Identification of the individual cardiac contraction threshold during high-frame-rate stress echocardiography

The clinical assessment of cardiovascular function during exercise using stress echocardiography is essential for accurate cardiac diagnosis. However, normal limitations of cardiac deformation responses to increasing physical exertion remain poorly understood. We introduce the individual cardiac contraction threshold (iCarT) as a novel marker of mechanical cardiac limitation and examine its relationship to standard cardiovascular measures.High-frame-rate (121-276 frames/s) exercise echocardiography was performed during stepwise cardiopulmonary exercise testing on a recumbent cycle ergometer in 19 healthy, recreationally active adults. Visit 1 was used to individualie exercise protocols, applied in Visit 2. Left ventricular (LV) peak twist, longitudinal strain (LS), and apical circumferential strain (ACS) were assessed at each exercise stage. Polynomial curve fitting was used to identify plateaus in deformation parameters. iCarT was defined as the relative heart rate, expressed as a percentage of maximum heart rate (% HR_{m a x}), at which the final deformation parameter reached its maximum. Associations between iCarT and the timing of plateau onset in volumetric and vascular measures were analysed using Spearman and Pearson correlations.At the group level (n = 19), LS plateaued first (76% HR_{m a x}), followed by ACS (82% HR_{m a x}) and LV twist (83% HR_{m a x}). iCarT occurred at 89 ± 13% HR_{m a x}. At the individual level, among participants with complete deformation data (n = 12), 50% did not show a clear deformation plateau. In participants displaying iCarT (n = 6), LV twist was the final parameter to level off in 83%. iCarT showed no association with stroke volume, cardiac output, end-systolic volume, or ejection fraction. iCarT identifies individual limits of cardiac deformation during exercise, characterized by marked interindividual variability. Its independence from conventional cardiovascular measures highlights the added value of deformation-based thresholds, with LV twist emerging as the most robust parameter during increasing effort.