7.3 Cardiac Arrhythmias, Conduction Blocks & EKG Interpretation
Key Takeaways
- In non-valvular atrial fibrillation, oral anticoagulation decision-making is guided by the CHA2DS2-VASc score; direct oral anticoagulants (DOACs: apixaban, rivaroxaban, dabigatran) are preferred over warfarin for patients with a score >=2 in men or >=3 in women.
- Warfarin (target INR 2.0-3.0 or 2.5-3.5) remains strictly mandatory for stroke prevention in patients with atrial fibrillation who have moderate-to-severe mitral stenosis or mechanical heart valves.
- Hemodynamically unstable atrial fibrillation or supraventricular tachycardia with hypotension, altered mental status, acute pulmonary edema, or ischemic chest pain mandates immediate synchronized direct-current cardioversion.
- Administration of AV nodal blocking agents (beta-blockers, CCBs, digoxin, adenosine) is strictly contraindicated in Wolff-Parkinson-White (WPW) syndrome presenting with atrial fibrillation, as blocking the AV node forces rapid conduction down the accessory pathway, precipitating ventricular fibrillation; treatment requires IV procainamide or ibutilide.
- Second-degree Mobitz Type II AV block and Third-degree (complete) AV block carry high risks of sudden cardiac arrest and require urgent transcutaneous/transvenous pacing as a bridge to permanent pacemaker implantation.
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
| CHA2DS2-VASc Risk Criterion | Point Value | Clinical Definitions & Cutoffs |
|---|---|---|
| C — Congestive Heart Failure | 1 | Signs/symptoms of HF or LVEF $\le 40%$. |
| H — Hypertension | 1 | Resting BP $>140/90\text{ mmHg}$ or on antihypertensives. |
| A2 — Age $\ge 75$ Years | 2 | Age 75 or older. |
| D — Diabetes Mellitus | 1 | Fasting glucose $>126\text{ mg/dL}$ or on anti-diabetics. |
| S2 — Stroke / TIA / Thromboembolism | 2 | History of TIA, ischemic stroke, or systemic embolus. |
| V — Vascular Disease | 1 | Prior MI, peripheral arterial disease, or complex aortic plaque. |
| A — Age 65–74 Years | 1 | Age 65 to 74. |
| Sc — Sex Category (Female) | 1 | Female biological sex. |
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).
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).
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.
Atrioventricular (AV) Conduction Blocks
| AV Block Type | EKG Manifestations | Anatomic Site | Management & Prognosis |
|---|---|---|---|
| 1st-Degree AV Block | Prolonged PR interval ($>200\text{ ms}$ or $>5\text{ small boxes}$), constant PR, every P wave followed by QRS. | AV Node | Benign. Observation. No treatment required. |
| 2nd-Degree Mobitz I (Wenckebach) | Progressive PR lengthening until a P wave fails to conduct ("long, longer, longest, drop"). | AV Node | Benign. Usually asymptomatic. Observe; atropine if symptomatic. |
| 2nd-Degree Mobitz II | Constant PR interval with unexpected non-conducted P waves ("random dropped QRS"). | Sub-nodal (His-Purkinje) | High risk of progression to 3rd-degree block. Permanent Pacemaker required. |
| 3rd-Degree (Complete) AV Block | Complete dissociation between P waves and QRS complexes. Regular P-P and R-R intervals. Ventricular rate 20–40 bpm. | Sub-nodal / His-Purkinje | High sudden death risk. Urgent Transcutaneous Pacing bridge to Permanent Pacemaker. |
A 34-year-old man with a history of Wolff-Parkinson-White syndrome presents to the emergency department with severe palpitations, dizziness, and lightheadedness. His blood pressure is 118/74 mmHg and pulse is 180/min. EKG demonstrates an irregularly irregular wide-complex tachycardia with marked variation in QRS morphology and axis. Which of the following medications is strictly contraindicated in this patient?
A 76-year-old woman is brought to the emergency department after experiencing a syncopal episode while gardening. She reports feeling increasingly fatigued over the past two weeks. Her blood pressure is 92/56 mmHg and heart rate is 32/min. EKG shows regular P waves at a rate of 75/min and regular QRS complexes with a wide escape morphology at a rate of 32/min, with complete absence of correlation between P waves and QRS complexes. Which of the following is the definitive long-term treatment for this patient?
A 72-year-old woman with a history of hypertension, type 2 diabetes mellitus, and a prior ischemic stroke 3 years ago is diagnosed with newly discovered non-valvular atrial fibrillation during an outpatient visit. She has no history of heart failure or vascular disease. Echocardiogram shows normal mitral valve structure and an ejection fraction of 60%. Her CHA2DS2-VASc score is calculated as 5. Which of the following is the most appropriate long-term anticoagulation strategy?