Atrial Fibrillation
Irregularly irregular narrow-complex rhythm with absent P waves; rate control (Metoprolol, Diltiazem) + anticoagulation per CHA2DS2-VASc (≥2 men, ≥3 women).
Atrial Flutter
Regular narrow-complex rhythm with 'sawtooth' flutter F-waves at 300 bpm (ventricular rate 150 bpm in 2:1 block); curative cavo-tricuspid isthmus radiofrequency ablation.
Ventricular Tachycardia
Regular wide-complex (QRS >120 ms) tachycardia with AV dissociation and fusion/capture beats; unstable → synchronized cardioversion; pulseless → defibrillation.
Third-Degree AV Block
Complete AV dissociation (P waves march through QRS complexes at independent rates; atrial rate > ventricular escape rate); requires permanent pacemaker.
WPW & Accessory Pathway
Short PR (<120 ms) + slurred upstroke Delta wave + wide QRS via Bundle of Kent; if AFib + WPW, give IV Procainamide (NEVER ABCD nodal blockers!).
Torsades de Pointes
Polymorphic VT with cyclical twisting of QRS axis around baseline in the setting of QTc >500 ms; first-line treatment is immediate IV Magnesium Sulfate.
Brugada Syndrome
Autosomal dominant SCN5A cardiac Na+ channel mutation; pseudo-RBBB with coved ST-segment elevation in leads V1–V3; requires Implantable Cardioverter-Defibrillator (ICD).
Pacemaker Mandates
Permanent pacemaker indicated for symptomatic bradycardia (Sick Sinus), Mobitz Type II 2nd-degree AV block, Complete 3rd-degree AV block, and alternating BBB.
Narrow vs. Wide Complex Tachyarrhythmias
Tachyarrhythmias (heart rate >100 bpm) are triaged algorithmically by two immediate binary questions: (1) Is the patient hemodynamically unstable (hypotension, ischemic chest pain, acute pulmonary edema, altered mentation, or shock)—in which case immediate Synchronized Direct-Current Cardioversion is performed—and (2) If stable, is the QRS complex Narrow (<120 ms, supraventricular) or Wide (≥120 ms, ventricular tachycardia vs. SVT with aberrancy/WPW) AND regular vs. irregular?
| Arrhythmia | ECG Characteristics | Hemodynamically Stable Therapy | Unstable Emergency Protocol |
|---|---|---|---|
| Atrial Fibrillation (AFib) | No distinct P waves; fibrillatory baseline; irregularly irregular RR intervals | Rate control (Metoprolol, Diltiazem); anticoagulation based on CHA2DS2-VASc (≥ 2 in men, ≥ 3 in women -> DOAC) | Immediate synchronized electrical cardioversion (100–200 J biphasic) |
| Supraventricular Tachycardia (AVNRT) | Regular narrow-complex tachycardia (150–220 bpm); retrograde P waves buried within or immediately after QRS | Vagal maneuvers (Valsalva, modified ice immersion) -> IV Adenosine 6 mg rapid push (follow with 12 mg if unsuccessful) | Synchronized electrical cardioversion (50–100 J) |
| Ventricular Tachycardia (Monomorphic VT) | Wide QRS complexes (> 120 ms), regular rhythm, rate 140–250 bpm; AV dissociation, capture beats, fusion beats | IV Amiodarone 150 mg infusion over 10 min, or IV Procainamide | Pulse present: Synchronized cardioversion (100 J); Pulseless: CPR + unsynchronized defibrillation (200 J) |
| Torsades de Pointes | Polymorphic VT twisting around the isoelectric line; preceded by prolonged QT interval (> 450 ms men, > 470 ms women) | IV Magnesium sulfate (2 g IV push); discontinue all QT-prolonging drugs; temporary pacing if bradycardia-dependent | Immediate defibrillation; emergent overdrive transvenous pacing |
Narrow-Complex Tachycardias: AVNRT/AVRT Vagal & Adenosine Protocol vs. Atrial Fibrillation Rate/Rhythm & CHA2DS2-VASc
For a hemodynamically stable Regular Narrow-Complex Tachycardia (Paroxysmal SVT, rate 150–250 bpm)—most commonly AV Nodal Reentrant Tachycardia (AVNRT) (dual slow/fast pathways within the AV node with retrograde P waves buried in the QRS) or orthodromic AV Reentrant Tachycardia (AVRT)—first-line therapy is Vagal Maneuvers (modified Valsalva maneuver or carotid sinus massage after auscultating to exclude a carotid bruit), which increase vagal acetylcholine release at M2 receptors to slow AV nodal conduction. If vagal maneuvers fail, administer rapid-push IV Adenosine (6 mg rapid IV push followed by 12 mg), which activates purinergic A1 receptors (Gi-coupled) to open K+ channels and block Ca2+ channels, causing transient complete AV nodal block (NOTE: adenosine is antagonized by theophylline and caffeine, potentiated by dipyridamole and in denervated heart transplant recipients, and causes transient flushing, chest pressure, and bronchospasm!). For Irregularly Irregular Narrow-Complex Tachycardias: Atrial Fibrillation (absent P waves, chaotic fibrillatory baseline; #1 cardiac cause of embolic stroke from the left atrial appendage) is rate-controlled in stable patients with IV/PO Beta-Blockers (metoprolol) or Non-Dihydropyridine CCBs (diltiazem/verapamil) (avoid CCBs/beta-blockers in decompensated HFrEF, where digoxin or amiodarone is used), and requires chronic oral anticoagulation (Apixaban/Rivaroxaban) when the CHA2DS2-VASc score is ≥2 in men or ≥3 in women. If AFib duration is >48 hours (or unknown), perform 3 weeks of therapeutic anticoagulation OR a Transesophageal Echocardiogram (TEE) to exclude left atrial thrombus before cardioversion, followed by 4 weeks of post-cardioversion anticoagulation!
The Wolff-Parkinson-White (WPW) Bundle of Kent Trap & Polymorphic VT (Torsades de Pointes vs. Ischemic VT)
Two wide-complex arrhythmia scenarios carry lethal pharmacologic traps on board exams: (1) Wolff-Parkinson-White (WPW) Syndrome with Atrial Fibrillation: WPW is caused by a congenital accessory pathway (Bundle of Kent) bypassing the AV node, producing a baseline triad of a short PR interval (<120 ms), a slurred upstroke of the QRS complex ('Delta wave'), and a widened QRS. If a WPW patient develops Atrial Fibrillation with pre-excitation (an irregularly irregular wide-complex tachycardia), AV nodal blocking agents—Adenosine, Beta-Blockers, Diltiazem/Verapamil, and Digoxin (ABCD)—are STRICTLY CONTRAINDICATED! Blocking the AV node removes competitive retrograde concealment and shunts 100% of chaotic atrial impulses (300 bpm) straight down the Bundle of Kent into the ventricles, precipitating immediate Ventricular Fibrillation! Instead, treat stable pre-excited AFib with IV Procainamide or Ibutilide (which slow accessory pathway conduction; definitive cure = catheter radiofrequency ablation). (2) Monomorphic vs. Polymorphic Ventricular Tachycardia: any wide-complex regular tachycardia in a patient with structural heart disease (prior MI scar) is treated as Monomorphic VT until proven otherwise (stable → IV Amiodarone or Procainamide; unstable → Synchronized Cardioversion; pulseless VT or VF → Unsynchronized Defibrillation). When Polymorphic VT occurs in the setting of baseline QT prolongation (Torsades de Pointes, 'twisting of the points,' triggered by macrolides, fluoroquinolones, antipsychotics, ondansetron, methadone, sotalol, hypokalemia, or hypomagnesemia), first-line treatment in a stable patient is Intravenous Magnesium Sulfate (2 g IV) regardless of serum magnesium level (with overdrive pacing or isoproterenol for refractory bradycardia-dependent Torsades).
Atrioventricular (AV) Blocks Master Algorithm
Atrioventricular (AV) conduction blocks are stratified into First-Degree AV Block, Second-Degree Mobitz Type I (Wenckebach), Second-Degree Mobitz Type II, and Third-Degree (Complete) Heart Block based on the anatomical site of conduction delay—either above/within the AV node (supranodal/nodal, responsive to atropine) or below the AV node within the His-Purkinje system (infranodal, requiring permanent pacemaker implantation).
| AV Block Type | PR Interval Behavior | QRS & Drop Pattern | Anatomical Level & Clinical Disposition |
|---|---|---|---|
| First-Degree AV Block | Fixed prolonged PR interval (> 200 ms or 5 small boxes) | Every P wave followed by QRS; no dropped beats | AV node delay; benign; observation, review AV nodal blocking drugs |
| Mobitz Type I (Wenckebach) | Progressive PR lengthening with each beat until dropped QRS | Regularly irregular; grouped beating | AV node; typically benign, inferior MI / high vagal tone; atropine if symptomatic |
| Mobitz Type II | Constant, fixed PR interval prior to sudden, unpredictable dropped QRS | Intermittent dropped QRS without PR warning; high risk of progression to 3rd degree | His-Purkinje system (infranodal); anterior septal MI; permanent pacemaker indicated |
| Third-Degree (Complete) AV Block | PR intervals completely variable and inconsistent | P waves and QRS complexes beat completely independently; slow escape rhythm (30–45 bpm) | Complete failure of AV conduction; transcutaneous/transvenous pacing -> permanent pacemaker |
Anatomical Level of Block: Nodal Mobitz Type I (Wenckebach) vs. Infranodal His-Purkinje Mobitz Type II
Distinguishing Second-Degree Mobitz Type I (Wenckebach) from Mobitz Type II on an ECG strip is one of the highest-yield cardiology board tasks because their prognosis and pacemaker indications are diametrically opposite: (1) First-Degree AV Block features a fixed, prolonged PR interval >200 ms (>1 large box) with every P wave conducting to a QRS (1:1 conduction; benign, no treatment). (2) Second-Degree Mobitz Type I (Wenckebach) occurs within the AV node itself (supplied by the RCA; common in inferior MI, high vagal tone in athletes, or beta-blocker/CCB/digoxin use) and displays progressive lengthening of the PR interval from beat to beat (coupled with progressive shortening of the R-R interval) until a P wave fails to conduct ('dropped QRS'), followed by a reset to a shorter PR interval; the QRS is typically narrow, progression to complete heart block is rare, and asymptomatic patients require observation only (symptomatic bradycardia responds well to IV Atropine). (3) Second-Degree Mobitz Type II occurs below the AV node in the Bundle of His or bundle branches (infranodal) (common in anterior LAD MI or calcific conduction disease) and displays a CONSTANT, non-lengthening PR interval across all conducted beats with sudden, unpredictable dropped QRS complexes (often accompanied by a wide bundle-branch-block QRS). Because Mobitz Type II carries a high risk of sudden progression to lethal ventricular standstill or complete heart block—and Atropine is ineffective or worsens infranodal block by accelerating the sinus node and increasing the ratio of blocked impulses—Mobitz Type II ALWAYS requires a Permanent Pacemaker, even if the patient is completely asymptomatic!
Third-Degree (Complete) Heart Block (AV Dissociation) & Lyme Carditis Ceftriaxone Exception
In Third-Degree (Complete) AV Block, zero atrial impulses conduct through to the ventricles, resulting in complete Atrioventricular (AV) Dissociation: on the 12-lead ECG, P waves march through at their own regular, faster sinus rate (P–P interval constant, e.g., 80–100 bpm), while QRS complexes march through independently at a slow, regular escape rate (R–R interval constant, e.g., 30–45 bpm) with completely variable PR relationships (some P waves deform T waves or fall inside QRS complexes). If the escape pacemaker arises near the His bundle, the QRS is narrow (40–60 bpm); if it arises in the distal Purkinje fibers, the QRS is wide and dangerously slow (20–40 bpm), causing syncope (Stokes-Adams attacks), dizziness, cannon A waves on JVP, and heart failure. Like Mobitz Type II, Third-Degree AV Block is an absolute indication for Permanent Pacemaker implantation (stabilized acutely with transcutaneous/transvenous pacing or dopamine/epinephrine infusion). CRITICAL BOARD EXCEPTION: Always check whether a young hiker from the Northeast/Midwest presenting with new-onset high-grade AV block, facial nerve palsy, or erythema migrans has Lyme Carditis (Borrelia burgdorferi)—Lyme carditis causes reversible AV nodal inflammation that resolves completely with Intravenous Ceftriaxone (for high-grade block / PR >300 ms, with temporary pacing if needed) and does NOT require a permanent pacemaker!
Atrial Fibrillation & Atrial Flutter: Comprehensive Rate vs. Rhythm Control & Anticoagulation
Atrial Fibrillation (AFib) is the most common sustained cardiac arrhythmia, driven by rapid, disorganized ectopic electrical firing originating primarily from muscular sleeves within the pulmonary veins in a dilated or fibrotic left atrium (triggered by hypertension, heart failure, mitral valve disease, hyperthyroidism, alcohol binge ['Holiday Heart syndrome'], or pulmonary embolism). The ECG demonstrates an irregularly irregular narrow-complex rhythm with complete absence of discrete P waves and an undulating fibrillatory baseline. Loss of coordinated atrial contraction ('atrial kick') reduces left ventricular end-diastolic filling by 15–25% (which precipitates acute decompensation in patients with stiff ventricles from hypertrophic cardiomyopathy, aortic stenosis, or diastolic heart failure) and promotes blood stasis in the left atrial appendage (LAA), creating a high risk of cardioembolic stroke, acute mesenteric ischemia, and acute limb ischemia.
| Clinical Scenario / Decision Node | Diagnostic Assessment / Score | First-Line Management Strategy | Critical Contraindications & Board Traps |
|---|---|---|---|
| Hemodynamically Unstable AFib / Flutter (Hypotension, Shock, Angina, Pulmonary Edema) | Immediate bedside vital signs and 12-lead ECG | Emergent Synchronized Direct-Current Cardioversion (do NOT delay for anticoagulation or TEE, though start IV heparin concurrently) | Never administer IV diltiazem or metoprolol to a hypotensive patient in cardiogenic shock |
| Stable AFib with RVR (Heart Rate >110 bpm, Normal LVEF) | TSH, free T4, electrolytes (K+, Mg2+), CBC, transthoracic echocardiogram | Rate Control (AFFIRM trial shows equal survival to rhythm control): IV/oral Beta-blocker (Metoprolol) OR Non-dihydropyridine CCB (Diltiazem, Verapamil); target resting HR <80–110 bpm | Always check TSH to rule out occult thyrotoxicosis (where propranolol is preferred) |
| Stable AFib in Decompensated HFrEF (LVEF ≤40%) | Echocardiogram showing reduced systolic function and volume overload | IV Amiodarone or Digoxin for acute rate control (once euvolemic, transition to evidence-based Beta-blocker: metoprolol succinate, carvedilol, or bisoprolol) | Non-dihydropyridine CCBs (Diltiazem, Verapamil) are CONTRAINDICATED in HFrEF due to potent negative inotropy that worsens heart failure! |
| Elective Rhythm Control (Symptomatic despite rate control or young patient) | Assess duration of AFib (<48 hours vs. ≥48 hours / unknown duration) and structural heart disease | If ≥48 hr: Either (1) TEE to rule out LAA thrombus -> cardioversion, OR (2) 3 weeks therapeutic anticoagulation -> cardioversion -> 4 weeks anticoagulation; maintain sinus with Amiodarone, Sotalol, Dofetilide, or Flecainide/Propafenone, or Pulmonary Vein Isolation ablation | Flecainide and Propafenone ('pill-in-the-pocket') are strictly restricted to patients with ZERO structural heart disease or CAD |
| Chronic Thromboembolic Stroke Prophylaxis | Calculate CHA2DS2-VASc score: CHF (1), HTN (1), Age ≥75 (2), Diabetes (1), Stroke/TIA/embolism (2), Vascular disease (1), Age 65–74 (1), Sex female (1) | Score ≥2 in men or ≥3 in women: Lifelong Oral Anticoagulation with a DOAC (Apixaban, Rivaroxaban, Dabigatran — preferred over Warfarin) | If patient has a Mechanical Heart Valve OR Moderate-to-Severe Mitral Stenosis ('Valvular AFib'), DOACs are contraindicated — MUST use Warfarin (INR 2.0–3.0 or 2.5–3.5) |
| Typical Atrial Flutter | Counterclockwise macro-reentrant circuit around the tricuspid annulus via the Cavo-Tricuspid Isthmus (CTI) | 'Sawtooth' flutter (F) waves in inferior leads (II, III, aVF) at 300 bpm with 2:1 AV block (ventricular rate ~150 bpm); identical CHA2DS2-VASc anticoagulation rules as AFib; Definitive cure: CTI Radiofrequency Catheter Ablation | Giving Class IC drugs (flecainide) without an AV nodal blocker in atrial flutter can slow atrial rate from 300 to 200 bpm and trigger 1:1 AV conduction at 200 bpm! |
CHA2DS2-VASc Scoring & Valvular vs. Non-Valvular Anticoagulation
Every patient with Atrial Fibrillation or Atrial Flutter—regardless of whether their rhythm is paroxysmal, persistent, or permanent, and even if they have been pharmacologically restored to sinus rhythm—must be risk-stratified for stroke using the CHA2DS2-VASc score: Congestive heart failure (+1), Hypertension (+1), Age ≥75 years (+2), Diabetes mellitus (+1), prior Stroke/TIA/thromboembolism (+2), Vascular disease (prior MI, PAD, or aortic plaque, +1), Age 65–74 years (+1), and Sc (Female sex category, +1—note that female sex alone without another risk factor does not warrant anticoagulation). Oral anticoagulation is indicated for a score of ≥2 in men or ≥3 in women (and considered for 1 in men / 2 in women). First-line therapy is a Direct Oral Anticoagulant (DOAC: apixaban, rivaroxaban, edoxaban, or dabigatran) due to lower intracranial hemorrhage risk than warfarin, unless the patient has a mechanical prosthetic valve or moderate-to-severe mitral stenosis, which mandate Warfarin.
The 48-Hour Cardioversion Rule & Left Atrial Appendage Thrombus
When Atrial Fibrillation persists beyond 48 hours (or when the exact time of onset is unknown), stasis in the left atrial appendage permits organized mural thrombus formation. Restoring mechanical atrial systole via electrical or pharmacologic cardioversion can eject a pre-existing thrombus into the systemic circulation, causing an embolic stroke. Moreover, even if no clot is present before cardioversion, the newly cardioverted atrium suffers from transient mechanical stunning ('atrial stunning') for several weeks, during which new clots can form. Therefore, for AFib ≥48 hours or unknown duration in a stable patient, you must EITHER (1) perform a Transesophageal Echocardiogram (TEE) to confirm the absence of LAA thrombus and cardiovert immediately on heparin/DOAC, OR (2) anticoagulate therapeutically for 3 full weeks BEFORE cardioversion—and in both pathways, continue therapeutic anticoagulation for at least 4 weeks AFTER cardioversion.
Atrial Flutter Cavo-Tricuspid Isthmus Circuit & Definitive Ablation
Unlike the chaotic pulmonary vein triggers of AFib, Typical Atrial Flutter is driven by a single organized, macro-reentrant electrical loop circulating counterclockwise around the tricuspid valve annulus in the right atrium, passing through a narrow anatomical bottleneck between the inferior vena cava and the tricuspid ring called the cavo-tricuspid isthmus (CTI). The atrial circuit fires at 250–350 bpm (classically 300 bpm), producing negatively deflected 'sawtooth' flutter waves in leads II, III, and aVF. Because the healthy AV node cannot conduct 300 impulses per minute, it typically conducts every second impulse (2:1 AV block), yielding a classic board clue: a regular narrow-complex tachycardia locked at a ventricular rate of exactly 150 bpm. While acute rate control and CHA2DS2-VASc anticoagulation mirror AFib, radiofrequency catheter ablation of the cavo-tricuspid isthmus is the definitive first-line curative intervention (>95% success).
Supraventricular Tachycardias: AVNRT, WPW (AVRT) & Multifocal Atrial Tachycardia
Paroxysmal Supraventricular Tachycardias (SVTs) present in young or middle-aged patients with abrupt-onset palpitations, dizziness, neck pounding, or dyspnea and an ECG showing a regular, narrow-complex tachycardia (QRS <120 ms, rate 150–250 bpm). The two most common mechanisms are AV Nodal Reentrant Tachycardia (AVNRT)—where the reentrant circuit lies entirely within the AV node via a slow pathway (short refractory period) and a fast pathway (long refractory period), burying retrograde P waves inside or immediately at the tail of the QRS complex—and Atrioventricular Reciprocating Tachycardia (AVRT), which utilizes an anatomically separate congenital accessory muscular bridge connecting the atria and ventricles called the Bundle of Kent (Wolff-Parkinson-White [WPW] syndrome).
| SVT Subtype | Electrophysiologic Mechanism | Classic ECG Appearance | Stepwise Acute & Definitive Management |
|---|---|---|---|
| AV Nodal Reentrant Tachycardia (AVNRT) | Micro-reentrant loop within the AV node (anterograde down slow alpha pathway, retrograde up fast beta pathway); most common regular SVT | Regular narrow QRS at 160–220 bpm; P waves invisible (buried in QRS) or visible as pseudo-r' in V1 / pseudo-S in inferior leads | 1st: Vagal maneuvers (Valsalva, carotid massage); 2nd: Rapid IV push Adenosine (6 mg -> 12 mg); 3rd: IV Diltiazem or Metoprolol; Definitive: Catheter ablation of slow AV nodal pathway |
| Orthodromic AVRT (in WPW) | Macro-reentrant loop traveling Anterograde down the normal AV node and Retrograde up the Bundle of Kent (95% of AVRT) | During SVT: Regular NARROW QRS tachycardia (no delta wave visible during tachycardia because ventricular activation occurs via normal His-Purkinje!); Delta wave appears ONLY after conversion to sinus rhythm | Acute: Vagal maneuvers -> IV Adenosine (safe in orthodromic narrow-complex AVRT because the AV node is the anterograde limb of the circuit); Definitive: Bundle of Kent catheter ablation |
| Antidromic AVRT (in WPW) | Macro-reentrant loop traveling Anterograde down the Bundle of Kent and Retrograde up the AV node (5% of AVRT) | Regular WIDE-complex tachycardia (mimics monomorphic VT) because the entire ventricle is depolarized cell-to-cell via the accessory pathway | Acute: IV Procainamide or Ibutilide (or synchronized cardioversion); AVOID AV nodal blockers; Definitive: Radiofrequency ablation of the Bundle of Kent |
| Pre-Excited Atrial Fibrillation in WPW | Chaotic atrial fibrillatory impulses (300–600 bpm) conducting directly down the non-decremental Bundle of Kent into the ventricles | IRREGULARLY IRREGULAR WIDE-complex tachycardia with varying QRS morphologies and extreme ventricular rates (200–300 bpm) | Stable: IV Procainamide or Ibutilide; Unstable: Synchronized cardioversion; NEVER give ABCD (Adenosine, Beta-blockers, CCBs, Digoxin) — triggers lethal Ventricular Fibrillation! |
| Multifocal Atrial Tachycardia (MAT) | Multiple ectopic atrial pacemakers firing automatically in a chronically hypoxic, hypercapnic, or right-heart-strained atrium | Irregularly irregular narrow-complex tachycardia (>100 bpm) with ≥3 distinct P-wave morphologies and varying PR and R-R intervals | Strongly associated with severe COPD exacerbations; 1st: Correct hypoxia, hypercapnia, K+/Mg2+, and stop theophylline; 2nd: Non-dihydropyridine CCB (Verapamil or Diltiazem — avoid non-selective beta-blockers in severe bronchospastic COPD; cardioversion and adenosine are ineffective) |
Adenosine Mechanism of Action, Flushing Side Effects & Heart Transplant Sensitivity
Adenosine is an ultra-short-acting endogenous purine nucleoside (half-life <10 seconds due to rapid uptake by erythrocytes and vascular endothelium) that binds cardiac A1 receptors coupled to Gi proteins. This activates inward-rectifier potassium currents (IKACh/Ado), causing profound membrane hyperpolarization, and inhibits L-type Ca2+ channels—transiently blocking conduction through the AV node for 3 to 6 seconds to break reentrant SVT circuits. Patients should be warned before rapid IV push that adenosine routinely causes 5–10 seconds of intense facial flushing, chest pressure/tightness, dyspnea (bronchospasm via A2B receptors—use caution in severe asthma), and a brief period of sinus pause/asystole on the monitor. Note two pharmacologic interactions: theophylline and caffeine competitively antagonize adenosine receptors (requiring higher adenosine doses), whereas dipyridamole (blocks cellular adenosine uptake) and denervated orthotopic heart transplants (receptor supersensitivity) markedly potentiate adenosine, requiring a 50–75% dose reduction.
Why ABCD Nodal Blockers Kill Patients with WPW + Atrial Fibrillation
The normal AV node exhibits decremental conduction—the faster it is bombarded with atrial impulses, the slower it conducts, protecting the ventricles from 300+ bpm atrial rates. In contrast, the Bundle of Kent in Wolff-Parkinson-White syndrome consists of working myocardial fibers governed by fast Na+ channels that lack decremental conduction. If a patient with WPW develops Atrial Fibrillation (producing an irregularly irregular wide-complex tachycardia), administering an AV nodal blocker (ABCD: Adenosine, Beta-blockers, Calcium channel blockers [verapamil/diltiazem], or Digoxin) blocks the normal AV node, eliminates retrograde concealed penetration into the accessory pathway, and shortens the refractory period of the Bundle of Kent. All 300+ bpm fibrillatory impulses then pour unimpeded down the Bundle of Kent, immediately degenerating into Ventricular Fibrillation and cardiac arrest. Always treat stable WPW + AFib with IV Procainamide (Class IA Na+ channel blocker that slows conduction directly in the accessory pathway).
Multifocal Atrial Tachycardia (MAT) vs. Atrial Fibrillation in COPD
Because both Atrial Fibrillation and Multifocal Atrial Tachycardia (MAT) present as an irregularly irregular narrow-complex tachycardia, exam takers must inspect the baseline ahead of every QRS complex. In AFib, discrete P waves are absent. In MAT, discrete P waves precede every QRS complex, but there are at least 3 distinct P-wave morphologies in the same ECG lead accompanied by varying P-P, P-R, and R-R intervals. MAT occurs almost exclusively in elderly patients with severe decompensated COPD, cor pulmonale, pneumonia, or methylxanthine (theophylline) toxicity. First-line management is treating the underlying hypoxia and respiratory acidosis and repleting potassium and magnesium; if pharmacologic rate control is needed, use a non-dihydropyridine calcium channel blocker (verapamil or diltiazem) rather than non-selective beta-blockers (which exacerbate bronchospasm).
Ventricular Arrhythmias, Torsades de Pointes & Inherited Channelopathies
Ventricular Tachycardia (VT) is defined as ≥3 consecutive premature ventricular complexes at a rate >100 bpm with a wide QRS complex (>120 ms). Monomorphic VT (uniform QRS morphology from beat to beat) most commonly arises from a stable macro-reentrant circuit circling around a healed myocardial infarction scar in patients with ischemic cardiomyopathy. ECG hallmarks that confirm VT over SVT with aberrancy include AV dissociation (independent P waves marching at a slower rate than the wide QRS complexes), fusion beats (a hybrid QRS formed when a supraventricular impulse and a ventricular impulse activate the ventricles simultaneously), capture beats (an occasional narrow QRS complex conducted normally through the AV node), and extreme northwest axis or precordial concordance across V1–V6.
| Syndrome / Arrhythmia | Genetic or Acquired Etiology | Pathognomonic ECG & Electrophysiology | Acute & Long-Term Management |
|---|---|---|---|
| Sustained Monomorphic VT | Reentry around post-MI myocardial scar or dilated/hypertrophic cardiomyopathy | Regular wide-complex tachycardia (QRS >120 ms) with uniform morphology, AV dissociation, fusion beats, and capture beats | Stable with pulse: IV Amiodarone, Procainamide, or Lidocaine (or elective synchronized cardioversion); Unstable with pulse: Emergent Synchronized Cardioversion; Pulseless VT: Unsynchronized Defibrillation + CPR + ICD for secondary prevention |
| Torsades de Pointes (Acquired Long QT) | Drug-induced IKr (hERG) K+ channel blockade + Hypokalemia, Hypomagnesemia, or Hypocalcemia | Prolonged baseline QTc (>450 ms men, >470 ms women; high risk >500 ms) triggering Early Afterdepolarizations (EADs) in Phase 3 -> twisting polymorphic VT | Stable/Recurrent: Immediate IV Magnesium Sulfate (2 g bolus — even if serum Mg2+ is normal!) + K+ repletion (>4.0 mEq/L) + overdrive pacing / isoproterenol (shortens QT by increasing HR); Unstable: Unsynchronized Defibrillation |
| Romano-Ward Syndrome (Congenital LQT1–3) | Autosomal DOMINANT loss-of-function K+ channel (KCNQ1 in LQT1, KCNH2 in LQT2) or gain-of-function Na+ channel (SCN5A in LQT3) | Marked baseline QTc prolongation with NORMAL hearing; syncope or sudden cardiac arrest triggered by swimming/exercise (LQT1) or sudden loud noises/alarm clocks (LQT2) | First-line: Nadolol or Propranolol (non-selective beta-blockers) + avoid all QT-prolonging drugs + strenuous swimming restriction; ICD if prior cardiac arrest or syncope on beta-blocker |
| Jervell and Lange-Nielsen Syndrome | Autosomal RECESSIVE homozygous mutation in KCNQ1 or KCNE1 (impairs endolymph K+ secretion in inner ear stria vascularis AND cardiac IKs) | Severe congenital QTc prolongation (>500–550 ms) + Bilateral Congenital Sensorineural Deafness | High-dose Beta-blocker + Implantable Cardioverter-Defibrillator (ICD) + cochlear implantation |
| Brugada Syndrome | Autosomal dominant loss-of-function mutation in SCN5A (cardiac fast voltage-gated Na+ channel); highest prevalence in young Southeast Asian men | Pseudo-Right Bundle Branch Block (rSR') with coved ≥2 mm ST-segment elevation and inverted T waves in leads V1–V3; ventricular fibrillation occurs during sleep or febrile illness | Implantable Cardioverter-Defibrillator (ICD) is the ONLY proven therapy to prevent sudden cardiac death; aggressively treat all fevers with acetaminophen (fever unmasks SCN5A channel inactivation!) |
| Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC) | Autosomal dominant mutation in cardiac desmosomal proteins (Plakoglobin, Desmoplakin, Plakophilin-2) | Fibrofatty replacement of the right ventricular myocardium; T-wave inversions in V1–V3 and 'Epsilon wave' (terminal notch at the end of the QRS complex in V1) | Exercise restriction + Beta-blockers + ICD implantation to prevent exercise-induced VT/VF |
Drug-Induced Long QT Syndrome: The ABCDE Mnemonic & Early Afterdepolarizations
Prolongation of the ventricular action potential duration (Phase 2 and Phase 3 repolarization, primarily via blockade of the rapid delayed rectifier potassium current IKr [hERG channel]) manifests on the surface ECG as a prolonged QTc interval and predisposes cardiomyocytes to spontaneous depolarizations during late Phase 2 or Phase 3 called Early Afterdepolarizations (EADs), which trigger Torsades de Pointes (contrast this with Delayed Afterdepolarizations [DADs] occurring during Phase 4 due to intracellular Ca2+ overload in Digoxin toxicity). Memorize the high-yield ABCDE classes of QT-prolonging drugs: Antiarrhythmics (Class IA: quinidine, procainamide, disopyramide; Class III: sotalol, dofetilide, ibutilide—note that amiodarone prolongs QT universally but has the lowest incidence of Torsades because it homogenizes transmural repolarization), Antibiotics (Macrolides: azithromycin, erythromycin; Fluoroquinolones: levofloxacin, moxifloxacin; Azoles: fluconazole), Antipsychotics (haloperidol, ziprasidone, thioridazine, quetiapine), Antidepressants (TCAs, citalopram, escitalopram), and Antiemetics (ondansetron, droperidol, metoclopramide), plus Methadone and Hydroxychloroquine.
Why IV Magnesium Sulfate Works in Torsades & Overdrive Pacing Physiology
Intravenous Magnesium Sulfate (2 g IV bolus over 1–2 minutes) is the undisputed first-line pharmacologic therapy for Torsades de Pointes, regardless of the patient's baseline serum magnesium concentration. Magnesium suppresses early afterdepolarizations (EADs) by inhibiting L-type calcium influx across the sarcolemma without shortening the QT interval itself. Simultaneously, serum potassium must be repleted to the high-normal range (4.5–5.0 mEq/L) and all QT-prolonging medications stopped. If Torsades recurs despite magnesium in a patient with acquired (bradycardia- or pause-dependent) Long QT, increasing the heart rate via transvenous overdrive pacing (to 100–110 bpm) or IV isoproterenol infusion shortens the refractory period and shrinks the QTc interval, preventing pause-dependent EADs (note: isoproterenol is contraindicated in congenital Long QT syndrome, where sympathetic surges trigger Torsades!).
Implantable Cardioverter-Defibrillator (ICD) Primary & Secondary Prevention Criteria
Board vignettes frequently ask when to implant an Implantable Cardioverter-Defibrillator (ICD) to prevent Sudden Cardiac Death (SCD). Secondary prevention (Class I indication) applies to any survivor of a prior ventricular fibrillation or hemodynamically unstable sustained VT arrest not caused by a completely reversible trigger (such as acute STEMI within 48 hours or severe electrolyte derangement). Primary prevention (Class I indication) requires an LVEF ≤35% despite at least 3 months of optimal Guideline-Directed Medical Therapy (GDMT) (and at least 40 days post-MI or 90 days post-CABG/PCI) in patients with NYHA Class II or III heart failure (or LVEF ≤30% in NYHA Class I ischemic cardiomyopathy) who have a life expectancy exceeding 1 year. If the patient also has a Left Bundle Branch Block (LBBB) with QRS ≥150 ms in sinus rhythm, upgrade to Cardiac Resynchronization Therapy with Defibrillator (CRT-D / biventricular pacing).
Sinus Node Dysfunction, Atrioventricular Blocks, Bundle Branch Blocks & Pacemaker Indications
Bradyarrhythmias and conduction blocks arise from failure of impulse generation in the sinoatrial (SA) node (supplied in 60% of individuals by the right coronary artery and 40% by the left circumflex) or failure of impulse propagation through the atrioventricular (AV) node (supplied in 90% by the AV nodal branch of the posterior descending artery from the Right Coronary Artery—explaining why inferior wall STEMIs in leads II, III, and aVF frequently cause sinus bradycardia and AV block) and the His-Purkinje system (supplied primarily by septal perforators of the Left Anterior Descending [LAD] artery).
| Conduction Disorder | Anatomic Site of Block | Diagnostic ECG Criteria | Prognosis & Definitive Management |
|---|---|---|---|
| Sick Sinus Syndrome (Bradycardia-Tachycardia Syndrome) | Fibrodegenerative fibrosis of the Sinoatrial (SA) node in older adults | Sinus bradycardia (<50 bpm), sinus pauses/arrest (>3 seconds), sinoatrial exit block, and alternating paroxysms of supraventricular tachycardia (AFib/Flutter) followed by long conversion pauses | Discontinue offending drugs (beta-blockers, CCBs, digoxin); if symptomatic (syncope, presyncope, chronotropic incompetence), implant a Permanent Pacemaker (which then permits safe beta-blocker therapy for the tachy-phase!) |
| First-Degree AV Block | AV Node (enhanced vagal tone, beta-blockers, CCBs, digoxin, Lyme carditis, inferior MI) | Fixed prolongation of the PR interval >200 ms (>1 large box) on every beat, with 1:1 conduction (zero dropped QRS complexes) | Benign and asymptomatic; no treatment required other than reviewing rate-slowing medications |
| Second-Degree AV Block: Mobitz Type I (Wenckebach) | AV Node (supranodal/intranodal — responsive to vagolytic atropine) | Progressive lengthening of the PR interval from beat to beat (with shortening R-R intervals) until a P wave fails to conduct (dropped QRS), followed by reset of the cycle | Usually benign and transient (athletes, sleep, inferior MI); observe unless symptomatic bradycardia occurs (treat acute symptomatic Wenckebach with IV Atropine) |
| Second-Degree AV Block: Mobitz Type II | Infranodal: Bundle of His or Bundle Branches (structural fibrosis or anterior LAD MI) | CONSTANT, fixed PR interval across conducted beats with sudden, unpredictable dropping of QRS complexes (often accompanied by a wide QRS / bundle branch block) | High-risk malignant rhythm that rapidly progresses to Complete 3rd-Degree Heart Block and sudden cardiac arrest; Atropine is INEFFECTIVE (and can worsen block); requires immediate transcutaneous/transvenous pacing -> Permanent Pacemaker! |
| Third-Degree (Complete) AV Block | Complete electrical block at AV node or His-Purkinje system (Lyme disease, sarcoidosis, endocarditis aortic root abscess, MI, maternal anti-Ro/SSA) | Complete AV Dissociation: constant P-P interval and constant R-R interval, but zero relationship between P waves and QRS complexes (Atrial rate > Ventricular escape rate of 30–40 bpm) | Immediate transcutaneous/transvenous pacing followed by Permanent Pacemaker implantation (Exception: Lyme carditis with complete heart block resolves with IV Ceftriaxone + temporary pacing) |
| Right vs. Left Bundle Branch Block (RBBB vs. LBBB) | Right Bundle Branch vs. Left Main Bundle (anterior + posterior fascicles) | Both have QRS >120 ms. RBBB ('MaRRoW'): rsR' ('bunny ears') in V1–V2 and wide slurred S wave in I, V5–V6. LBBB ('WiLLiaM'): deep broad QS/rS in V1 and broad notched R wave in I, aVL, V5–V6 | Isolated RBBB is often benign (or seen in RV strain/PE/ASD); New-onset LBBB with chest pain is a STEMI equivalent (or sign of dilated cardiomyopathy) and obscures standard ST-elevation interpretation (use Sgarbossa criteria) |
Differentiating Mobitz Type I (Wenckebach) vs. Mobitz Type II on Boards
When an ECG rhythm strip shows P waves that intermittently fail to produce a QRS complex (Second-Degree AV Block), immediately measure the PR interval of the beats immediately before and after the dropped QRS. If the PR interval immediately following the dropped beat is noticeably shorter than the PR interval immediately preceding the dropped beat, the diagnosis is Mobitz Type I (Wenckebach). Because Mobitz I resides within the AV node (mediated by L-type Ca2+ channels and heavy vagal innervation), it is usually benign and responds to IV Atropine. Conversely, if the PR intervals of all conducted beats remain identically constant before and after the dropped beat, the diagnosis is Mobitz Type II. Because Mobitz II resides below the AV node in the His-Purkinje system, administering atropine increases sinus node firing without improving His-Purkinje conduction, paradoxically worsening the ratio of conducted beats; Mobitz II is a Class I mandate for a permanent pacemaker even in an asymptomatic patient.
Reversible Causes of High-Grade AV Block: Lyme Carditis & Aortic Root Abscess
Before implanting a permanent pacemaker in a young patient with new second- or third-degree AV block, two critical infectious etiologies must be evaluated: (1) Lyme Carditis (Borrelia burgdorferi) occurs 3 to 6 weeks after an Ixodes scapularis tick bite (early disseminated stage, often alongside erythema migrans or bilateral Bell palsy) and causes fluctuating first-, second-, or complete third-degree AV block; treat with IV Ceftriaxone and temporary cardiac pacing—do NOT implant a permanent pacemaker, as conduction recovers completely within 1 to 2 weeks of antibiotic therapy; (2) In any patient with Infective Endocarditis (especially involving the aortic valve) who develops a new PR prolongation, bundle branch block, or complete heart block, immediately suspect a perivalvular aortic root abscess extending into the adjacent interventricular membranous septum and Bundle of His—confirm with Transesophageal Echocardiography (TEE) and consult cardiothoracic surgery for emergent debridement and valve replacement.
Class I Permanent Pacemaker Indications Summary
Commit the five core Class I indications for Permanent Pacemaker implantation to memory: (1) Symptomatic Sinus Bradycardia or Sick Sinus Syndrome (documented correlation between bradycardia/pauses and syncope, presyncope, dizziness, or exercise intolerance, after excluding reversible drug/metabolic causes or when the offending rate-slowing drug is medically essential), (2) Mobitz Type II Second-Degree AV Block (whether symptomatic or asymptomatic), (3) High-Grade (2:1 or 3:1 infranodal) or Complete Third-Degree AV Block (whether symptomatic or asymptomatic), (4) Alternating Bundle Branch Block (ECG alternating between RBBB and LBBB, indicating trifascicular disease), and (5) Carotid Sinus Hypersensitivity with recurrent syncope and documented ventricular pauses >3 seconds.
COMLEX / OMM Integration
NBOME High-Yield Correlate
Cardiac Conduction Viscerosomatics: Laterality of Autonomic Innervation & Vagal Maneuvers
Osteopathic principles provide a precise neuroanatomic map of cardiac rhythm disturbances based on the laterality of both sympathetic and parasympathetic innervation. Cardiac sympathetic preganglionic fibers originate from spinal segments T1–T5: right-sided T1–T5 sympathetics project predominantly to the sinoatrial (SA) node (accelerating heart rate / positive chronotropy), whereas left-sided T1–T5 sympathetics project predominantly to the atrioventricular (AV) node and ventricular myocardium (enhancing AV conduction and contractility). Remarkably, parasympathetic fibers within the vagus nerve (CN X) mirror this exact lateralization: the right vagus nerve primarily innervates the SA node (right vagal stimulation causes sinus bradycardia or sinus arrest), whereas the left vagus nerve primarily innervates the AV node (left vagal stimulation prolongs PR interval and induces AV nodal block). Consequently, a patient presenting with paroxysmal supraventricular tachycardia (AVNRT) or atrial fibrillation frequently exhibits acute somatic dysfunction at T1–T5 alongside cranial base compression at the occipitoatlantal (OA) and occipitomastoid (OM) sutures (jugular foramen). Bedside vagal maneuvers—such as the modified Valsalva maneuver or unilateral carotid sinus massage (only after auscultating to exclude a carotid bruit!)—increase efferent vagal acetylcholine release onto M2 muscarinic receptors, activating Gi and opening K+ channels (IKACh) to hyperpolarize the AV node and terminate AVNRT. Gentle suboccipital inhibition and rib raising help restore autonomic equilibrium, though carotid massage is strictly contraindicated in patients with carotid stenosis or recent TIA/stroke.
Board Traps & Common Distractors
- In patients with Wolff-Parkinson-White (WPW) syndrome presenting with pre-excited Atrial Fibrillation (irregularly irregular wide-complex tachycardia), NEVER administer AV-nodal blocking agents (ABCD: Adenosine, Beta-blockers, Calcium channel blockers, Digoxin); blocking the AV node forces all rapid atrial impulses down the non-decremental Bundle of Kent, precipitating Ventricular Fibrillation. Use IV Procainamide or synchronized cardioversion.
- Class IC antiarrhythmics (Flecainide and Propafenone) are STRICTLY CONTRAINDICATED in any patient with prior myocardial infarction, coronary artery disease, left ventricular hypertrophy, or structural heart disease (CAST trial) because they provoke lethal proarrhythmic ventricular tachycardia.
- Mobitz Type II second-degree AV block (dropped QRS beats WITHOUT progressive PR prolongation, localized infranodally in the bundle of His or Purkinje fibers) is highly unstable and rapidly degenerates into third-degree complete heart block and sudden cardiac arrest; place transcutaneous pacing pads and implant a permanent pacemaker even if the patient is currently asymptomatic.
- In any patient presenting with unstable tachycardia (hypotension, ischemic chest pain, acute pulmonary edema, or altered mental status), the immediate next step is ALWAYS Synchronized Direct-Current Cardioversion—except in Pulseless VT, Ventricular Fibrillation, or Polymorphic VT (Torsades), which require Unsynchronized Defibrillation.
- When performing elective cardioversion for Atrial Fibrillation or Atrial Flutter lasting >48 hours (or of unknown duration), NEVER cardiovert before either (1) obtaining a Transesophageal Echocardiogram (TEE) confirming no left atrial appendage thrombus, OR (2) completing ≥3 weeks of therapeutic oral anticoagulation prior to cardioversion, followed by ≥4 weeks of anticoagulation afterward.
- Amiodarone (Class III K+ channel blocker) has an elimination half-life of up to 60 days and causes cumulative multi-organ toxicity requiring baseline and serial screening: Pulmonary fibrosis (PFTs/DLCO — most lethal toxicity), Hyper- or Hypothyroidism (TSH/free T4 — contains 37% iodine by weight), Hepatotoxicity (AST/ALT), Blue-gray skin discoloration, and Corneal microdeposits.
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Volume 2: Organ System Clinical Pathology • Chapter 23 of 60