Why is an implantable defibrillator not useful soon after a heart attack?

Why is an implantable defibrillator not useful soon after a heart attack?

Implantable defibrillator is a life saving device which automatically detects and treats life threatening heart rhythm disorders either by giving fast electrical signals or a shock when needed. Heart attack is a disease prone for life threatening heart rhythm disorders. Still why is an implantable defibrillator said to be not very useful soon after a heart attack?

DINAMIT study checked out why this is so. 653 patients 6 to 40 days after a heart attack and a low heart rate variability were given either an implantable cardioverter defibrillator (ICD) or just standard medical treatment. It was a negative study in that ICD therapy failed to improve survival. This was because those who get death due to heart rhythm abnormality averted by an ICD have a greater decrease in blood supply to heart muscle and heart failure risk. This leads to higher mortality in the subsequent weeks or months.

On the other hand, the lower risk group have less chance of ICD shocks as well as subsequent mortality. There was also a higher proportion of ventricular fibrillation than ventricular tachycardia, among the causes for appropriate ICD shocks in this study. This may also be a marker of higher mortality risk. Ventricular fibrillation is the most dangerous heart rhythm disorder which causes the heart to stop functioning. Ventricular tachycardia is a fast rhythm originating from the lower chambers.

The study was not able to identify whether the heart rhythm disorder needing the ICD shock caused deterioration of heart function or an additional factor caused both together. It could also not show whether the shock itself directly or indirectly lead to higher mortality. This was because the small number of deaths in subgroups limited the role of any such analysis. This study also highlights the fact that ICD therapy may not always mean averting a death.

A competing risks analysis showed that those factors which increased the risk of arrhythmic death also increased the risk of nonarrhythmic deaths. Arrhythmia is the technical term for heart rhythm disorder. Reduction in sudden cardiac death by the implantable cardioverter defibrillator was fully offset by an increase in nonarrhythmic deaths, which were greatest in patients receiving defibrillator shocks.

Similarly the IRIS study also showed that preventive implantation of an ICD did not reduce overall mortality among patients with heart attack and high risk clinical features. IRIS study had 898 patients 5 to 31 days after heart attack. The high risk features needed for enrollment were a lowered pumping function of the heart, higher heart rate and non-sustained ventricular tachycardia on 24 hour Holter monitoring.

Non-sustained ventricular tachycardia is an abnormal heart rhythm originating from the lower chambers lasting less than 30 seconds. If it is longer than 30 seconds, it is called sustained ventricular tachycardia, which is more dangerous and has been shown to benefit from ICD in earlier studies. Hence such patients were excluded from this study as they anyway need an ICD.