Sudden Cardiac Death

Excerpt

Cardiac arrest is defined as a sudden cessation of cardiac activity resulting in hemodynamic collapse. Sudden cardiac death (SCD) is defined as death presumed to be of a cardiac cause that occurs within 1 hour of the onset of cardiac symptoms or 24 hours of last being seen healthy and alive. Autopsies may reveal a cardiac etiology, though not all SCD cases have an identifiable cause. SCD may be the first presentation of cardiovascular diseases and accounts for half of cardiovascular deaths. SCD has a strong association with age. Men are at a higher risk of SCD as compared to age-matched women. SCD has a low incidence in infancy, but the condition's annual incidence reaches as high as 200 per 100,000 person-years in the 8th decade of life.

Coronary artery disease (CAD) is responsible for more than 75% of SCD cases in the developed world. The incidence of CAD has increased over the last few decades. However, a significant decline in cardiovascular mortality is also evident. Early CAD treatment is the most effective SCD preventive method. Studies show that cardiac arrest and SCD may be the first presentation of CAD in genetically predisposed individuals. Myocardial infarction or ischemia is the typical diagnosis in these patients. Early CAD identification and management of atherosclerotic cardiovascular disease (ASCVD) risk factors are the best strategies for minimizing the risk of cardiac arrest and SCD. Early cardiopulmonary resuscitation (CPR) is paramount in preventing SCD in patients with witnessed cardiac arrest.

In younger individuals with inherited arrhythmias, identifying and appropriately treating the underlying condition can effectively prevent SCD. Implantable cardioverter-defibrillator (ICD) use is the only way to prevent SCD in most inherited cardiac arrhythmias.

Heart Anatomy

The human heart is a muscular organ in the thoracic cavity, slightly left of the center. The heart circulates blood, supplying oxygen and nutrients to tissues and organs. Structurally, the heart consists of 4 chambers: 2 atria and 2 ventricles. The right atrium receives deoxygenated blood returning from the body via the superior and inferior vena cavae, while the left atrium receives oxygenated blood from the lungs through the pulmonary veins. Blood flows from the atria into the ventricles through the atrioventricular valves—the tricuspid valve on the right side and mitral valve on the left. Ventricular contraction pumps blood out of the heart through the semilunar valves—the pulmonary valve on the right ventricle and aortic valve on the left ventricle—into the pulmonary artery and aorta, respectively.

The coronary arteries supply oxygen-rich blood to the heart muscle (myocardium). The left coronary artery arises from the left side of the aorta and branches into 2 main arteries. The left anterior descending artery supplies the anterior surfaces of the left ventricle and interventricular septum. The left circumflex artery supplies the left atrium and posterolateral side of the left ventricle. The right coronary artery, originating from the aorta's right side, supplies the right atrium and ventricle and part of the left ventricle's posterior wall. The coronary arteries ensure the heart receives an adequate blood supply to meet its high metabolic demands.

The cardiac conduction system comprises specialized cells that generate and transmit electrical impulses regulating the heart's rhythm and coordinating its contractions. The sinoatrial node, located in the right atrium near the entrance of the superior vena cava, serves as the heart's natural pacemaker. The electrical impulses then travel through the atria, causing them to contract and forcing blood into the ventricles. The impulses reach the atrioventricular (AV) node, situated at the junction of the atria and ventricles. The impulses are momentarily delayed in the AV node, allowing the ventricles to fill completely before contracting. From the AV node, the impulses travel through specialized conduction pathways—the bundle of His and Purkinje fibers—stimulating the ventricles to contract and pump blood to the lungs and the rest of the body.