Sudden cardiac death - who benefits most from an implantable cardioverter defibrillator?

DZHK: Who is at risk of sudden cardiac death?

Axel Bauer: The identification of those at risk of sudden cardiac death is an unsolved problem in cardiology. However, there are known high-risk groups. These are, for example, people with severely restricted heart pumping capacity. The risk of sudden cardiac death is relatively high in this group. However, the majority of people affected by sudden cardiac death do not belong to any of the known risk groups.

That sounds paradoxical, how can that be?

That is the difference between relative and absolute numbers. A relatively large number of people in a risk group will die of sudden cardiac death. However, this does not mean that in absolute terms most victims must be in this group. In fact, most cases of sudden cardiac death do not belong to any of the known risk groups. And in reality, many people die suddenly without knowing that they have a heart condition.

How many people are affected?

Sudden cardiac death is difficult to quantify, but we estimate that there are about 70,000 cases in Germany every year. Most people die before we can diagnose them as ‘at risk’.

So there are ‘hidden’ heart diseases that make people susceptible to sudden cardiac death. What are these and can they be prevented?

Most sudden cardiac deaths, in whatever form, can be traced back to an underlying coronary heart disease like a circulatory disorder of the heart. This doesn’t always have symptoms. This means that the same recommendations apply to the prevention of sudden cardiac death that are generally made for maintaining good heart health. You should eat a healthy diet, exercise, keep cholesterol levels and blood pressure within recommended limits, recognize and treat diabetes. A resting ECG at the GP may sometimes show abnormalities and this can lead to calls for population-wide screening, but there is currently not enough evidence for this as it could result in too many false positives.

Are there families where sudden cardiac death is more common?

Yes, there are congenital and inherited diseases such as hereditary channelopathies. These are diseases that affect the function of the ion channels in the heart and have a fundamental influence on the electrophysiology of the heart. These include, for example, Long QT syndrome. Or there is hypertrophic cardiomyopathy, a congenital thickening of the heart muscle. You are probably familiar with the media reports of young athletes who suddenly die during training. Often these young people have an undetected congenital heart disease. Such a thing is very tragic, but in terms of numbers it is the smallest group of people affected by sudden cardiac death.

An implantable cardioverter defibrillator (ICD) can protect against sudden cardiac death by bringing the heart back into rhythm with a small current surge. When does it make sense to have an ICD fitted?

This only makes sense for people with a high risk of life-threatening rhythm disturbances. On the one hand, these are patients who have already had a serious rhythm event – those who have survived sudden cardiac death only because someone close-by has revived them or the emergency services were on the scene very quickly. Then there are certain groups of people with heart diseases who are known to carry a high risk - those with a severely impaired heart function. For these high-risk groups, the guidelines recommend fitting an ICD. Unfortunately, this type of risk assessment is rather inaccurate for two reasons: firstly, only a few people who have an ICD fitted according to these guidelines will suffer from a very serious rhythm disorder. Secondly, most people who die of sudden cardiac death have a heart function that is too good to qualify for an ICD. In clinical studies at the DZHK we try to better characterize and treat both groups with respect to their risk. An ICD can have serious side effects and complications - it can lead to unnecessary electrical surges or infections.

How will you work out who really benefits from an ICD?

One of our studies deals with the classic risk group, the patient with limited pumping capacity. We know that the risk in this group is varied. We have developed so-called digital biomarkers that can determine the risk of life-threatening rhythm disturbances from signals of the heart such as the ECG. In a recent pan-European study, we demonstrated that a biomarker we developed can identify people who benefit most from an ICD.

And what about people who were previously thought to be less at risk?

We are also investigating them. These are, for example, patients who seem to have recovered well after a heart attack and whose pumping capacity is not severely restricted. Nevertheless, there are many high-risk people in this large group that we can identify with our biomarkers. As part of the SMART-MI study, we fit a so-called ‘event monitor’ for these high-risk people. This is a small, elongated chip that is ‘injected’ under the skin and allows someone to receive remote monitoring for several years. If the monitoring brings something to our attention, we can ask our patient to come and see us to investigate the underlying problem.

Are there myths about sudden cardiac death?

Hmm, actually more about the ICD. Some people fear that it will condemn them to an eternal life by stimulating the heart forever- Here you can reassure them that this isn’t the case. You can of course die, even if you have an ICD, because the mechanical performance of the heart can fail independently of electrical stimulation. However, during end-of-life care it is important to switch off the shock function of the ICD.

Kontakt: Christine Vollgraf, Presse- und Öffentlichkeitsarbeit, Deutsches Zentrum für Herz-Kreislauf-Forschung (DZHK), Tel.: 030 3465 529 02, presse(at)dzhk.de