From respiratory limitation to dynamic depletion of mechanical-ventilatory reserves: a paradigm shift to probe exertional dyspnea with clinical exercise testing

Cardiopulmonary exercise testing (CPET) is frequently requested in the hope that detecting the maximal limits of cardiovascular or respiratory function will provide clinically relevant information on the genesis of exertional dyspnea. We provide a concise review of emerging evidence that analyzing whole-test data, accounting for the dynamic (mis)match between requirements and capabilities (i.e., progressive reserve depletion), is more accurate and valuable for clinical decision-making than the traditional respiratory limitation paradigm. In this context, a pattern of excessive breathing emerges when heightened inspiratory muscle activation is fully translated into increased ventilation in the absence of mechanical restraints, such as reduced PaCO 2 , increased physiological dead space, or high CO 2 output. Conversely, constrained breathing results from impediments to tidal volume expansion, imposed by the prevailing inspiratory capacity, which hinders ventilation despite increased inspiratory muscle activation. Based on sex- and age-adjusted standards for submaximal 0-10 Borg dyspnea-work rate and dyspnea-ventilation, dynamic ventilatory reserve-work rate, and dynamic inspiratory reserve-ventilation, the practitioner can readily identify whether excessive and/or constrained breathing can explain the subject’s exertional dyspnea. Regardless of the precise mechanism of excessive breathing, therapeutic efforts should primarily focus on reducing the sources of increased afferent ventilatory stimuli. The identification of constrained breathing should prompt interventions to improve inspiratory reserve volume. This pragmatic approach to clinical CPET interpretation focuses on dyspnea as a treatable trait across physiological and disease states, aiming at providing cogent explanations for the symptom in light of the pre-test likelihood of abnormality.