Why is an Implantable Defibrillator NOT Useful Soon After Myocardial Infarction?

Here is the transcript of the video: Implantable defibrillator is an important life saving device. It can automatically detect life threatening ventricular arrhythmias and treat them, either with a shock or, sometimes by overdrive pacing. Then, why is it mentioned that, implanting a defibrillator soon after an acute myocardial infarction, in those with left ventricular dysfunction and prone for ventricular arrhythmias and sudden cardiac death, is not useful?

This a representative image, showing the implanted defibrillator, and two shock coils, one in the superior vena cava and another in the right ventricle. These are high voltage, defibrillator shock coils. And, after the shock, if there is bradycardia, it can be covered by these two pacing electrodes, one at the tip, and one proximal to it. You can see the screw here, which means that it is a screw in type of electrode. This is the basic single chamber defibrillator. There are dual chamber defibrillators, where an additional lead can be placed in the right atrium, which can detect, atrial activity and guide the defibrillator to discrimate between supraventricular and ventricular arrhythmias. You may be knowing that one of the causes for inappropriate ICD shocks, is supraventricular arrhythmia, wrongly detected by the defibrillator as a ventricular arrhythmia and giving a shock. It can be about 25% of the shocks and that can cause a lot of psychological distress to the person. So in that case, you can use a dual chamber defibrillator with an additional atrial lead. Naturally, that will be more expensive and presence of two leads will cause more of complications. And another factor is that, battery life also may be slightly lower if you have an additional lead in the atrium. Most of the defibrillators implanted in this region would be single chamber defibrillators. Mostly for protection against sudden cardiac death. So, this is how an ICD works. Then, why is it mentioned that, an implantable defibrillator, is not useful, soon after myocardial infarction? For this, you have to assess the clinical study data of two important trials.

DINAMIT and IRIS, were the two important, negative studies, which showed that implanting a defibrillator, soon after myocardial infarction, typically within 40 days after a myocardial infarction, for primary prevention, for prophylactic implantation. These are not evaluation ICD for those already having ventricular arrhythmias and sudden cardiac death. That is called as secondary prevention. Secondary prevention is always a class I indication ICD. These are prophylactic or primary prevention studies. Both were negative trials. So what is the reason why, ICD, even though there is a high risk of ventricular arrhythmias in the early period after myocardial infarction, is found to be not useful? Some of the reasons cited are that, those who are likely to develop ventricular arrhythmias, that is basically those with left ventricular dysfunction after myocardial infarction, are also more prone for heart failure deaths as well as higher ischemic burden, soon after myocardial infarction. So, after myocardial infarction, the risk is more of heart failure, than arrhythmic death. That is one reason. Secondly, that is, in those who are having basic left ventricular dysfunction, after myocardial infarction, residual LV dysfunction. Second reason is that they have more of ventricular fibrillation than ventricular tachycardia. And another important aspect which could not be proved by these studies, mainly because, the number of deaths noted were lower, is whether, the shock itself is causing deterioration of ventricular function and mortality or, shock is needed in those who are having a higher risk of mortality due to other reasons. Whatever be the cause, these studies showed that, implantation of a defibrillator, soon after myocardial infarction, typically it is 40 days, is the cut off in which it is considered as a class III indication. Implantation of a defibrillator is considered class III. Then, what do we do for these patients, who are at higher risk, of arrhythmic mortality, but ICD is not indicated as per the trial data?

There comes the role of WCD or wearable cardioverter defibrillator. It is a device which is worn and the electrodes are within the vest which is worn as a vest and this can automatically detect arrhythmias and give shocks, just like the ICD. But pacing may not be a good option in such situation because the device is external and overdrive pacing is not an option as in subcutaneous ICDs, which are the other less invasive form of ICDs. And it has been sometimes called as a “Life vest till the life boat”, that is the ICD, “arrives”, meaning that, in the window period, when there is risk of arrhythmia, but ICD is not indicated as per the clinical guidelines. The role of ICD (error: WCD) was evaluated by VEST trial, but problem was that, intention to treat analysis of VEST trial did not show significant reduction in mortality. So there was dilemma. As treated protocol and per protocol analysis was made and it was shown that, WCD protects those who are really wearing it. Since it is an external device, unlike the implanted device, compliance was the most important Achilles heel! Of the 48 patients who died, in the VEST trial, only 12 were wearing WCD at the time of death. So that can be considered as an Achilles heel of the WCD. When you take it out for some other reason, then if an arrhythmia occurs at that period, naturally, there is no protection, unlike an ICD, which is already inside, and continuous protection is available for an ICD, while for an implantable, sorry, for an external wearable cardioverter defibrillator, there is obviously no protection when the VEST is not worn. Leave alone the factors that the shocks may be less effective when it is given externally. But with modern devices which give biphasic shocks, instead of the monophasic shocks which the original devices had, the rates of defibrillation and successful defibrillation are better with wearable defibrillators. And, in a post myocardial infarction patient with left ventricular dysfunction who are prone for arrhythmias, guidelines give a class IIb indication for wearing a defibrillator vest known as WCD or wearable cardioverter defibrillator. So in that way, you can give some protection for those post myocardial infarction patients early in their course who are not having indication for ICD, but still having risk for arrhythmic mortality.