5.3 Arrhythmias & Conduction System Disorders

Arrhythmias & Conduction System Disorders

Step 3 Clinical Focus: The management of cardiac arrhythmias requires a structured approach to rate versus rhythm control in atrial fibrillation, the acute and chronic management of ventricular tachyarrhythmias, systematic identification and treatment of heart blocks, and understanding the specific clinical indications for pacemakers and implantable cardioverter-defibrillators (ICDs).

Atrial Fibrillation and Flutter

Atrial fibrillation (AF) is characterized by disorganized atrial electrical activity leading to an irregular, rapid ventricular rate. Atrial flutter is characterized by a reentrant loop around the tricuspid annulus, presenting with a classic "sawtooth" pattern in inferior leads. Both present with palpitations, fatigue, dyspnea, or thromboembolic events (stroke). Management of AF involves rate control, rhythm control, and anticoagulation.

• Rate vs. Rhythm Control: The AFFIRM trial demonstrated no survival advantage of rhythm control. Rate control is preferred for most patients, utilizing beta-blockers (e.g., metoprolol) or non-dihydropyridine calcium channel blockers (diltiazem) to achieve a resting heart rate < 110 bpm. CCBs must be avoided in patients with HFrEF due to negative inotropic effects. Digoxin is a second-line agent, useful in patients with heart failure and low blood pressure. Combining beta-blockers and calcium channel blockers is avoided due to the risk of bradycardia and profound heart block. Rhythm control (antiarrhythmics like amiodarone, sotalol, flecainide; catheter ablation) is reserved for patients who remain symptomatic despite rate control. Class IC antiarrhythmics (flecainide, propafenone) can be used as a "pill-in-the-pocket" strategy for self-cardioversion, but they are contraindicated in patients with structural or ischemic heart disease due to proarrhythmic risk.
• Anticoagulation: Driven by the CHA2DS2-VASc score. Anticoagulation is indicated for a score of ≥ 2 in men or ≥ 3 in women. Direct Oral Anticoagulants (DOACs, such as apixaban, rivaroxaban, or dabigatran) are preferred over warfarin due to a lower risk of intracranial hemorrhage. Warfarin remains mandatory for mechanical prosthetic heart valves or moderate-to-severe mitral stenosis.
• Cardioversion Protocol: If AF has lasted ≥ 48 hours (or is of unknown duration), the patient must receive therapeutic anticoagulation for at least 3 weeks before cardioversion, and continue for at least 4 weeks after. Alternatively, a transesophageal echocardiogram (TEE) can exclude a left atrial thrombus; if absent, cardioversion can proceed, followed by 4 weeks of anticoagulation. If hemodynamically unstable (e.g., hypotension, ischemia), immediate synchronized cardioversion (typically 100-200 Joules) is indicated regardless of duration.

Ventricular Tachyarrhythmias

Ventricular tachycardia (VT) is defined by three or more consecutive premature ventricular contractions at a rate > 100 bpm with a wide QRS complex (≥ 120 ms). Management depends on hemodynamic stability. Hemodynamically unstable VT with a pulse is treated with immediate synchronized cardioversion. Pulseless VT or ventricular fibrillation (VF) requires immediate unsynchronized defibrillation (typically 200 Joules biphasic) and high-quality CPR. Hemodynamically stable VT is treated pharmacologically. Procainamide is preferred in patients with Wolff-Parkinson-White syndrome and pre-excited AF, whereas amiodarone or lidocaine is preferred in patients with structural heart disease or LVEF ≤ 40%. Torsades de Pointes (polymorphic VT with prolonged QT) is treated with intravenous magnesium sulfate, correction of electrolyte disturbances (hypokalemia, hypomagnesemia), and withdrawal of offending medications (sotalol, fluoroquinolones, haloperidol). If refractory, overdrive pacing or isoproterenol is used to shorten the QT interval.

Heart Blocks

Conduction delays are classified by severity and anatomical site. First-degree AV block is characterized by a constant PR interval > 200 ms without dropped beats; it is benign and requires no treatment. Second-degree Mobitz Type I (Wenckebach) features progressive PR interval prolongation until a QRS complex is dropped; it is typically benign, and treatment (atropine) is indicated only if symptomatic. Second-degree Mobitz Type II features a constant PR interval with sudden dropped QRS complexes. It occurs below the AV node and carries a high risk of progression to complete heart block. Of clinical note, administering atropine to a patient with Mobitz II can worsen the block because increasing the sinus rate increases the burden on the diseased infra-nodal conduction system, leading to more dropped beats. All patients with Mobitz Type II require a permanent pacemaker, even if asymptomatic. Third-degree (complete) AV block is characterized by complete AV dissociation, with independent P waves and QRS complexes. It requires temporary pacing (transcutaneous followed by transvenous) and permanent pacemaker insertion.

Pacemakers & Implantable Cardioverter-Defibrillators (ICDs)

Understanding device indications is critical:

• Permanent Pacemakers: Indicated for symptomatic sinus node dysfunction (sick sinus syndrome), acquired second-degree Mobitz Type II block, third-degree AV block, and symptomatic bradycardia without reversible causes.
• Implantable Cardioverter-Defibrillators (ICDs): Used for prevention of sudden cardiac death. Primary prevention is indicated in patients with HFrEF (LVEF ≤ 35%) on optimal GDMT for at least 3 months (non-ischemic) or at least 40 days post-MI, with NYHA Class II or III symptoms. Secondary prevention is indicated in survivors of cardiac arrest due to VF or unstable VT without reversible causes. Prior to surgery utilizing electrocautery, a magnet is placed over an active pacemaker/ICD to convert it to asynchronous mode, preventing inappropriate sensing of electrocautery as ventricular fibrillation.
• Cardiac Resynchronization Therapy (CRT): Indicated for patients with LVEF ≤ 35%, sinus rhythm, NYHA Class II-IV symptoms despite optimal GDMT, and LBBB with QRS duration ≥ 150 ms (biventricular pacing improves ventricular synchronization).