CHD is the major structural abnormality found in most SCA victims. In 80% of patients who have had an SCA, CAD is present. SCA is often the first manifestation of CHD.¹⁰,¹³ Pathology studies of SCD patients have shown that coronary atherosclerosis is the major predisposing cause. Plaque rupture and plaque erosion are the underlying pathologies in the majority of cases of SCD. Evidence shows a difference in the mechanism of MI and death between men and woman. Men tend to have coronary plaque rupture, while women tend to have plaque erosion.¹⁴ Data from the Nurses’ Health Study of 121,701 women have shown that 94% of women who had SCD had one risk factor for heart disease.⁹ The evolution and clinical manifestation of CHD leading to SCA has been identified as four separate stages (Fig. 27-1). The first stage is atherogenesis, which is the beginning of plaque formation and occurs over a long period of time. This stage should be thought of as the stage that determines risks for CHD. The
second stage is the transitional stage where changes in plaque anatomy and pathophysiology are taking place. During this stage, the disease has moved from quiet to active. The third level of the process is the acute coronary syndrome phase when the acute ischemic event may be triggered by plaque disruption and the onset of the thrombotic process. The time to a fatal arrhythmia is close, leaving less time for preventative actions. The final phase is the arrhythmogenesis, when an interaction is occurring between the active ischemic process and the onset of cardiac arrhythmias.¹⁵

Changes in coronary artery blood flow from other causes such as coronary vasospasms or nonatherosclerotic coronary artery abnormalities can also provoke ischemia and create myocardial electrical disturbances and the development of VF. Structural coronary artery abnormalities without atherosclerosis are rare and include congenital lesions, coronary artery emboli from aortic valve endocarditis, or from thrombotic material released from prosthetic aortic or mitral valve.¹⁶

The second largest group of patients who experience SCD includes patients with cardiomyopathy. Severely depressed left ventricular function is an independent predictor of SCD in patients with ischemic and nonischemic cardiomyopathy. An ejection fraction equal to or less than 0.35 is considered the most powerful predictor of SCD. Improved treatment options for patients with heart failure provide them with better long-term survival. However, there is an increasing proportion of patients with heart failure who die suddenly.¹⁶ Three primary prevention studies have shown reduction in total mortality for ischemic and nonischemic cardiomyopathy patients. The Multicenter Automatic Defibrillator Implantation Trial (MADIT II) has shown that the implantable cardioverter defibrillator (ICD) will reduce total mortality in patients with ischemic cardiomyopathy with ejection fractions less that 0.30. The Sudden Cardiac Death in Heart Failure Trial (SCD-HeFT) enrolled patients with either ischemic or nonischemic cardiomyopathy with ejection fractions less than 0.35, and NYHA classes II and III heart failure. The results confirmed reduced mortality in the ischemic patients, but also in the nonischemic cardiomyopathy patients. The Defibrillators in Nonischemic Cardiomyopathy Treatment Evaluation (DEFINITE) trial also showed reduction in death from arrhythmias with ICD therapy.¹⁷, ¹⁸, ¹⁹

LVH has been established as an independent risk factor for SCD. ECG changes consistent with LVH and echocardiography evidence have both been associated with sudden and unexpected cardiac death.⁸,²⁰ The underlying causes for myocardial hypertrophy include hypertensive or valvular heart disease, obstructive and nonobstructive hypertrophic cardiomyopathy (HCM), and right ventricular hypertrophy secondary to pulmonary hypertension or congenital heart disease. All of these conditions are associated with increased risk of SCD, but it has been suggested that people with severely hypertrophic ventricles are especially susceptible to SCD.¹⁶

HCM is a familial cardiac disease that occurs in 1 out of 500 people. Often the disease goes undiagnosed and is the most common cause of sudden death in people under 30 years of age.²¹,²² Sudden death often occurs with vigorous exercise is this group of patients. Various risk factors for SCD with HCM include family history of sudden death, documented nonsustained VT, recurrent and unexplained syncope, and extreme thickness of 30 mm or more of the left ventricle. Polymorphic VT and VF are thought to be the initial rhythm for patients with HCM who experience SCD.⁷,¹⁶ Genetic studies of HCM have confirmed autosomal dominate inheritance patterns. Typically HCM is caused by mutations in any one of 10 genes that encode proteins of the cardiac sarcomere. DNA testing can identify patients with HCM at an early onset, helping to modify medical therapy and recommendations for placement of an implantable defibrillator, or withdrawing from intense physical activities and competitive sports.²²

The risk of sudden death in patients with valve disease is low but present. After prosthetic or heterograft aortic valve replacements, patients are at risk for SCD caused by arrhythmias, prosthetic valve dysfunction, or existing CHD. The risk of SCD after surgery peaks at 3 weeks and plateaus after 8 weeks. Sudden death can also occur with exertion in young adults with congenital aortic stenosis. The mechanism is uncertain but thought to be from sudden changes in ventricular filling or aortic obstruction with secondary arrhythmias.⁷ Mitral valve prolapse is associated with a high incidence of symptomatic atrial and ventricular arrhythmias; however, whether it causes SCD is unresolved.¹⁶

Rare reports of sudden death have been reported from coronary embolism due to valvular vegetations, which trigger a fatal ischemic arrhythmia. Endocarditis of the aortic or mitral valve may cause deterioration of the valvular apparatus, and abscesses of the valvular rings or septum leading to sudden death.¹⁶

Patients with the classic long QT intervals corrected for heart rate (QTc interval) are at increased risk for torsades.²⁰,²³,²⁴ Congenital long QT is hereditary; mutations in eight genes have been identified. The most common mutations are LQT1 (42%), LQT2 (45%), and LQT3 (8%). Most mutations affect the cardiac potassium-channel genes KCNQ1 and KCNH2 and cause the most frequent forms of long QT. An impaired sodium channel gene, SCN5A, is the cause for LQT3. However, the mutation LQT4 has been identified as a mutation from a membrane adaptor protein, and not an ion channel.¹²,²³ QT interval duration was identified as the strongest predictor of SCD, even before genetic mutation identification. A QTc greater than 500 ms in a patient identified with an affected gene carries the highest risk of syncope or SCD by the age of 40.²⁰ Two patterns of long QT have been reported: the Romano Ward syndrome, which is an autosomal dominant syndrome, and the Jervell and Lange-Nielsen syndrome. The latter is autosomal recessive, more severe, and often associated with congenital deafness.²⁰,²⁵

In individuals with long QT, ventricular arrhythmias often occur in the setting of stress and activity, but also can occur during rest and sleep. Exercise, particularly swimming, is often a trigger for arrhythmias in the LQT1 patients. LQT2 patients have arrhythmias that are triggered during both rest and emotion, often associated with acoustic stimuli. The LQT3 patients are more prone to having arrhythmias at rest and during sleep.²⁰,²⁶ Avoidance of competitive sports is recommended for LQT1 and LQT2 patients, but not for LQT3 patients.²²

Brugada syndrome is an inherited disease that is associated with a high incidence of sudden death. A mutation of cardiac sodiumchannel gene SCN5A, has been identified in about 20% of persons with the syndrome, and occurs more often in men. The Brugada syndrome is confirmed with ST-segment elevation in the precordial leads, right bundle-branch block (RBBB) conduction pattern, and history of SCD. The ECG changes may be present all the time or elicited when antiarrhythmic drugs are given that block sodium channels (Chapter 15).²⁰,²⁷,²⁸ Fever has been reported to unmask ECG changes of Brugada syndrome and elicit an electrical VT storm.²⁹ Globally, the syndrome is more prevalent in areas of southeast Asia.²³

Wolff-Parkinson-White (WPW) is associated with an accessory pathway that allows for conduction between the atria and ventricles. Normally, WPW is associated with nonlethal arrhythmias. However, if atrial fibrillation develops and conduction is rapid over the accessory pathway, the ventricular rate can become so fast that the rhythm degenerates into VF. SCD in the setting of WPW is quite low, and has been reported at 0.39% per year.²³,³⁰

CVPT is a catecholamine-dependent arrhythmia that occurs in the absence of a long QT interval. The arrhythmia often occurs during physical activity or emotional distress; therefore, β-adrenoceptorblocking agents can be helpful in treating this condition. The first episode often occurs during childhood. The arrhythmia is characterized by bidirectional and polymorphic VT. The condition is rare, and is related to a genetic disorder. CPVT is caused by mutations involving the cardiac ryanodine receptor, and calsequestrin, a calcium-buffering protein.²⁰,²³,³¹

Defibrillation and cardioversion are a delivery of electrical energy that totally depolarizes and stuns the myocardium, which allows the sinus node to resume its function as the pacemaker for the heart. Defibrillation is, by definition, the therapy for VF. Cardioversion, which is a synchronized shock, is the electrical therapy for all other tachyarrhythmias. Transcutaneous and transvenous pacing are additional types of electrical therapy used in ACLS
for patients with hemodynamically compromised bradycardias (Chapter 28).

VT and VF are the most common arrhythmias during cardiac arrest, although the incidence of VF seems to be declining as reported by two studies from European cities, and from analysis of cardiac arrest events in Seattle, Washington from 1980 to 2000.³³,³⁴ Defibrillation is the definitive therapy for cardiac arrest caused by VF. Rapid, early defibrillation is a key step and the most important intervention likely to save lives. Survival rates are best when immediate bystander CPR is provided and defibrillation occurs within 3 to 5 minutes.³⁵,³⁶ A major obstacle to rapid, early defibrillation is that most cardiac arrests occur outside of the hospital, indicating a need for public health initiatives to improve early recognition of heart attack signs and symptoms.¹ The widespread use of automated external defibrillators (AEDs) assists in making early defibrillation a reality by expanding the number of rescuers available to treat SCD. The AHA integrates the use of AEDs with basic life support skills because VF is the most common rhythm found in adults with witnessed, nontraumatic SCA.³²,³³