1.4 Supraventricular Tachycardias, Ventricular Arrhythmias & Bradycardias

1. Supraventricular Tachycardias (SVT)

AV Nodal Reentrant Tachycardia (AVNRT)

• Mechanism: Dual AV nodal pathways (slow pathway with short refractory period; fast pathway with long refractory period) creating a micro-reentrant circuit within the AV node.
• ECG: Regular narrow-complex tachycardia (140-220 bpm) with P waves buried within or immediately following the QRS complex (pseudo-r' in lead V1, pseudo-S in inferior leads).
• Acute Treatment: (1) Vagal maneuvers (modified Valsalva with supine leg elevation); (2) IV Adenosine (6 mg rapid IV push through large antecubital vein followed by 20 mL saline flush; if no response within 1-2 min, administer 12 mg IV); (3) IV Diltiazem or Metoprolol if adenosine fails; (4) Synchronized cardioversion if unstable.
• Definitive Treatment: Radiofrequency catheter ablation of the slow pathway (curative in >95% of cases).

AV Reentrant Tachycardia (AVRT) & Wolff-Parkinson-White (WPW) Syndrome

• Mechanism: Congenital extranodal accessory pathway (Bundle of Kent) connecting atrium and ventricle, bypassing the AV node.
• Baseline ECG Triad: Short PR interval (<120 ms), slurred QRS upstroke (delta wave), and widened QRS (>120 ms).
• Orthodromic AVRT (90%): Conduction travels anterograde down the AV node and retrograde up the accessory pathway -> Regular narrow-complex tachycardia. Treat identical to AVNRT (vagal maneuvers, IV Adenosine).
• Antidromic AVRT (10%): Conduction travels anterograde down the accessory pathway and retrograde up the AV node -> Regular wide-complex tachycardia.
• Pre-Excited Atrial Fibrillation (AF + WPW):
  • ECG: Irregularly irregular, wide-complex tachycardia with bizarre, polymorphous QRS morphology and extremely rapid ventricular rates (>250-300 bpm).
  • CRITICAL BOARD CONTRAINDICATION: AV nodal blocking agents ("ABCD": Adenosine, Beta-blockers, Calcium channel blockers [Diltiazem/Verapamil], Digoxin) are STRICTLY CONTRAINDICATED. Blocking the AV node diverts 100% of electrical impulses down the accessory pathway, which has a very short refractory period, precipitating ventricular fibrillation and cardiac arrest.
  • Treatment: Hemodynamically stable -> IV Procainamide (or IV Ibutilide) which prolongs the refractory period of the accessory pathway. Hemodynamically unstable -> Emergency Synchronized Direct-Current Cardioversion.
  • Definitive: Catheter ablation of the accessory pathway.

2. Ventricular Arrhythmias & Channelopathies

Monomorphic Ventricular Tachycardia (VT)

• Defined as >=3 consecutive ventricular beats at a rate >100 bpm with identical QRS morphology. Wide QRS complex (>120 ms).
• ECG Signs of VT vs. SVT with Aberrancy:
  • AV Dissociation: P waves marching independently of wide QRS complexes (pathognomonic).
  • Capture Beats & Fusion Beats: Normal narrow QRS (capture) or hybrid hybrid QRS (fusion) demonstrating dual supraventricular and ventricular activation.
  • Concordance: Uniformly positive or uniformly negative QRS complexes across leads V1-V6.
  • Brugada Sign: Interval from onset of R wave to deepest nadir of S wave >100 ms in any precordial lead.
• Management:
  • Pulseless VT: Immediate CPR + Defibrillation (unsynchronized 200 J biphasic) + Epinephrine + Amiodarone/Lidocaine.
  • Unstable with Pulse: Synchronized cardioversion (100-200 J).
  • Stable with Pulse: IV Amiodarone (150 mg over 10 min, then 1 mg/min for 6 hours) or IV Procainamide (20-50 mg/min until arrhythmia suppressed, hypotension ensues, or QRS widens >50%).

Polymorphic VT & Torsades de Pointes (TdP)

• Torsades de Pointes: Polymorphic VT characterized by QRS complexes continuously twisting around the isoelectric baseline, occurring in the setting of prolonged baseline QT interval (QTc >500 ms).
• Causes of Prolonged QTc:
  • Electrolytes: Hypokalemia, Hypomagnesemia, Hypocalcemia.
  • Drugs: Antiarrhythmics (Sotalol, Dofetilide, Procainamide), Psychotropics (Haloperidol, Citalopram, Methadone), Antimicrobials (Macrolides, Fluoroquinolones, Azole antifungals), Antiemetics (Ondansetron).
  • Congenital Long QT Syndromes (LQTS): Romano-Ward (autosomal dominant), Jervell and Lange-Nielsen (autosomal recessive with sensorineural deafness).
• Acute Management of Torsades de Pointes:
  1. IV Magnesium Sulfate: 2 grams IV push over 1-2 minutes, repeated if necessary, regardless of baseline serum magnesium level.
  2. Withdraw all QT-prolonging medications and vigorously recheck/correct potassium to >4.0-4.5 mEq/L.
  3. Overdrive Transvenous Pacing or Isoproterenol Infusion: Increases heart rate to 100-110 bpm, which shortens ventricular repolarization and eliminates pauses that trigger TdP.
  4. Unsynchronized defibrillation if degenerates to VF or hemodynamic collapse.

3. Bradycardias and Conduction Blocks

Sinus Node Dysfunction (Sick Sinus Syndrome)

• Manifests as persistent sinus bradycardia, sinus pauses/arrest (>3 seconds), or Tachycardia-Bradycardia Syndrome (paroxysms of AF alternating with severe sinus pauses). Permanent Pacemaker (PPM) is indicated when bradycardia is symptomatic.

Atrioventricular (AV) Blocks & Pacing Indications

Acute Symptomatic Bradycardia Protocol:

1. Atropine: 1 mg IV push q3-5min (max 3 mg total). Note: Atropine is effective for AV nodal block (Mobitz I), but is INEFFECTIVE for infranodal block (Mobitz II / wide-QRS complete block).
2. If atropine fails or is contraindicated: Transcutaneous Pacing or IV Chronotropic Infusions (Dopamine 5-20 mcg/kg/min or Epinephrine 2-10 mcg/min).
3. Emergent Transvenous Temporary Pacing while preparing for permanent pacemaker implantation.