonary disease and pulmonary hypertension, display different profiles of VE efficiency. Additionally, we discuss different methodologies for analyzing the VE/VCO2 relationship, including measuring the slope up to and beyond the respiratory compensation point, assessing the ventilatory equivalents for CO2 (EqCO2) at defined time points, and evaluating the EqCO2 nadir. Lastly, we aimed to highlight the prognostic implications of identifying specific ventilatory response profiles alongside meet the defined cut-offs based on different methodologies of assessment. All the provided information is aimed at helping clinicians in understanding the VE efficiency concept, measures and its application in clinical reasoning for improved patient management. Indeed, when assessing a cardiopulmonary exercise test VE efficiency must be assessed, quantified and its causes investigated to allow physicians to modify patients' treatment accordingly.