How to Interpret an ECG

The Cardiac Conduction System (foundation)

The ECG Waveforms

Systematic Approach - 14 Parameters (Harrison's)

1. Standardization and Technical Features

• Standard calibration: 1 mV = 10 mm (2 large squares vertically)
• Paper speed: 25 mm/s - 1 small box = 40 ms; 1 large box = 200 ms
• Check for lead placement errors and artifact

2. Rhythm

• Is it regular or irregular?
• Is there a P wave before every QRS and a QRS after every P?
• Normal sinus rhythm: P wave in lead II positive, rate 60-100 bpm

3. Heart Rate

• Large box method: 300 ÷ number of large boxes between R waves (e.g., 2 boxes = 150, 3 = 100, 4 = 75, 5 = 60)
• Small box method: 1500 ÷ number of small boxes between R waves
• Normal: 60-100 bpm; Bradycardia: <60; Tachycardia: >100

4. PR Interval

• Normal: 120-200 ms (3-5 small boxes)
• Short PR (<120 ms): pre-excitation (WPW), AV junctional rhythm
• Long PR (>200 ms): 1st degree AV block
• Progressively lengthening PR until dropped QRS: 2nd degree AV block (Mobitz I/Wenckebach)
• Fixed PR with randomly dropped QRS: 2nd degree AV block (Mobitz II)
• No relationship between P and QRS: 3rd degree (complete) AV block

5. QRS Interval (Duration)

• Normal: ≤100-110 ms (≤2.5 small boxes)
• Wide QRS (>120 ms): bundle branch block, ventricular rhythm, hyperkalemia, drug toxicity (e.g., flecainide, TCAs)

6. QT/QTc Interval

• Varies with heart rate - always correct for rate (QTc)
• Framingham formula: QTc = QT + 0.154 × (1000 - RR) [in ms]
• Upper normal limits: ≤460 ms in women, ≤450 ms in men
• Prolonged QT: hypokalemia, hypomagnesemia, hypocalcemia, drugs (amiodarone, quinidine, antipsychotics), congenital long QT syndrome - risk of torsades de pointes

7. Mean QRS Electrical Axis

• Normal axis: -30° to +90°
• Left axis deviation (LAD) < -30°: left anterior fascicular block, inferior MI, LVH
• Right axis deviation (RAD) > +90°: right ventricular hypertrophy, left posterior fascicular block, PE, lateral MI
• Quick method: If lead I is positive and aVF is positive = normal axis. If lead I positive and aVF negative = LAD. If lead I negative and aVF positive = RAD.

8. P Waves

• Normal: <120 ms wide, <2.5 mm tall, upright in leads I, II, V4-V6, inverted in aVR
• Tall peaked P in II (P pulmonale, >2.5 mm): right atrial enlargement
• Broad notched P in II (P mitrale, >120 ms): left atrial enlargement
• Absent P waves: atrial fibrillation, flutter, junctional rhythm

9. QRS Voltages

• LVH criteria (Sokolow-Lyon): S in V1 + R in V5 or V6 >35 mm; or R in aVL >11 mm
• Low voltage (<5 mm in all limb leads): pericardial effusion, cardiac tamponade, obesity, emphysema, hypothyroidism

10. Precordial R-Wave Progression

• R wave should grow from V1 to V5 (small in V1, tall in V5-V6)
• Poor R-wave progression (dominant S to V4 or beyond): anterior MI, LBBB, RVH
• Transition zone (R = S) normally at V3 or V4

11. Abnormal Q Waves

• Pathological Q wave: >40 ms wide OR >25% of the R wave height in the same lead
• Location indicates territory of infarction:
  • Q in II, III, aVF = inferior MI (RCA territory)
  • Q in V1-V4 = anterior MI (LAD territory)
  • Q in I, aVL, V5-V6 = lateral MI (LCx territory)

12. ST Segments

• Should be at baseline (isoelectric)
• ST elevation: acute STEMI, pericarditis (diffuse saddle-shaped), Prinzmetal angina, early repolarization, LV aneurysm
• ST depression: subendocardial ischemia/NSTEMI, posterior MI (V1-V2), LVH strain pattern, digoxin effect
• The J point separates QRS from ST - ST is measured 60-80 ms after J point

13. T Waves

• Should be upright in I, II, V3-V6; inverted in aVR, V1
• Hyperacute T waves (tall, symmetrical): earliest sign of STEMI
• Deep symmetrical T inversions: myocardial ischemia, Wellens sign (critical LAD stenosis when in V1-V4)
• Peaked narrow T waves: hyperkalemia
• Flat/inverted T: ischemia, LVH, bundle branch block, electrolyte abnormalities

14. U Waves

• Small positive deflections after T wave, best seen in V2-V3
• Prominent U waves: hypokalemia, bradycardia, class IA antiarrhythmics
• Negative U waves: ischemia, hypertension, LVH

The 12 Leads at a Glance

Common Patterns and Their Meaning

Summary Mnemonic: "R-RATE-P-PR-QRS-QT-AXIS-ST-T"

1. R - Rhythm (regular/irregular, P before QRS?)
2. Rate - Calculate bpm
3. P - P wave morphology
4. PR - PR interval (120-200 ms)
5. QRS - Duration (<110 ms), axis, voltages, Q waves, R progression
6. QT - Rate-corrected QTc (<450 ms men, <460 ms women)
7. ST - Elevation or depression
8. T - T wave morphology
9. U - U waves