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Sudden Cardiac Arrest (SCA)

Sudden Cardiac Arrest  (SCA), sometimes referred to Sudden Arrhythmic Death Syndrome (SADS), is the term used to describe a group of medical conditions that lead to sudden, unexpected and life threatening instability of the heart’s rhythm. 

In various forms it is also known as Sudden Cardiac Death (SCD), Sudden Infant Death (SID) or Sudden Unexplained Death (SUD). 

In the majority of cases the unstable heart rhythm (arrhythmia) that develops is a rhythm called Ventricular Fibrillation (VF), where the main pumping chambers of the heart (the ventricles) lose all rhythm and regularity and start beating at heart rates in excess of 250 beats per minute. Ventricular fibrillation is incompatible with life and will cause sudden collapse, seizure like activity and cardiac arrest (total loss of heart function). If diagnosed quickly then cardiac massage (CPR) and a shock from a defibrillator can quickly restore the normal rhythm of the heart and signs of life.

Sudden Cardiac Arrest is uncommon in young people. In the general population the chance of SCA is highest in people with known angina, heart attacks or furred arteries but because these conditions are rare in the under 35’s sudden cardiac arrest in the young tends to be caused by other diseases. 

What Causes Sudden Cardiac Arrest (SCA)?

Conditions causing SCA can be split into four main categories;

  • Abnormalities of the heart blood supply
  • The heart muscle
  • The heart’s electrics  
  • 'Other conditions’ (that although not specifically heart conditions can have a ‘knock on’ effect on the heart’s function)

All conditions have a single common feature; the ability to suddenly destabilise the heart’s rhythm and cause SCA. 

Blood Supply

The heart is kept nourished by a series of arteries (coronary arteries) supplying its muscle and electrical circuits.  Sudden blockage of these arteries (as in heart attack) can destabilise either heart pumping strength or heart rhythm as previously described.  Blocked arteries tends to be a disease of the over 35’s.  It is more common in males, smokers and people with a strong family history of blocked arteries (angina, stents or bypass surgery).  Conditions such as sugar diabetes, high blood pressure and high cholesterol also cause blocked arteries.

Other than blockages, disruption to heart arteries can also occur in the form of spasm (sudden narrowing of arteries reducing blood supply), sudden rupture and congenital abnormalities of the origin of the arteries (can lead to compression of the blood supply).  All can cause SCA but are very rare.

Other than SCA, coronary artery disease can cause chest pain (chest, shoulder, throat, arm, jaw or upper abdomen) or tightness on exertion.  It can sometimes cause excessive belching, shortness of breath or clamminess.  Rest will often cause symptomatic relief within 5-10 minutes. 

Heart Muscle Diseases

Diseases of the heart muscle (cardiomyopathies) are the commonest cause of SCA in the young.  Two conditions Hypertrophic Cardiomyopathy and Arrhythmogenic Right Ventricular Cardiomyopathy are the commonest causes of sports field related deaths (approx. 95%).  Other conditions such as Dilated Cardiomyopathy, Myocarditis and Infiltrative Cardiomyopathies are also known to cause SCA.

Many of these conditions will be silent (i.e. no symptoms before SCA) however exertional dizziness, faints, shortness of breath or chest pain can occur.

All of these conditions disrupt the muscle structure of the heart thereby disturbing the normal passage of electricity though the heart.  Under certain circumstances (e.g. illness, extreme exertion) this can lead to SCA.

Electrical Diseases

Electrical diseases relate to the way in which the heart is ‘wired’ or the way in which the heart cells manage electricity.   Wiring problems that can lead to SCA relate predominantly to a condition called Wolff-Parkinson-White syndrome in which an extra wire exists connecting the top and bottom chambers of the heart (atria and ventricles).  Other ‘electrical’ diseases alter the way in which each heart beat electrical signal is generated, conducted and re-generated and under certain circumstances can lead to SCA.  These conditions include Long QT syndrome (LQT), Catecholaminergic Polymorphic VT (CPVT), Brugada syndrome (BS), Early Repolarization Syndrome (ERS), Short QT syndrome (SQT) and idiopathic ventricular fibrillation (IVF).

Most of these conditions can cause dizziness or faints but will largely go unrecognized until SCA occurs.  Certain triggers for fainting (other than exertion) such as large meals, fever, swimming, alarm clocks, sleep and use of antibiotic or antihistamine medication can raise suspicion of the presence of one of these conditions. 

Other Causes

In certain sports, sudden impact to the chest (e.g. ball or physical contact) that happens at a specific time during a normal heart rhythm can cause SCA.  This happens even when no underlying heart problems are present (i.e. completely healthy muscle, arteries and electrics).  This condition, called Commotio cordis, has been seen most commonly in sports with high speed, small, solid balls (e.g. baseball).  It cannot be medically screened for but can be avoided by the use of impact protecting vests.

Other conditions that lead to SCA are rare and may have no direct relation to the heart.  These can include conditions such as the rupture of major blood vessels (aortic dissection), to epilepsy and bleeding in the brain or imbalance of salts and minerals in the body.  All these conditions can alter the way in which oxygen and minerals are delivered to the heart or brain leading to a ‘knock-on’ effect of SCA.