Systematic Approach to ECG Interpretation

1. Standardization and Technical Features

• Calibration: Standard is 10 mm/mV amplitude and 25 mm/sec paper speed
• Lead placement: Limb lead reversal is a common cause of pseudo-axis deviation or pseudo-STEMI
• Artifacts: Movement, electrode contact, and pacemaker interference can mimic arrhythmias or conduction defects (Braunwald's illustrates pseudo-VT and pseudo-pacemaker failure as pitfalls)

2. Heart Rate

• Rate by squares: Count large boxes between R waves, divide 300 by that number (e.g., 3 large boxes = 300/3 = 100 bpm)
• Count QRS complexes x 6: Count all QRS complexes on the 10-second strip and multiply by 6 - useful for irregular rhythms

3. Rhythm

• A P wave before every QRS
• A QRS after every P wave
• Regular P-P and R-R intervals
• P wave upright in leads I and II (confirming sinus origin)

• Narrow vs. wide QRS (< or > 100 ms)
• Regular vs. irregular

4. PR Interval / AV Conduction

• Normal: 120-200 ms (3-5 small boxes)
• Short PR (< 120 ms): Pre-excitation (WPW), junctional rhythm
• Long PR (> 200 ms): First-degree AV block
• Progressively lengthening PR then dropped beat: Second-degree AV block, Mobitz I (Wenckebach)
• Constant PR with randomly dropped beats: Second-degree AV block, Mobitz II
• No consistent PR relationship: Third-degree (complete) AV block

5. QRS Interval

• Normal: 60-100 ms (< 2.5 small boxes)
• Borderline: 100-120 ms (incomplete bundle branch block)
• Wide QRS (> 120 ms): Bundle branch block, ventricular paced rhythm, pre-excitation, hyperkalemia

6. QT / QTc Interval

• Normal QTc: < 440 ms in men, < 460 ms in women (Bazett's formula: QT / √RR in seconds)
• Prolonged QTc predisposes to Torsades de Pointes (TdP)

7. Mean QRS Electrical Axis

8. P Waves

• Normal: Upright in I and II; biphasic in V1; duration < 120 ms; amplitude < 2.5 mm
• Broad, notched P in II (P mitrale): Left atrial enlargement (LAE) - seen in mitral stenosis
• Tall, peaked P in II > 2.5 mm (P pulmonale): Right atrial enlargement - seen in COPD, pulmonary hypertension
• Absent P waves: AF, junctional rhythm
• Retrograde P waves (inverted in II): Junctional or low atrial rhythms

9. QRS Voltage / Hypertrophy

• Low voltage (< 5 mm in all limb leads or < 10 mm in all precordial leads): Pericardial effusion, cardiac tamponade, obesity, emphysema, hypothyroidism, amyloidosis
• LVH criteria (e.g., Sokolow-Lyon): S in V1 + R in V5 or V6 > 35 mm; commonly associated with ST/T wave strain pattern
• RVH: Tall R in V1, right axis deviation, right ventricular strain

10. Precordial R-Wave Progression

• Normally R waves increase from V1 to V5/V6
• Poor R-wave progression (PRWP) - R wave < 3 mm through V1-V3: Anterior MI, LBBB, LVH, normal variant
• Transition zone: Normally R > S by V3-V4; early transition suggests posterior MI; late transition seen with PRWP

11. Abnormal Q Waves

• Pathological Q wave: Width ≥ 40 ms (1 small box) OR depth ≥ 25% of the R wave in the same lead
• Present in ≥ 2 contiguous leads in a vascular territory = prior MI (the territory helps localize):

• Septal Q waves (small q in I, aVL, V5-V6) are normal

12. ST Segments

• Normal: Isoelectric (flat), may have < 1 mm elevation in early repolarization (convex)
• ST elevation (≥ 1 mm in limb leads, ≥ 2 mm in precordial leads, in 2 contiguous leads) - causes:
  • STEMI (convex/tombstone elevation - emergency)
  • Pericarditis (saddle-shaped, diffuse)
  • Myocarditis
  • Left ventricular aneurysm
  • Brugada pattern (V1-V2)
  • Hyperkalemia
  • Early repolarization (benign, concave)
• ST depression - causes:
  • NSTEMI / unstable angina (ischemia)
  • Reciprocal changes in STEMI
  • Digoxin effect (reverse tick or scooped pattern)
  • LVH or LBBB strain pattern

13. T Waves

• Normal: Upright in I, II, V3-V6; inverted in aVR; variable in III, aVL, V1-V2
• T-wave inversion - broad differential (Kaplan & Sadock):
  • Ischemia/NSTEMI (new, dynamic inversions)
  • PE ("right heart strain" pattern: S1Q3T3 + anterior T inversions V1-V4)
  • Subarachnoid hemorrhage (diffuse deep symmetric inversions)
  • Myocarditis / pericarditis
  • RBBB or LVH (strain)
  • Hypokalemia, hypercalcemia
  • Drug effects (antipsychotics, lithium)
• Peaked T waves (tall, symmetrical): Hyperkalemia, hyperacute T waves in early MI
• T-wave alternans: Alternating T-wave morphology = electrical instability, risk of life-threatening arrhythmia

14. U Waves

• Normal: Small positive deflection after T wave, best seen in V2-V3; same direction as T
• Prominent U waves: Hypokalemia, bradycardia, drugs (quinidine, amiodarone)
• Inverted U waves: Ischemia, hypertension, LVH
• Large U waves that merge with T can cause apparent QT prolongation (actually QU prolongation)

Summary: The 14-Step ECG Checklist

1. Calibration and technical features
2. Rhythm (regular/irregular, P wave relationship to QRS)
3. Heart rate
4. PR interval
5. QRS duration
6. QTc interval
7. Electrical axis
8. P wave morphology
9. QRS voltage
10. R-wave progression (V1-V6)
11. Pathological Q waves
12. ST segments
13. T waves
14. U waves

• Harrison's Principles of Internal Medicine 22E (2025), Ch. 247 - Electrocardiography
• Kaplan & Sadock's Comprehensive Textbook of Psychiatry, Ch. 27.2 - Electrophysiology and ECG Interpretation
• Braunwald's Heart Disease 15E, Ch. 61 - Long-term Electrocardiography
• Tintinalli's Emergency Medicine 9E - ECG in chest pain evaluation