The hearts has its own electrical system to control the rate and rhythm of the heart beat. Impulses are generated in the sinoatrial node (SA node) and immediately cause the muscles of the atria to contract. This pushes blood into the ventricles. The impulse travels along the internodal pathways to the atrioventricular node (AV) node where it is delayed. This ensures that the ventricles do not contract at the same time as the atria. After a few milliseconds, the impulses leave the AV node and dissipate to the muscles of the ventricles thereby causing it to contract. Blood is then pushed out of the ventricles and into the pulmonary or systemic circulation.
The electrical system can increase or decrease the heart rate as needed throughout the day. The rhythm, however, stays the same – atria contract and then the ventricles a short while later. This process is continuous throughout life. Sometimes heart disease and other conditions may change the rate and rhythm of the heart activity and this causes various disturbances in circulation. These abnormal alterations of the rate and rhythm is known as an arrhythmia. Some arrhythmias are life-threatening and can disrupt heart activity to such a degree that normal heart function ceases altogether.
What is sudden cardiac death?
Sudden cardiac death (SCD) is the sudden and unexpected cessation of heart function in a person with no symptomatic heart disease or within the early onset of symptoms, usually within an hour. Simply this means that a person may not have had any typical symptoms of heart disease like cardiac pain, palpitations, dizziness, fainting or breathlessness and then cardiac (heart) activity suddenly stops. Alternatively, these symptoms start up suddenly and within an hour death occurs due to cessation of cardiac activity. Often a person will hear of a death that arose in an otherwise ‘healthy’ person where the heart suddenly stopped and the person had no previous complaints of heart disease. This is sudden cardiac death.
Despite the lack of symptoms or short duration of symptoms, sudden cardiac death does not occur for no known reason. The electrical activity of the heart becomes abnormal leading to irregularities in the rate and rhythm of the heart beat known as arrhythmias. This can reach a point where cardiac function ceases – fatal arrhythmia.
Most cases of sudden cardiac death is associated with ischemic heart disease (IHD) where there is a disruption to the blood supply to heart wall leading to a deficiency of oxygen and nutrients. This causes damage of the heart muscle (myocardium) referred to as myocardial ischemia. IHD is mainly due to coronary artery disease, where there is an obstruction of the arteries supplying blood to the heart wall. However, if it is minor blockage, typically less than 75% blockage of the arterial lumen, a person may be asymptomatic for long periods of time. In this regard, sudden cardiac death occurs in a person with no previous symptoms of heart disease or within a short duration from the onset of symptoms. Essentially the heart disease was present but asymptomatic.
Causes of Sudden Cardiac Death
Sudden cardiac death is often associated with tachyarrhythmias. These are irregular heart beats associated with a disruption in the electrical system of the heart that causes the heart to beat rapidly and abnormally. The most common tachyarrhymias associated with sudden cardiac death is ventricular tachycardia (VT) and ventricular fibrillation (VF).
Ventricular tachycardia arises when there is irregular discharges in the condition system to the ventricles. Ventricular fibrillation is where impulses spontaneously arise in the ventricular conduction system and causes the heart to quiver. In both cases, the ventricles are unable to fill sufficiently with blood and pump out the blood completely. Most cases of fatal arrhythmias that cause sudden cardiac death is often not due to defects within the electrical conduction system of the heart. Instead it is due to structural defects of the heart muscle (myocardium) which affects the discharge of impulses.
As previously mentioned, most cases are associated with coronary artery disease. This is a narrowing of the coronary arteries which supply blood to the heart most commonly due to the build up of fatty plaques (atherosclerosis) and further compounded by a blood clot or vasospasm.
Other causes of sudden cardiac death includes :
- Heart valve disease like aortic valve stenosis and mitral valve prolapse.
- Inflammation of the heart muscle (myocarditis)
- Structural defects of the coronary arteries from birth (congenital)
- Heart muscle disease (cardiomyopathy)
- Family history
- Previous myocardial infarctions (heart attacks)
- Pulmonary hypertension
- Cardiac arrhythmias
- Enlarged heart (cardiomegaly)
- Severe blood loss leading to shock
- Oxygen deprivation
- Drug abuse particularly with the use of cocaine and methamphetamines
Various other systemic and metabolic diseases may also be responsible for sudden cardiac arrest. Due to the frequency of coronary artery disease causing sudden cardiac death, its associated risk factors also need to be considered. This includes :
- Hypertension – high blood pressure
- Hyperlipidemia – high level of lipids (cholesterol, triglycerides) in the blood.
- Diabetes mellitus
- Cigarette smoking
Read more on First Aid in Sudden Cardiac Death.
Article reviewed by Dr. Greg. Last updated on July 9, 2011