Prediction of mortality in clinical practice for medicare patients undergoing defibrillator implantation for primary prevention of sudden cardiac death

Abstract

Objectives: The aim of this study was to derive and validate a practical risk model to predict death within 4 years of primary prevention implantable cardioverter-defibrillator (ICD) implantation.

Background: ICDs for the primary prevention of sudden cardiac death improve survival, but recent data suggest that a patient subset with high mortality and minimal ICD benefit may be identified.

Methods: Data from a development cohort (n = 17,991) and validation cohort (n = 27,893) of Medicare beneficiaries receiving primary prevention ICDs from 2005 to 2007 were merged with outcomes data through mid-2010 to construct and validate complete and abbreviated risk models for all-cause mortality using Cox proportional hazards regression.

Results: Over a median follow-up period of 4 years, 6,741 (37.5%) development and 8,595 (30.8%) validation cohort patients died. The abbreviated model was based on 7 clinically relevant predictors of mortality identified from complete model results, referred to as the "SHOCKED" predictors: 75 years of age or older (hazard ratio [HR]: 1.70; 95% confidence interval [CI]: 1.62 to 1.79), heart failure (New York Heart Association functional class III) (HR: 1.35; 95% CI: 1.29 to 1.42), out of rhythm because of atrial fibrillation (HR: 1.26; 95% CI: 1.19 to 1.33), chronic obstructive pulmonary disease (HR: 1.70; 95% CI: 1.61 to 1.80), kidney disease (chronic) (HR: 2.33; 95% CI: 2.20 to 2.47), ejection fraction (left ventricular) ≤ 20% (HR: 1.26; 95% CI: 1.20 to 1.33), and diabetes mellitus (HR: 1.43; 95% CI: 1.36 to 1.50). This model had C-statistics of 0.75 (95% CI: 0.75 to 0.76) and 0.74 (95% CI: 0.74 to 0.75) in the development and validation cohorts, respectively. Validation patients in the highest risk decile on the basis of the SHOCKED predictors had a 65% 3-year mortality rate. A nomogram is provided for survival probabilities 1 to 4 years after ICD implantation.

Conclusions: This useful model, based on more than 45,000 primary prevention ICD patients, accurately identifies patients at highest risk for death after device implantation and may significantly influence clinical decision making.

Figure 1. Hosmer-Lemeshow Test Statistics and Model Calibration Plots

The relationship between observed and expected numbers of deaths as a percent of risk group deciles for events during 2 years (A), 3 years (B), and 4 years (C) of follow-up after implantable cardioverter-defibrillator implantation is shown. The linear association between observed and expected percents is plotted as a solid line, along with a dashed line identifying the ideal association. HL = Hosmer-Lemeshow.

Figure 2. Nomogram for Determination of Survival Probabilities After ICD Implantation

A nomogram is presented for the estimation of survival 1 to 4 years after implantable cardioverter-defibrillator (ICD) implantation on the basis of the 7 “SHOCKED” risk factors from the abbreviated model. To calculate patient survival probabilities, obtain points for each covariate value by dropping a vertical line from the points axis to the value of each covariate, calculate the total points obtained from all 7 covariate values, and then drop a vertical line from the total points axis to locate the associated probability of survival for the patient at the time point of interest after the procedure. AF = atrial fibrillation; CKD = chronic kidney disease; COPD = chronic obstructive pulmonary disease; DM = diabetes mellitus; LVEF = left ventricular ejection fraction; NYHA = New York Heart Association.

Figure 3. Distribution of the Risk Score in the Validation Cohort

The frequency distribution of the nomogram-based risk score (derived from the abbreviated model) is shown. The score ranges from 0 to 360.

Figure 4. Mortality Rates by Quintile of Risk Score in the Validation Cohort

The mortality rates at 1, 2, and 3 years in the validation cohort on the basis of quintile of the nomogram-based risk score are shown (A). In addition, mortality rates are also shown, with the highest quintile split into 4 groups in ascending order of risk (B). Group 5A represents percentiles 80 to 84, while group 5D represents percentiles 95 to 99.