1. Fibrillation (both AF and VF) - classification, epidemiology, ECG features, clinical features, management
2. Defibrillator (ICD and external defibrillator) - components, indications, types, operation

MBBS Final Year - Clinical Cardiology: 5-Mark Theory Answers

1. FIBRILLATION

ATRIAL FIBRILLATION (AF)

• Absence of distinct P waves; replaced by rapid, irregular fibrillatory (f) waves at 350-600/min
• Irregularly irregular RR intervals (ventricular response is irregular)
• Narrow QRS complexes (unless aberrant conduction)

• Hypertension (most common)
• Valvular heart disease (especially mitral stenosis)
• Coronary artery disease
• Thyrotoxicosis
• Alcohol (holiday heart syndrome)
• Heart failure, cardiomyopathy
• Idiopathic (lone AF - in patients <60 years without structural heart disease)

• Stroke: AF confers a ~5-fold increased risk of stroke due to left atrial thrombus formation (especially in the left atrial appendage). The risk is especially high with concomitant mitral stenosis.
• Heart failure: Loss of atrial contribution to ventricular filling and rapid heart rates reduce cardiac output
• Increased all-cause mortality: ~2-fold increase

1. Rate Control (goal: resting HR <80 bpm or lenient <110 bpm):
   • Beta blockers (metoprolol, bisoprolol)
   • Non-dihydropyridine calcium channel blockers (diltiazem, verapamil)
   • Digoxin (especially in heart failure with reduced EF)
2. Rhythm Control (restoration and maintenance of sinus rhythm):
   • Electrical cardioversion (synchronized DC cardioversion: 120-200 J biphasic)
   • Pharmacological cardioversion: flecainide, propafenone (no structural disease); amiodarone, sotalol (structural disease)
   • Catheter ablation (pulmonary vein isolation)
3. Anticoagulation (stroke prevention):
   • Assess thromboembolic risk using CHA₂DS₂-VASc score
   • Score ≥2 (males) or ≥3 (females): anticoagulate with DOAC (apixaban, rivaroxaban, dabigatran) or warfarin
   • Warfarin (not DOACs) is mandatory in AF with mitral stenosis (rheumatic)

VENTRICULAR FIBRILLATION (VF)

• Irregular undulations of varying contour and amplitude
• No distinct QRS complexes, no P waves, no T waves
• Chaotic baseline

• Acute myocardial infarction / ischemia (most common)
• Cardiomyopathy
• Long QT syndrome (Torsades de Pointes degenerating to VF)
• Wolff-Parkinson-White syndrome with very rapid ventricular rates
• Commotio cordis (precordial impact during T wave)
• Electrolyte imbalances (severe hypokalemia, hypomagnesemia)
• Electrical shock

1. Immediate unsynchronized DC defibrillation (200-360 J monophasic; 150-200 J biphasic) - this is the ONLY definitive treatment
2. CPR (cardiopulmonary resuscitation) continued until defibrillation is possible; resumed immediately after each shock
3. If first shock is ineffective: deliver shocks at maximum defibrillator output
4. IV amiodarone 300 mg bolus if initial shocks fail (while CPR continues)
5. IV epinephrine 1 mg every 3-5 minutes
6. Identify and treat the reversible cause (4 H's and 4 T's)

2. DEFIBRILLATOR

TYPES OF DEFIBRILLATORS

• Delivers electrical current across the chest wall through paddles or self-adhesive electrode pads
• Two waveform types:
  • Monophasic: current flows in one direction; higher energy (360 J); older devices
  • Biphasic: current flows in two directions; lower energy (150-200 J); more effective, less myocardial damage; current standard

• Portable, user-friendly device with voice prompts
• Automatically analyzes cardiac rhythm and advises shock delivery
• Designed for use by lay public in out-of-hospital cardiac arrest
• Used for VF and pulseless VT only

• Implanted subcutaneously (generator in subpectoral/abdominal region) with transvenous sensing and shocking electrodes
• Components: pulse generator, lead system (sensing + shocking electrodes), rhythm analysis circuitry, power supply
• Tiered response to ventricular arrhythmias:
  1. Antitachycardia pacing (ATP) - for tolerated VT
  2. Low-energy cardioversion
  3. High-energy defibrillation
• Newer ICDs discriminate supraventricular from ventricular tachycardia, reducing inappropriate shocks
• Battery life: approximately 6-9 years
• Subcutaneous ICD (S-ICD): no pacing capability; lead tunneled to left sternal region; generator in left axillary region
• ICD reduces mortality from sudden cardiac death from ~30-45%/year to <2%/year

• Secondary prevention: survivors of cardiac arrest due to VF/VT
• Primary prevention: EF ≤35% despite optimal medical therapy (NYHA Class II-III)
• Hypertrophic cardiomyopathy with high risk features
• Inherited arrhythmia syndromes (Long QT, Brugada, ARVC)

SYNCHRONIZED CARDIOVERSION vs. DEFIBRILLATION

1. Sedate the patient (ensure IV access, oxygen, monitoring)
2. Apply electrode pads in correct position (right subclavicular + left lateral)
3. Select "Sync" mode on defibrillator
4. Select appropriate energy level
5. Announce "Stand clear" and charge
6. Deliver shock; reactivate sync mode after each shock (devices revert to async mode automatically)

• Myocardial injury (especially high-energy repeated shocks)
• Burns at electrode sites
• Arrhythmias induced by shock (bradycardia, asystole)
• Failure to restore rhythm if underlying cause not treated

Sources: Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine (11th Ed.); Tintinalli's Emergency Medicine; Morgan & Mikhail's Clinical Anesthesiology