Cardiology

Background 

Sudden cardiac death (SCD) is an unexpected death due to cardiac causes occurring within a short time period, generally within one hour of symptom onset in a person with known or unknown cardiac disease. It results from abrupt cessation of cardiac mechanical activity, leading to loss of effective circulation and hemodynamic collapse. SCD is a leading cause of mortality worldwide, often due to fatal arrhythmias such as ventricular fibrillation or pulseless ventricular tachycardia. 

II) Classification/Types

By Mechanism: 

• Arrhythmic SCD: Due to fatal arrhythmias such as ventricular fibrillation (VF), pulseless ventricular tachycardia (VT), or asystole. 

• Non-arrhythmic SCD: Resulting from acute mechanical failure (e.g., massive myocardial infarction causing pump failure). 

By Underlying Cardiac Condition: 

• Ischemic heart disease (most common cause) 

• Cardiomyopathies (hypertrophic, dilated, arrhythmogenic right ventricular) 

• Inherited channelopathies (Long QT syndrome, Brugada syndrome) 

• Structural abnormalities (valvular disease, congenital heart disease) 

• Myocarditis 

Pathophysiology 

Sudden cardiac death arises from a sudden loss of effective cardiac output due to electrical or mechanical failure of the heart. The predominant mechanism is ventricular arrhythmias—ventricular fibrillation or pulseless ventricular tachycardia—triggered by myocardial ischemia, fibrosis, or genetic electrical disorders causing abnormal impulse formation or conduction. 

Ischemic myocardial injury creates a substrate for reentrant arrhythmias through areas of scar and heterogeneous conduction. Channelopathies cause dysfunction of ion channels, leading to prolonged or shortened repolarization and increased arrhythmia risk. In some cases, mechanical causes like acute pump failure during massive infarction lead to SCD. 

Epidemiology 

• SCD accounts for approximately 15-20% of all deaths worldwide. 

• Incidence varies between 50 to 100 per 100,000 persons annually in the general population. 

• Most cases occur in adults over 35 years with underlying coronary artery disease. 

• Male sex is a risk factor; males have a higher incidence than females. 

• Survivors of SCD are a growing population due to advances in resuscitation and defibrillation. 

Etiology 

I) Causes

Cardiac Causes: 

• Coronary artery disease with acute myocardial infarction or ischemia (most common) 

• Cardiomyopathies (hypertrophic, dilated, arrhythmogenic right ventricular) 

• Inherited channelopathies (Long QT syndrome, Brugada syndrome, catecholaminergic polymorphic VT) 

• Structural heart diseases (valvular heart disease, congenital anomalies) 

• Myocarditis 

Non-Cardiac Causes: 

• Electrolyte disturbances (hyperkalemia, hypokalemia) 

• Drug toxicity (e.g., QT-prolonging agents, cocaine) 

• Severe metabolic derangements 

II) Risk Factors

• Prior myocardial infarction or known coronary artery disease 

• Reduced left ventricular ejection fraction (<35%) 

• Heart failure 

• Family history of SCD or inherited arrhythmias 

• Smoking, hypertension, diabetes mellitus 

• History of syncope or documented ventricular arrhythmias 

• Male sex and increasing age 

Clinical Presentation 

I) History (Symptoms)

• Often presents as sudden collapse, usually witnessed. 

• May have prodromal symptoms such as chest pain, palpitations, syncope, or dyspnea. 

• Family history may reveal sudden unexplained death or inherited arrhythmia syndromes. 

II) Physical Exam (Signs)

• Unresponsive, pulseless state at presentation. 

• Signs of underlying cardiac disease may be present (e.g., murmurs, signs of heart failure). 

• Post-resuscitation exam may show neurological deficits if prolonged downtime occurred. 

Differential Diagnosis (DDx) 

• Non-cardiac causes of sudden death (e.g., pulmonary embolism, intracranial hemorrhage) 

• Non-arrhythmic cardiac arrest causes (e.g., severe bradycardia, electromechanical dissociation) 

• Other causes of sudden collapse (seizure, stroke, hypoglycemia) 

Diagnostic Tests 

Initial Work-Up (During resuscitation) 

• ECG: To identify arrhythmia type and ischemic changes. 

• Point-of-care ultrasound (POCUS): To assess cardiac contractility, tamponade, pneumothorax. 

• Capnography: End-tidal CO₂ monitoring during CPR to assess quality of resuscitation. 

• Blood tests: Electrolytes, cardiac enzymes, toxicology screen, arterial blood gases. 

Advanced Testing (Post-resuscitation) 

• Echocardiogram: Evaluate structural heart disease and cardiac function. 

• Coronary angiography: If acute coronary syndrome is suspected. 

• MRI of the heart: To assess fibrosis and cardiomyopathy. 

• Genetic testing: For inherited arrhythmia syndromes. 

• Electrophysiology studies: To evaluate arrhythmogenic foci. 

Treatment 

I) Acute Management

• Immediate high-quality CPR and rapid defibrillation if shockable rhythm (VF/VT). 

• Advanced cardiac life support (ACLS) protocols: airway management, IV access, medications (epinephrine, amiodarone). 

• Identify and treat reversible causes (Hs & Ts: Hypoxia, Hypovolemia, Hypothermia, Toxins, Thrombosis, Tamponade, Tension pneumothorax). 

• Early coronary reperfusion therapy if ischemic cause suspected. 

II) Chronic/Post-Resuscitation Management

• Hemodynamic support and monitoring in ICU. 

• Targeted treatment of underlying cardiac disease (e.g., revascularization, heart failure management). 

• Implantable cardioverter-defibrillator (ICD) placement in patients at high risk for recurrent arrhythmias. 

• Beta blockers and antiarrhythmic medications as indicated. 

• Lifestyle modification and cardiac rehabilitation. 

Medications 

Device Therapy 

• Implantable cardioverter-defibrillator (ICD): Indicated for secondary prevention in survivors of SCD and primary prevention in patients with low EF. 

• Pacemakers: If bradyarrhythmias or conduction disease coexist. 

• Mechanical circulatory support: ECMO or ventricular assist devices in refractory cardiogenic shock. 

Patient Education, Screening, Vaccines 

• Educate patients on symptom recognition and early emergency response. 

• Screening family members in inherited arrhythmia syndromes. 

• Promote lifestyle changes: smoking cessation, diet, exercise. 

• Ensure vaccination against influenza and pneumococcus to prevent infections that may exacerbate cardiac disease. 

Consults/Referrals 

• Cardiology: For comprehensive cardiac evaluation and management. 

• Electrophysiology: For arrhythmia evaluation and ICD placement. 

• Genetics: If inherited arrhythmia or cardiomyopathy suspected. 

• Critical Care: For post-arrest management. 

• Psychology/Psychiatry: For post-traumatic stress and counseling. 

Follow-Up 

Short-Term 

• Monitoring for recurrent arrhythmias. 

• Neurological assessment and rehabilitation. 

• Optimization of medical therapy. 

Long-Term 

• Regular cardiology follow-up for device checks and medication adjustments. 

• Cardiac rehabilitation. 

• Family screening when indicated. 

• Psychological support. 

Prognosis 

• Overall prognosis depends on cause, timeliness of resuscitation, and neurological outcome. 

• Survival to hospital discharge after out-of-hospital SCD is approximately 10-20%. 

• Patients with reversible causes and preserved neurologic function have better outcomes. 

• ICD therapy significantly reduces mortality in high-risk populations.