Introduction: Mean Intrinsic Heart Rate (MIHR) and the Heart Rate Score (HRS), defined as the percent of all atrial sensed and paced beats in the single largest 10 beats/min bin on a device histogram and a measure of chronotropic incompetence, predict survival in ICD populations but possibly by different mechanisms.
Hypothesis: We sought to investigate if MIHR and HRS are independent predictors of outcomes in a large, prospective, ICD population.
Methods: The INTRINSIC RV trial enrolled 1530 patients receiving dual chamber ICDs. This retrospective analysis involves all patients (n= 1471) for whom both MIHR and HRS were available. The relationship between baseline MIHR and HR score on the primary end point of all-cause mortality and heart failure (HF) hospitalization was assessed using multivariate regression and Cox modelling. As categorical variables, MIHR >70 bpm and a HRS >70% were considered high. Regression analyses were used to assess correlation between HR score and MIHR.
Results: The median baseline MIHR and HRS were 74 (IQR=16) and 50% (IQR=20) respectively. HRS was associated with hypertension and coronary artery disease but not with gender, diabetes, beta-blocker use, or atrial pacing percentage whereas MIHR was associated with NYHA class but not with gender or beta-blocker use. Regression analysis showed baseline MIHR to be associated with HRS (p=0.01); for every 1 beats/min increase in MIHR, HRS increased by 0.018. A MIHR >70 bpm and HRS ≥70% predicted but were independently associated with the primary end point (HR 1.39; CI95: 1.10-1.76, p=0.005 for MIHR and 1.654; CI95: 1.111-2.461, p=0.01 for HRS).
Conclusions: In this large, prospectively studied ICD population, high MIHR and high HRS predicted outcomes independent of each other. This is novel evidence that HRS, a marker of chronotropic incompetence, can be a unique predictor of death/hospitalization in ICD patients and deserves further study regarding underlying mechanisms.