Cardiac arrhythmia Study Guide

📖 Core Concepts Cardiac arrhythmia – any abnormal heart rhythm (too fast, too slow, or irregular). Normal sinus rhythm – regular rhythm originating from the sino‑atrial (SA) node. Tachycardia – resting HR > 100 bpm (adults). Bradycardia – resting HR < 60 bpm (adults). Automaticity – spontaneous impulse generation by pacemaker cells; ectopic automaticity → premature beats. Re‑entry – impulse circles a pathway with heterogeneous refractoriness, repeatedly re‑exciting tissue. Triggered activity – after‑depolarizations reach threshold and fire extra beats. Fibrillation – chaotic, rapid electrical activity of an entire chamber (atrial vs ventricular). Heart block – impaired AV conduction (1° prolonged PR, 2° Mobitz I progressive PR, 2° Mobitz II non‑conducted beats, 3° complete block). --- 📌 Must Remember Ventricular arrhythmias cause 80 % of sudden cardiac death. Atrial fibrillation (AF) prevalence: 2–3 % of adults in Europe/North America. Non‑sustained run: ≥3 premature beats < 30 s; sustained ≥30 s. Vagal maneuvers terminate most supraventricular tachycardias (SVT). Beta‑blockers → ↓ heart rate; first‑line for many tachyarrhythmias. Anticoagulation (warfarin, heparin, DOACs) reduces stroke risk in AF. Cardioversion = synchronized shock for SVT/AF; defibrillation = unsynchronized shock for VF/pulseless VT. ICD = automatic detection & shock for life‑threatening ventricular arrhythmias. Pacemaker indicated for persistent bradycardia or complete heart block. --- 🔄 Key Processes Normal conduction pathway SA node → atria → AV node → Bundle of His → Purkinje fibers → ventricles. Re‑entry circuit formation (a) Two pathways with different conduction speeds. (b) One pathway blocked (long refractory period). (c) Impulse travels slow pathway, re‑enters fast pathway when recovered → loop repeats. Vagal maneuver algorithm Perform Valsalva → carotid sinus massage → cold stimulus → reassess rhythm. Management decision tree Hemodynamically unstable? → Immediate synchronized cardioversion (SVT) or unsynchronized defibrillation (VF/VT). Stable SVT? → Vagal → Adenosine → β‑blocker → consider ablation. AF with risk factors? → Anticoagulation + rate control (β‑blocker) ± rhythm control (antiarrhythmic or ablation). --- 🔍 Key Comparisons Automaticity vs Re‑entry vs Triggered Activity Automaticity: impulse originates spontaneously; seen in ectopic pacemakers. Re‑entry: impulse re‑circulates a pre‑existing circuit; requires heterogeneous refractory periods. Triggered activity: extra beats arise from after‑depolarizations (early or delayed). Mobitz I (Wenckebach) vs Mobitz II Mobitz I: progressive PR prolongation → dropped beat; usually supra‑His. Mobitz II: constant PR, intermittent non‑conducted beats; usually infra‑His, higher risk of progressing to 3°. Cardioversion vs Defibrillation Cardioversion: synchronized with R‑wave; used for SVT, AF with pulse. Defibrillation: unsynchronized; emergency for VF or pulseless VT. Beta‑blocker vs Calcium‑channel blocker (non‑dihydropyridine) Both slow AV conduction; β‑blocker also reduces contractility & sympathetic tone; CCB (e.g., diltiazem) preferential for rate control in AF when β‑blocker contraindicated. --- ⚠️ Common Misunderstandings “All fast rhythms are tachycardia.” Only resting HR > 100 bpm qualifies; exercise‑induced rates are not arrhythmias. “Premature beats always need treatment.” Isolated premature atrial/ventricular contractions are often benign. “Defibrillation can be used for any arrhythmia.” Only unsynchronized shocks for VF/pulseless VT; synchronized shocks required for organized rhythms. “A‑block always needs a pacemaker.” First‑degree block is usually benign; pacing reserved for symptomatic high‑grade block. --- 🧠 Mental Models / Intuition “Circuit‑breaker” model for re‑entry: imagine a race track with two lanes—one fast, one slow. A car (impulse) stuck in the slow lane can re‑enter the fast lane when it clears, creating endless laps. “After‑depolarization ripple” – think of a wave that bumps the membrane after the main wave; if the bump is big enough, it triggers a new wave. “Rate‑vs‑Rhythm” – treat rate first (β‑blocker, diltiazem) to stabilize patient; address rhythm (cardioversion, ablation) if symptoms persist or stroke risk is high. --- 🚩 Exceptions & Edge Cases Mobitz I can progress to 2° Mobitz II or 3° in the setting of ischemia or drug toxicity. AF with rapid ventricular response may be refractory to β‑blockers alone; consider digoxin or combination therapy. Drug‑induced QT prolongation can precipitate torsades de pointes even in patients with normal baseline QT. Athlete’s sinus bradycardia (< 60 bpm) is physiologic, not pathologic, if asymptomatic. --- 📍 When to Use Which Vagal maneuver → first‑line for SVT with narrow QRS and stable patient. Adenosine → if vagal fails and SVT is still suspected (rapid, transient AV block). β‑blocker → rate control in AF, VT suppression, and prophylaxis in ischemic heart disease. Procainamide → preferred for wide‑complex tachycardia when ventricular origin is suspected and no structural heart disease. Anticoagulation → any AF lasting >48 h or with CHA₂DS₂‑VASc ≥2 (men) / ≥3 (women). ICD → documented ventricular fibrillation, sustained VT with LVEF ≤ 35 % despite optimal medical therapy. Pacemaker → symptomatic complete heart block, high‑grade AV block, or sinus node dysfunction with pauses >3 s. --- 👀 Patterns to Recognize Progressive PR prolongation → dropped beat = Mobitz I. Constant PR with intermittent dropped beats = Mobitz II. Irregularly irregular rhythm with absent P‑waves = atrial fibrillation. Saw‑tooth flutter waves (≈300 bpm) with regular ventricular response = atrial flutter. Wide QRS, AV dissociation, chaotic baseline = ventricular fibrillation. --- 🗂️ Exam Traps “Irregularly irregular” → may be AF or sinus arrhythmia; look for absent P‑waves to confirm AF. “Synchronized shock for ventricular tachycardia” – wrong; VT with pulse gets synchronized cardioversion, pulseless VT → unsynchronized defibrillation. “First‑degree block needs a pacemaker” – false; only if symptomatic bradycardia or progression. “All QT‑prolonging drugs cause torsades” – overstatement; risk depends on dose, electrolyte status, and patient genetics. “Beta‑blockers are contraindicated in all heart blocks” – not true for first‑degree or Mobitz I; contraindicated in high‑grade AV block without pacing.