Cardiopulmonary exercise testing and heart failure: a tale born from oxygen uptake

Since 50 years, cardiopulmonary exercise testing (CPET) plays a central role in heart failure (HF) assessment. Oxygen uptake (VO₂) is one of the main HF prognostic indicators, then paralleled by ventilation to carbon dioxide (VE/VCO₂) relationship slope. Also anaerobic threshold retains a strong prognostic power in severe HF, especially if expressed as a percent of maximal VO₂ predicted value. Moving beyond its absolute value, a modern approach is to consider the percentage of predicted value for peak VO₂ and VE/VCO₂ slope, thus allowing a better comparison between genders, ages, and races. Several VO₂ equations have been adopted to predict peak VO₂, built considering different populations. A step forward was made possible by the introduction of reliable non-invasive methods able to calculate cardiac output during exercise: the inert gas rebreathing method and the thoracic electrical bioimpedance. These techniques made possible to calculate the artero-venous oxygen content differences (ΔC(a-v)O₂), a value related to haemoglobin concentration, pO₂, muscle perfusion, and oxygen extraction. The role of haemoglobin, frequently neglected, is however essential being anaemia a frequent HF comorbidity. Finally, peak VO₂ is traditionally obtained in a laboratory setting while performing a standardized physical effort. Recently, different wearable ergo-spirometers have been developed to allow an accurate metabolic data collection during different activities that better reproduce HF patients' everyday life. The evaluation of exercise performance is now part of the holistic approach to the HF syndrome, with the inclusion of CPET data into multiparametric prognostic scores, such as the MECKI score.