Electrocardiography Study Guide

📖 Core Concepts Electrocardiogram (ECG) – a voltage‑vs‑time trace recorded from skin electrodes that reflects cardiac depolarization and repolarization. Depolarization vs. Repolarization Deflections – moving toward the positive electrode → upward (positive) wave; moving away → downward (negative) wave. Normal Conduction Path – SA node → atria (P wave) → AV node → Bundle of His → right/left bundle branches → Purkinje network (QRS complex) → ventricular repolarization (T wave). Lead System – 12‑lead ECG = 3 limb leads (I, II, III), 3 augmented limb leads (aVR, aVL, aVF), 6 precordial (V1‑V6). Leads view the heart from different angles, forming the hexaxial reference system. Rhythm & Rate – Identify a 1:1 P‑to‑QRS relationship (sinus rhythm). Count QRS complexes to get heart rate; 60–100 bpm is normal. Axis – Mean direction of ventricular depolarization in the frontal plane; normal –30° to +105°. --- 📌 Must Remember Normal intervals: PR 120‑200 ms, QRS ≤ 120 ms, QT varies with rate (use QTc). Small box = 0.1 mV × 40 ms; large box = 0.5 mV × 200 ms. Rate formulas: 30‑second method: Count QRS in 6 s → multiply by 10. 300‑box method: 300 ÷ (# large boxes between QRS) = bpm. Axis quick check: Positive QRS in both I and II → normal axis. ST‑segment elevation ≥ 1 mm in ≥ 2 contiguous leads = STEMI. Hyperacute T wave = earliest sign of acute MI. Pathologic Q wave: > 0.04 s wide or > 25 % of R amplitude → myocardial necrosis. Lead I polarity: + left arm (LA), – right arm (RA); Lead II: + left leg (LL), – RA; Lead III: + LL, – LA. --- 🔄 Key Processes Calculate Heart Rate (300‑box method) Count large boxes between two consecutive QRS complexes → $HR = \frac{300}{\text{boxes}}$ (bpm). Determine Rhythm Verify regular spacing of QRS complexes. Check P‑wave relationship: 1:1 → sinus; absent P → consider AFib. Frontal‑plane Axis Determination Look at QRS polarity in leads I and aVF: (+,+) → normal (–30° to +90°). (+,–) → left axis deviation (< –30°). (–,+) → right axis deviation (> +105°). Identify ST‑segment Changes Measure from J point to isoelectric line; elevation = upward deviation, depression = downward. Assess QRS Morphology for Bundle Branch Blocks RSR′ in V1 → Right bundle branch block (RBBB). Wide, notched or slurred R in leads I, aVL, V5‑V6 → Left bundle branch block (LBBB). --- 🔍 Key Comparisons Atrial fibrillation vs. Atrial flutter AFib: No distinct P waves, “irregularly irregular” R‑R intervals. AFlutter: Saw‑tooth “flutter” waves, usually regular ventricular response. ST‑segment elevation vs. ST‑segment depression Elevation: ≥ 1 mm above baseline in ≥ 2 contiguous leads → acute injury (STEMI). Depression: ≥ 0.5 mm below baseline → subendocardial ischemia (NSTEMI) or demand ischemia. Right vs. Left Axis Deviation Right: Positive in lead III, negative in aVL; > +105°. Left: Positive in aVL, negative in lead III; < –30°. Normal Q wave vs. Pathologic Q wave Normal: ≤ 0.04 s wide, ≤ 25 % of ensuing R amplitude, seen in leads I, aVL, V5‑V6. Pathologic: Wider/deeper, indicates prior MI. --- ⚠️ Common Misunderstandings “All negative waves are abnormal.” Small Q waves in limb leads are normal; only large/deep Q waves are pathological. “ST elevation always means MI.” Early repolarization, pericarditis, and left ventricular hypertrophy can also cause elevation. Look at morphology and clinical context. “A flat T wave is normal.” T waves should be upright in most leads; flat or inverted T in multiple leads suggests ischemia or electrolyte abnormality. “Lead placement doesn’t matter much.” Misplaced leads can reverse polarity, producing false diagnoses (e.g., mistaking a lead reversal for limb‑lead axis deviation). --- 🧠 Mental Models / Intuition “Vector View” – Imagine the heart’s electrical vector sweeping from the atria (right‑to‑left) then down the septum (left‑to‑right), then toward the apex (upward) and finally back up the ventricles. Each swing creates the characteristic P‑Q‑R‑S‑T sequence. “Box Counting” – Treat the ECG paper as a grid: every large box = 0.2 s horizontally, 0.5 mV vertically. Quick mental conversion speeds up interval measurement. “Contiguity Rule” – For any ST‑segment change, it must appear in at least two anatomically adjacent leads to be considered significant. --- 🚩 Exceptions & Edge Cases Brugada pattern – Coved ST elevation in V1‑V3 with a right bundle branch block morphology; may be concealed at baseline. Early repolarization – Concave ST elevation with prominent J point; benign in young healthy individuals but can mimic STEMI. Left anterior fascicular block – Small Q in lead I and small R in aVF; may coexist with other blocks. Hyperkalemia – Tall peaked T waves, widened QRS, possible sine‑wave pattern at severe levels. --- 📍 When to Use Which Rate calculation method – Use the 300‑box method for regular rhythms; use the 30‑second method for irregular rhythms (e.g., AFib). Axis determination – Use the simple I/II check for quick screening; switch to the full hexaxial system if results are ambiguous. Lead choice for MI localization – Anterior wall: V1‑V4. Lateral wall: I, aVL, V5‑V6. Inferior wall: II, III, aVF. Posterior wall: V7‑V9 (or V7‑V9 if suspicion). When to order a 12‑lead vs. Holter – 12‑lead for acute evaluation (chest pain, suspected MI); Holter for intermittent symptoms, arrhythmia monitoring over 24‑48 h. --- 👀 Patterns to Recognize “Elevated ST in contiguous leads with reciprocal depression” → classic STEMI. “QRS width > 120 ms + wide, slurred S in V1‑V2” → Left bundle branch block. “Irregularly irregular R‑R, absent P waves” → Atrial fibrillation. “Saw‑tooth flutter waves at 300 bpm” → Atrial flutter. “R wave progression: small R in V1, increasing to V4 then decreasing” → Normal precordial pattern; reversal suggests posterior MI or RV hypertrophy. --- 🗂️ Exam Traps Lead reversal – Right arm ↔ left arm reversal flips lead I polarity; answer choices that assume normal lead I may be wrong. “Isolated ST elevation in aVR” – Often a sign of left main coronary artery occlusion, not a benign early repolarization pattern. Misreading a wide QRS – Could be bundle branch block or ventricular tachycardia; check for AV dissociation and morphology. QT interval – Remember to use the corrected QT (QTc) for rate‑adjusted comparison; raw QT can be misleading at tachycardia. Hyperacute T vs. Normal Tall T – Hyperacute T is unusually peaked and transient; a tall but smooth T in a healthy young adult is usually benign early repolarization. ---