ECG (Electrocardiogram) - Complete NEET PG Guide

1. What is an ECG?

• Standard paper speed: 25 mm/sec
• Standard calibration: 10 mm = 1 mV
• Grid: Small box = 1 mm = 0.04 sec; Large box = 5 mm = 0.2 sec
• 5 large boxes = 1 second

2. ECG Waves and Their Meaning

3. The Systematic 7-Step Approach to ECG

Step 1 - RATE

• Regular rhythm: Rate = 300 / (number of large boxes between R-R)
• Memory trick for large-box count: 300 - 150 - 100 - 75 - 60 - 50 (for 1, 2, 3, 4, 5, 6 large boxes)
• Irregular rhythm: Count QRS complexes in a 10-second strip and multiply by 6
• Normal: 60-100 bpm | Tachycardia: >100 | Bradycardia: <60

Step 2 - RHYTHM

• Is it regular or irregular?
• Is there a P wave before every QRS? Is there a QRS after every P?
• Normal sinus rhythm: P wave upright in II, inverted in aVR, followed by QRS

Step 3 - AXIS

• Normal axis: -30° to +90°
• Quick method: If QRS is positive in lead I and positive in aVF → Normal axis
• Left axis deviation (LAD): Positive I, negative aVF (left anterior hemiblock, LVH, inferior MI)
• Right axis deviation (RAD): Negative I, positive aVF (RVH, PE, lateral MI)
• Extreme axis: Negative in both I and aVF

Step 4 - P WAVE

• Morphology, duration, axis
• P pulmonale: Peaked P >2.5 mm in II (RA enlargement)
• P mitrale: Broad notched P >120 ms (LA enlargement)

Step 5 - PR INTERVAL

• Short PR (<120 ms): WPW syndrome, LGL syndrome, junctional rhythm
• Long PR (>200 ms): First-degree AV block

Step 6 - QRS COMPLEX

• Duration >120 ms = bundle branch block or ventricular rhythm
• Pathological Q wave: >40 ms wide OR >25% of R wave height = old MI
• LBBB: Broad QRS, no septal q waves, RSR' (M shape) in V5/V6, deep S in V1
• RBBB: RSR' (rabbit ears) in V1, deep S in I, V5, V6

Step 7 - ST SEGMENT AND T WAVE

• ST elevation: STEMI, pericarditis, Brugada, early repolarization
• ST depression: NSTEMI, ischemia, digoxin effect, LVH strain
• T wave inversion: Ischemia, RBBB, PE (V1-V4), Wellens syndrome
• Peaked T waves: Hyperkalemia

4. AV Block Classification (High-Yield Table)

5. Most Commonly Asked NEET PG ECG Topics

A. Myocardial Infarction ECG Changes

• Anterior MI (LAD): ST elevation V1-V4
• Inferior MI (RCA): ST elevation II, III, aVF; reciprocal ST depression in I, aVL
• Lateral MI (LCx): ST elevation I, aVL, V5-V6
• Posterior MI: ST depression V1-V3 + tall R in V1 (mirror image); confirm with V7-V9
• RV MI (proximal RCA): ST elevation in V4R; associated with inferior MI

B. Atrial Fibrillation (AF)

• Irregularly irregular rhythm - no two R-R intervals equal
• No P waves - replaced by fibrillatory (f) baseline
• Narrow QRS (usually) unless aberrant conduction or WPW
• Classic association: Mitral stenosis, thyrotoxicosis, hypertension
• NEET PG pearl: AF + very fast irregular wide-complex tachycardia = WPW + AF (pre-excited AF) - do NOT give AV nodal blockers (digoxin, verapamil, adenosine)

C. Ventricular Tachycardia (VT) vs. SVT with Aberrancy

D. Hyperkalemia ECG Progression (Sequential Changes)

1. Peaked (tented) T waves - earliest
2. PR prolongation + P wave flattening
3. Wide QRS complex
4. Sine wave pattern - pre-terminal
5. VF/Asystole

E. Wolff-Parkinson-White (WPW) Syndrome

• Short PR (<120 ms)
• Delta wave (slurred upstroke of QRS)
• Wide QRS (>120 ms)
• Secondary ST-T changes
• Type A WPW: Positive delta in V1 (left accessory pathway)
• Type B WPW: Negative delta in V1 (right accessory pathway)

F. Bundle Branch Blocks

• QRS >120 ms
• RSR' pattern in V1 (M pattern)
• Broad S waves in I, V5, V6
• Causes: RV strain, PE, ASD, normal variant

• QRS >120 ms
• Broad R, no septal q, no normal R progression V1-V3
• RSR' (M shape) in I, aVL, V5, V6
• NEET pearl: New LBBB + chest pain = STEMI equivalent; treat as STEMI

G. Pericarditis ECG Changes

• Saddle-shaped (concave upward) ST elevation in multiple leads (diffuse, not regional)
• PR depression in multiple leads (most specific finding)
• No reciprocal ST depression (unlike STEMI)
• No Q waves
• Stage IV: T wave normalization

H. Long QT Syndrome

• QTc >440 ms (men), >460 ms (women)
• Risk of Torsades de Pointes (TdP)
• Causes: Congenital (Romano-Ward, Jervell-Lange-Nielsen), drugs (amiodarone, sotalol, haloperidol, erythromycin, fluoroquinolones), hypokalemia, hypomagnesemia
• TdP ECG: Twisting of QRS axis around isoelectric line

I. Pulmonary Embolism (PE) - S1Q3T3

• Sinus tachycardia (most common ECG finding in PE)
• S1Q3T3: Deep S in lead I, Q wave in III, inverted T in III
• RBBB (incomplete or complete)
• Right axis deviation
• T inversion V1-V4 (right heart strain pattern)
• NEET pearl: S1Q3T3 is classic but rare; sinus tachycardia is actually the most common finding

J. Brugada Syndrome

• Coved-type ST elevation in V1-V3 (≥2 mm downsloping ST elevation + T wave inversion)
• RBBB pattern
• Risk of sudden cardiac death (SCD) from VF
• Autosomal dominant (SCN5A mutation)
• Unmasked by: fever, flecainide test, sodium channel blockers

6. Normal ECG Intervals - Quick Reference Table

7. Quick Memory Tricks for NEET PG

• Rate rule: 300-150-100-75-60-50 for 1-6 large boxes
• Axis in 4 steps: Lead I (+) and aVF (+) = Normal; I(+) aVF(-) = LAD; I(-) aVF(+) = RAD; both negative = Extreme
• LBBB vs RBBB in V1: LBBB = W (rS), RBBB = M (RSR')
• Wenckebach = Benign, Mobitz II = Dangerous
• WPW triad: Short PR + Delta wave + Wide QRS
• Hyperkalemia: T peaks first, then P disappears, then QRS widens, then sine wave
• AF: Irregularly irregular + no P waves
• PE: Sinus tachycardia is most common; S1Q3T3 is classic but rare
• Pericarditis vs STEMI: Saddle-shaped vs convex ST elevation; PR depression in pericarditis; no reciprocal changes

8. NEET PG Image-Based ECG Questions - What to Expect

1. STEMI with territory identification - "ECG shows ST elevation in II, III, aVF - which artery?"
2. Complete heart block - AV dissociation with slow ventricular rate
3. Atrial fibrillation - irregular rate, no P waves
4. VT vs SVT - wide complex tachycardia differential
5. WPW - delta wave + short PR
6. Hyperkalemia - peaked T waves or sine wave
7. LBBB/RBBB - RSR' pattern identification
8. PE - S1Q3T3 pattern
9. Brugada - coved ST elevation V1-V3
10. Pericarditis - diffuse saddle-shaped ST elevation + PR depression