Introduction: Impaired chronotropic response (ICR) often affects the recovery of exercise capacity. The purpose of the present study is to investigate whether the persistence of ICR after the completion of phase II cardiac rehabilitation may impact on the future cardiovascular prognosis.
Methods: We enrolled 247 consecutive patients (69 y.o. on average) who participated in phase II cardiac rehabilitation program at least for 3 months (8.5 month on average). All underwent cardiopulmonary exercise test (CPX) at the initial and completion period of rehabilitation. Those who suffered from a life-threatening event within 90 days since rehabilitation started were excluded. Index of heart rate (HR) reserve (HRRI) from CPX data was calculated as the following formula: HRRI = (peak HR - resting HR) / (220 - age - resting HR). Using the established criteria, HRRI <0.80, or 0.62 on beta-blocker regimen, was regarded as “impaired”. Handgrip strength, daily step counts, the left ventricular ejection fraction and blood test data were obtained. The Cox proportional hazards regression analysis was used to assess the independent predictors of the future events. Impaired cases at completion were divided in 2 groups by the median of HRRI (severe and mild), and we finally compared peak HR, HRRI, HR recovery (at 1 minutes after the peak exercise) by CPX between 3 groups (“Normal chronotropic response”, “Mild ICR” and “Severe ICR”). A Kaplan-Meier curve was drawn to compare the incidence of future cardiovascular events.
Results: ICR was detected in 182 cases (74%) at the beginning of rehabilitation, and persisted in 128 (52%) at the completion. Peak HR (median 143 vs. 123 vs. 99/min), HRRI (median 0.895 vs. 0.639 vs. 0.409), HR recovery (median 19 vs. 15 vs. 8/min) were significantly different between groups at the completion (all p<0.001). Peak oxygen consumption had no significant difference. “Severe ICR” group had significantly higher incidence of cardiovascular events than “mild ICR” group (25.0% vs. 7.8%, p=0.008). Multivariate analysis revealed that “severely reduced HRRI” was an independent predictor of future events (hazard ratio 3.1, confident interval: 1.1–8.5). Kaplan-Meier analysis showed significant difference in event-free survival rate between groups (692 days on average, Log Rank p=0.017).
Conclusion: Incidence of cardiac events may partly depend on the severity of ICR at the chronic phase of cardiovascular disease. Persistence of ICR after phase II cardiac rehabilitation can be a prognostic indicator of the future adverse events.