If you are deciding on starting an exercise programme to get yourself fit and healthy, you should tick off some boxes to ensure that you reduce your risk of sudden cardiac death (SCD). SCD is sudden unexpected death caused by the onset of a life-threatening heart rhythm (sudden cardiac arrest). In Singapore, it is estimated that about 1,000 individuals die of SCD every year. A 2017 Singapore study on SCD reported that the median age of SCD victims was about 47 years in males and more than 50 years for women. The largest cause of natural death in many developed countries including the USA is sudden cardiac death (SCD). SCD accounts for about half of heart-related deaths in the USA. It is also the most frequent medical cause of sudden death in athletes.

In SCD, there is a malfunction of the electrical system in the heart and the heart rhythm suddenly becomes very fast and irregular. This life-threatening heart rhythm (ventricular fibrillation or VF) will cause the pumping chambers (ventricles) of the heart to “quiver” instead of pumping blood to the body. This will result in a large drop in blood pressure, severe reduction of blood flow to the brain, loss of consciousness and death if no emergency measures are instituted. In the USA, if sudden cardiac arrest occurs outside a hospital, the one-year survival rate after hospital discharge is only about 10 per cent.

Heart attack does not equal sudden cardiac death In coronary artery disease (CAD), the coronary or heart artery is gradually narrowed and when the artery suddenly gets completely blocked (usually by a blood clot), a heart attack occurs resulting in damage to the heart muscle. This may result in instability of the heart rhythm and the development of VF. If VF occurs, and emergency measures are not instituted immediately, SCD ensues. However, not all heart attacks result in life-threatening heart rhythms. Hence, SCD can occur in seemingly “normal” individuals without any symptoms or warning in the absence of heart attacks. Causes of sudden cardiac death Coronary artery disease (CAD) is the most common cause of SCD and accounts for about 80 per cent of all SCD.

What is interesting is that in a recent Australian study on SCD in the young (published in October 2020 in the Circulation: Cardiovascular Quality and Outcomes journal) the most common cause of SCD in those 35 years or younger was CAD. accounting for 40 per cent of SCD. If this is further segmented into different age groups, CAD accounted for 50 per cent of SCD in those aged 26 years to 35 years. In those 5 years and below, inflammation of the heart muscle (myocarditis) was the main cause of SCD and in those 6 to 15 years, myocarditis accounted for 25 per cent of SCD. Other than CAD and myocarditis, the other two main underlying causes of SCD were:
* those who die from an electrical malfunction (arrhythmia) with a structurally normal heart but no other cause found (otherwise known as sudden arrhythmic death syndrome or SADS)
* those with a structurally abnormal heart (swollen heart such as dilated cardiomyopathy or abnormal thickening of the heart muscle such as hypertrophic cardiomyopathy).

Less common causes of SCD include tear of the aorta (aortic dissection), heart valve disease and congenital heart disease. Sudden cardiac death in athletes The SCD profile is different in competitive athletes with most SCDs occurring during intense exertion, and the  common causes of SCD being SADS, heart muscle disease and coronary anomaly (abnormal heart artery). In one study, it was reported that in competitive athletes, SCD was due to SADS in 42 per cent of the cases followed by heart muscle disease (40 per cent). An American College of Cardiology article in December 2019 stated that recent data from the National Collegiate Athletic Association suggested that the most common finding at autopsy for SCD cases was a structurally normal heart (25 per cent), implying that abnormal heart rhythm is likely to be the most common cause of SCD. The second commonest cause was coronary anomaly followed by hypertrophic cardiomyopathy.

This is different from Italian studies, where the most common cause of SCD in athletes was arrhythmogenic right ventricular cardiomyopathy (ARVC), which accounted for about 25 per cent. ARVC is a genetically inherited heart muscle disease where the right ventricle muscle wall has been replaced with abnormal scar tissue causing electrical instability and an increased disposition to VF. The risk of VF is increased by exercise. Generally, in athletes, the data is consistent that electrical malfunction of the heart, heart muscle disease and abnormal heart artery are the three commonest causes of SCD, unlike the general population where CAD is the commonest cause of SCD. Checklist to reduce risk of sudden cardiac death As the consequences of SCD are often devastating, preventing SCD is a key priority.

This means that we need to identify those at risk of SCD. As CAD is the main cause of SCD even for the young , identifying those at risk of CAD is one of the best ways to prevent SCD. Check out this pre-exercise risk identifier list before you embark on vigorous exercise:
* Do you have risk factors for CAD such as high cholesterol, smoking, hypertension, diabetes mellitus, or family history of heart disease?
* Did you have significant damage to your heart because of a previous heart attack or did you have a heart attack within the last 6 months?
* Did you have a previous episode of sudden cardiac arrest?
* Do you have a family history of sudden cardiac arrest or SCD?
* Do you have pre-existing abnormal heart rhythms?
* Do you have unexplained fainting episodes?
* Do you have exertional chest tightness or shortness of breath?
* Are you on medication that can potentially increase your risk of abnormal heart rhythm such as diuretics (can cause abnormally low blood potassium or magnesium levels) or rhythm controlling drugs?
* Do you have a history of congenital heart disease?

If your answer is yes to any of the questions above, you should seek your doctor’s advice before embarking on your exercise programme.

Pre-exercise screening
Both the American Heart Association and the European Society of Cardiology (ESC) advocate pre-participation screening (PPS) of young athletes as intense athletic activity can trigger SCD or disease progression in susceptible individuals. While both recommend a thorough medical and physical examination, only the ESC recommends a routine 12-lead electrocardiogram (ECG) to detect underlying electrical abnormalities or heart muscle disease. However, it is important to understand the limitations of the ECG. For example, the ECG is normal in most individuals with congenital coronary anomalies, in 5 to 10 per cent of those with hypertrophic cardiomyopathy, and in most individuals with catecholaminergic polymorphic ventricular tachycardia (life-threatening heart rhythm).

Those athletes with abnormal findings during the PPS will require further assessment. In addition, the American Heart Association recommends that for men who are 40 years and older and women who are 50 years and older, an exercise stress test may also be required depending on the assessment by their doctor. If heart problems are identified or suspected during the screening, the individual should be referred to a heart specialist for further evaluation before embarking on an exercise programme. As CAD accounts for about 80 per cent of SCD, it means that ticking off the boxes before embarking on exercise and getting appropriate medical advice can prevent SCD in most high-risk individuals.