Going the Distance: 6-Minute Walk Test for ICD Treatment Decisions | CardioSource WorldNews Interventions
JACC in a Flash | Recently, the Sudden Cardiac Death in Heart Failure Trial (SCD-HeFT) produced some perplexing results: while patients with New York Heart Association (NYHA) class III (moderate) symptoms did not benefit from implantable cardioverter-defibrillator (ICD) therapy and appeared to be harmed by amiodarone, patients with NYHA class II (mild) symptoms obtained significant survival benefit from ICD.
Could there be a more objective measure of functional capacity that might provide a better tool for selecting preventive therapies? In a recent article appearing in JACC, Daniel P. Fishbein, MD, and colleagues postulated that the 6-minute walk test (6MWT) could better assist with treatment decisions for patients with heart failure.
"[NYHA classification] has the advantage of wide familiarity, easy application, demonstrated prognostic importance and no cost," Dr. Fishbein and coauthors reasoned. "The 6-minute walk test shares many of those qualities but adds the additional advantage of being a more structured, objective measure of functional capacity." In the 2,397 patients enrolled in SCD-HeFT who underwent a baseline 6MWT, median distance walked was 342 meters.
As expected, patients with greater functional capacity, determined by longer 6MWT distance, had lower rates of all-cause mortality in all treatment arms (TABLE).
When taken in terms of overall risk, ICD therapy significantly reduced mortality compared to placebo in the lower tertiles:
- 0.42 (0.26, 0.66) for 6MWT distance >386 m (top tertile)
- 0.57 (0.39, 0.83) for 6MWT distance 288-386 m (middle tertile)
- 1.02 (0.75, 1.39) for 6MWT distance <288 m (bottom tertile)
Amiodarone seemed to have the opposite effect: the drug increased mortality in the lowest 6MWT distance tertile (HR = 1.56; p = 0.0028) and had a marginal trend toward survival benefit in the highest tertile (HR = 0.68; p = 0.061). "The reason for the adverse results for amiodarone therapy in the patients with more severe heart failure remains unclear," the investigators noted, "but the present analysis at least suggests that this finding was not dependent on NYHA class."
In terms of cause-specific mortality, heart failure-related mortality was inversely related to 6MWT distance— with lower 6MWT distance associated with higher mortality. "This study confirms and extends previous findings from the SCD-HeFT trial showing that the survival benefit of primary prevention ICD therapy varies importantly depending upon baseline functional capacity," Dr. Fishbein and colleagues concluded.
Overall, patients with more advanced heart failure, reflected by a 6MWT in the lowest tertile (<288 m) did not benefit from either prophylactic ICD implantation or amiodarone therapy. Patients with less-severe heart failure and with greater functional capacity, however, experienced favorable results with ICD therapy, but were harmed by amiodarone therapy.
What does this mean for the clinician? "At the individual patient level, clinicians must continue to exercise judgment as to the suitability of advanced heart failure patients for primary prevention ICD therapy, recognizing that sicker patients are much less likely to benefit," the authors added. A decision-making process that takes both NYHA class and the 6MWT distance into account may better assist clinicians in selecting patients who are "too sick to benefit."
Fishbein DP, Hellkamp AS, Mark DB, et al. J Am Coll Cardiol. 2014 March 27. [Epub ahead of print]
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