The empirical evidence suggests that ICDs are not effective for primary prevention of SCD when im- planted early after CABGS16-15 or an acute myocardial An analysis of individual patient lev- el data from 3 secondary prevention trialsS16-18 showed a significant variation (P=0.011) in the clinical effec- tiveness of ICDs between patients with an LVEF 35% (hazard ratio: 0.66) and an LVEF >35% (hazard ratio: 1.2). Some studies and simulation models suggest that ICDs might prolong life expectancy to a greater extent when used in higher-risk patients than in lower- In contrast, there is little evidence of variation in the effectiveness or cost-effectiveness of the ICD based on factors such as age or sex.S16-20 Most studies of ICD effectiveness and value have been per- formed on patients with reduced LV function due to prior MI or NICM. There are few data on the effec- tiveness or value of an ICD for other potential clinical indications, such as cardiac channelopathies or HCM, although studies have suggested that their potential cost effectiveness in such patients will depend on their underlying risk of SCD, with little evidence of value in 17. QUALITY OF LIFE ICD implantation has not had a significant effect on QoL in the overall population of patients enrolled in Several studies have, however, demon- strated that the subset of patients who receive inap- propriate ICD shocks have worse QoL than patients who have an ICD but have not had inappropriate Because an ICD is designed to prevent SCD rather than to reduce symptoms, it would not be ex- pected to improve QoL or functional status directly, but may have indirect, negative effects in some pa- tients due to device complications, or indirect, positive effects in some patients due to reassurance of having a protective device in place. Figure 17. 