7.3 Cardiac Arrhythmias, Conduction Blocks & EKG Interpretation

Cardiac Arrhythmias, Conduction Blocks & EKG Interpretation

Cardiac arrhythmias encompass a wide spectrum of disorders ranging from benign conduction delays to catastrophic ventricular arrest. On USMLE Step 2 CK, candidates must rapidly analyze EKGs, apply validated clinical scoring tools (such as CHA2DS2-VASc), select appropriate rate versus rhythm control strategies, and execute emergency ACLS resuscitation protocols.

Atrial Fibrillation & Atrial Flutter

Atrial Fibrillation (AF) EKG Features & Management

Atrial fibrillation is characterized by irregularly irregular ventricular response, absence of discrete P waves, and irregular fibrillatory baseline waves ($350\text{--}600\text{ bpm}$).

1. Hemodynamic Stability & Acute Resuscitation

• Hemodynamically Unstable (hypotension, acute pulmonary edema, ongoing myocardial ischemia): Immediate Synchronized Direct-Current Cardioversion (DCCV) starting at 120–200 J biphasic (with IV heparin bolus).
• Hemodynamically Stable: Focus on Rate Control initially using IV or oral beta-blockers (metoprolol, esmolol) or non-dihydropyridine calcium channel blockers (diltiazem, verapamil). Target resting heart rate $<110\text{ bpm}$.

2. Stroke Prevention: CHA2DS2-VASc Risk Stratification

Anticoagulation Decision Rules:

• Score 0 in men / 1 in women: No anticoagulation required.
• Score 1 in men / 2 in women: Consider oral anticoagulation based on individual clinical risk.
• Score $\ge 2$ in men / $\ge 3$ in women: Oral Anticoagulation Mandated.
• Drug Selection: Direct Oral Anticoagulants (DOACs: apixaban, rivaroxaban, dabigatran, edoxaban) are preferred over Warfarin due to lower intracranial hemorrhage rates and no monitoring requirements.
• Valvular AF Exception: Warfarin (target INR 2.0–3.0, or 2.5–3.5 for mechanical mitral valves) is strictly mandatory for patients with AF and moderate-to-severe mitral stenosis or mechanical heart valves. DOACs are contraindicated in these settings.

3. Cardioversion Timing & Anticoagulation Rules

If AF duration is $>48\text{ hours}$ (or unknown), cardioversion carries a high risk of thromboembolism. Recommendations:

• Perform Transesophageal Echocardiogram (TEE) to rule out left atrial/appendage thrombus prior to cardioversion, OR
• Anticoagulate for at least 3 consecutive weeks before cardioversion.
• All patients require therapeutic anticoagulation for at least 4 weeks post-cardioversion regardless of CHA2DS2-VASc score.

Atrial Flutter

EKG demonstrates regular "sawtooth" flutter waves (F waves) best visualized in inferior leads II, III, and aVF, typically with an atrial rate of $\sim 300\text{ bpm}$ and 2:1 AV block (ventricular rate $\sim 150\text{ bpm}$). Definitive treatment is radiofrequency catheter ablation of the cavotricuspid isthmus (CTI).

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Supraventricular Tachycardias & Pre-Excitation Syndromes

Regular Narrow-Complex Tachycardias (QRS $<120\text{ ms}$)

Encompasses AV Nodal Reentrant Tachycardia (AVNRT) and AV Reentrant Tachycardia (AVRT), presenting with sudden-onset palpitations and HR $150\text{--}250\text{ bpm}$.

• Stable Management: First line is Vagal Maneuvers (modified Valsalva maneuver or carotid sinus massage). Second line is Adenosine 6 mg IV rapid push (followed by 12 mg IV if non-responsive). Adenosine causes transient SA/AV nodal block.
• Unstable Management: Immediate Synchronized Cardioversion.

Wolff-Parkinson-White (WPW) Syndrome

Conduction via an accessory pathway (Bundle of Kent) bypassing the AV node.

• Classic Resting EKG Triad: Short PR interval ($<120\text{ ms}$), Delta wave (slurred upstroke of QRS), and widened QRS ($>120\text{ ms}$).
• WPW with Atrial Fibrillation: Results in an irregular, extremely rapid, wide-complex tachycardia.
• CRITICAL CONTRAINDICATION: NEVER administer AV nodal blocking agents (beta-blockers, diltiazem, verapamil, digoxin, adenosine) in WPW with AF! Blocking the AV node promotes 1:1 conduction down the accessory pathway, degenerating into Ventricular Fibrillation.
• Treatment of Choice: IV Procainamide or Ibutilide (stable patients), or Synchronized Cardioversion (unstable patients).

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Ventricular Arrhythmias & Channelopathies

Monomorphic Ventricular Tachycardia (VT)

Wide-complex tachycardia (QRS $\ge 120\text{ ms}$) at rates $>100\text{ bpm}$. EKG signs supporting VT over SVT with aberrancy include AV dissociation, capture beats, fusion beats, and concordant QRS pattern across precordial leads.

• Stable with Pulse: IV Amiodarone (150 mg IV infusion) or procainamide.
• Unstable with Pulse: Synchronized Cardioversion (100 J).
• Pulseless VT / Ventricular Fibrillation (VF): Unsynchronized Defibrillation (200 J biphasic) + High-Quality CPR + Epinephrine 1 mg IV q3–5min + Amiodarone 300 mg bolus.

Polymorphic VT / Torsades de Pointes

Associated with prolonged QTc interval ($>480\text{--}500\text{ ms}$). Causes include electrolyte abnormalities (hypokalemia, hypomagnesemia, hypocalcemia) and QTc-prolonging medications (macrolides, fluoroquinolones, antipsychotics, methadone, class IA/III antiarrhythmics).

• Treatment: Intravenous Magnesium Sulfate 1–2 g IV bolus (even if serum magnesium levels are normal). Discontinue offending agents and correct hypokalemia.

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Atrioventricular (AV) Conduction Blocks