Sudden Cardiac Death (SCD) - Causes and Mechanisms

Dateline: 10/18/98

Have you read my previous article on SCD syndrome - Sudden Cardiac Death - Who, What, Where ?

Etiology - So what causes SCD ?

• Far and away, the most important cause of SCD is Coronary Artery Disease (CAD). While coronary atherosclerosis causes most cases of SCD in older adults, other conditions may be operative in the younger adult and children.
• Birth defects of the coronary arteries sometimes cause SCD. Most important among these are ALCAPA (a disease where the left main coronary artery arises from the pulmonary artery instead of the aorta) and an abnormal course of the coronary artery between the great vessels.
• Rarely, displacement of blood clots or infective vegetations on the aortic valve into the coronary arteries ("embolism") can cause SCD.
• Inflammatory conditions of the coronaries (called "arteritis") - most frequently Kawasaki's disease and Polyarteritis Nodosa - can result in SCD.
• And other rare coronary causes of SCD are coronary artery spasm and myocardial bridges that block the artery.

There are many other disorders that can cause SCD. Here is a list of some of them:

• Hypertrophic Obstructive Cardiomyopathy (HOCM)
• Myocardial Disease and Heart Failure
• Infections of the heart, like Sarcoidosis
• Severe aortic valve stenosis
• Congenital heart disease like Eisenmenger's syndrome
• Arrhythmias late after surgical repair of complex birth defects
• Intra-ventricular rhythm conduction disturbances like Long QT syndrome
• Electrophysiologic disturbances caused by disease or hormones and chemicals
• SCD during extreme physical activity

Patho-physiology - What happens in SCD ?

To understand such a complex condition, where interaction between multiple factors determines the sequence of events, a biological model has been proposed to co-ordinate all the available knowledge.

• The Substrate - Underlying structural disease of the heart is the basis for any form of SCD. Most patients with SCD have been detected, on autopsy, to have had some abnormality of heart structure. Rarely, though, the heart may be absolutely "normal".
• Functional Changes - Dynamic changes in the functioning (physiology) of the heart and electric impulse conduction get superimposed on the structural substrate, precipitating SCD. These may be in the form of
  
  1. transient decreases in the blood flow to the heart
  2. systemic factors like low blood pressure, low oxygen content or electrolyte imbalances
  3. altered levels of hormones and chemical transmitters
  4. toxic effects of drugs
  
• The Trigger - When these dynamic effects are added on to the anatomical substrate, they act as a trigger to initiate abnormal electrical activity in the heart - called Premature Ventricular Complexes (PVC). In most instances, PVCs are benign. Rarely, though, they may deteriorate into more serious rhythm disturbances like Ventricular Tachycardia (VT) or Ventricular Fibrillation (VF), both of which may be lethal unless treated promptly.

Recommended Reading :

Sudden Cardiac Death : Prevalence, Mechanisms, and Approaches to Diagnosis and Management
Masood Akhtar, et al / Hardcover / Published 1994
Buy it from Amazon.com. Visit the Heart Bookstore for more titles on SCD

To learn more about Sudden Cardiac Death syndrome, read the next article in this series.