Miller School Cardiologist Authors Review Article for The New England Journal of Medicine

11/19/2008

Implantable cardioverter-defibrillators (ICDs) are medical devices designed to constantly monitor the heart beat, recognize life-threatening heart rhythm disturbances, and deliver a life-saving shock when needed.  Patients who have survived a cardiac arrest clearly benefit from these devices.

Robert Myerburg, M.D.

In addition, certain groups of patients who have survived a myocardial infarction (heart attack) - but have not had a cardiac arrest - can also benefit, but it is difficult to identify the individual patients within those groups who will achieve benefit from an ICD after a heart attack.  University of Miami Miller School of Medicine cardiologist Robert Myerburg, M.D., has authored a review article examining and dealing with this problem in the November 20 issue of The New England Journal of Medicine.

When a patient survives a heart attack that results in a significant degree of permanent loss of heart muscle, they are at increased risk for sudden death usually due to ventricular tachyarrhythmias, with the risk being highest in the first 12 months after a heart attack, but with risk continuing over time.

“The cumulative results of three secondary-prevention trials [studies in patients who have survived a cardiac arrest] have led to a general acceptance of ICD therapy for most survivors of [heart rhythm irregularities called] tachyarrhythmic cardiac arrest," writes Myerburg.  "In contrast to the fairly clear role of ICD therapy for secondary prevention, the optimal approach to the selection of patients for primary prevention [studies in patients who have survived a heart attack] has been more difficult to define."

In the review article titled "Implantable Cardioverter-Defibrillators after Myocardial Infarction," Myerburg reviews the available evidence, patient selection and areas of uncertainty before outlining his recommendations on when ICD therapy should be initiated for primary prevention of a cardiac event in a patient who has already suffered a heart attack.

“Because of the limitations of the available data and current guidelines, the selection of [individual] patients to receive an ICD for primary prevention is not uniform in clinical practice," Myerburg writes in the "Recommendations" section of his review article.

He goes on to suggest a reasonable approach is to "begin with an assessment of the ejection fraction and then consider the modifying factors." (The ejection fraction is the measurement of the pumping capacity of the heart that is used as a marker of risk of cardiac arrest when it is very low).  Myerburg recommends that patients with ejection fractions of 25% or less should generally be considered suitable candidates.  Low ejection fractions also lead to heart failure, which is associated with irregular heart rhythms and sudden death.

Myerburg says there is uncertainty about the magnitude of ICD benefit for patients with an ejection fraction in the range of 25% to 35%, especially for those with ejection fractions between 30% and 35%.  But he says that’s when modifying factors could play a role. “Modifying factors include symptomatic heart failure or a history of heart failure, documented nonsustained or inducible ventricular tachycardia and a prolonged duration of the QRS interval," Myerburg writes.  These factors, when present, help identify the potential for benefit when the ejection fraction is in a range of uncertainty.

Myerburg also cautions that his recommendations are "provisional and are likely to change as more information becomes available."   He believes we are in a transitional period while awaiting further research, with his final conclusion being that "ICD therapy should be neither denied nor overextended while we are awaiting better evidence-based algorithms. Physicians familiar with the current criteria and their limitations are in the best position to exercise reasonable judgment. Patients should be advised of the issues involved and included in the discussion of the appropriateness of ICD implantation for them."