ECG Interpretation Mastery: From Basics to Advanced

Master ECG interpretation with our systematic approach. Learn to identify normal variants, ischemic changes, arrhythmias, and subtle findings that matter.

ECG interpretation is a fundamental skill for every medical professional. A systematic approach will help you identify both common and subtle abnormalities.

The Systematic Approach

Follow this 10-step checklist for every ECG:

1. Rate and Rhythm

Normal rate: 60-100 bpm
Regular vs. irregular: Check R-R intervals
P waves: Present, upright in lead II?

2. Axis

Axis	Lead I	Lead aVF	Interpretation
Normal	+	+	-30° to +90°
Left Axis Deviation	+	-	-30° to -90°
Right Axis Deviation	-	+	+90° to +180°
Extreme Axis	-	-	-90° to -180°

3. Intervals

PR interval: 120-200 ms (3-5 small boxes)
QRS duration: < 120 ms (3 small boxes)
QTc: < 450 ms (men), < 460 ms (women)

4. Hypertrophy

Left Ventricular Hypertrophy (LVH):

S in V1 + R in V5/V6 > 35 mm
Strain pattern in lateral leads

Right Ventricular Hypertrophy (RVH):

R > S in V1-V2
RAD with right atrial enlargement

5. Ischemia and Infarction

ST Elevation Myocardial Infarction (STEMI)

Territory	Leads with ST Elevation	Culprit Vessel
Anterior	V1-V4	LAD
Inferior	II, III, aVF	RCA
Lateral	I, aVL, V5-V6	LCx
Posterior	V7-V9 (reciprocal)	RCA/LCx

NSTEMI vs Unstable Angina

NSTEMI: Troponin positive, ST depression/T-wave changes
Unstable angina: Troponin negative, ischemic symptoms

6. Arrhythmias

Supraventricular Arrhythmias

Sinus bradycardia: Rate < 60, normal P waves
Sinus tachycardia: Rate > 100, normal P waves
Atrial fibrillation: Irregularly irregular, no P waves
Atrial flutter: Sawtooth pattern in II, III, aVF

Ventricular Arrhythmias

PVCs: Wide QRS, early, compensatory pause
V-tach: Wide complex tachycardia, hemodynamic instability
V-fib: Chaotic, no organized activity

7. Conduction Blocks

Bundle Branch Blocks

RBBB: rsR’ in V1-V2, wide S in I, V6
LBBB: Broad notched R in I, aVL, V5-V6

AV Blocks

First degree: PR > 200 ms
Second degree Mobitz I: Progressive PR prolongation
Second degree Mobitz II: Constant PR with dropped QRS
Third degree: Complete AV dissociation

High-Yield ECG Findings

Brugada Syndrome

Coved ST elevation in V1-V2
Type 1: > 2mm ST elevation with T-wave inversion
Risk of sudden cardiac death

Hypokalemia

Flattened T waves
U waves
ST depression
QT prolongation

Pulmonary Embolism

Sinus tachycardia (most common)
S1Q3T3 pattern
Right heart strain
RBBB

Clinical Correlation

Always correlate ECG findings with:

Clinical presentation: Symptoms, vital signs
History: Prior ECGs, cardiac history
Laboratory data: Troponin, electrolytes
Imaging: Echo, CXR

Practice Tips

Read 10 ECGs daily: Pattern recognition improves with repetition
Compare with normal: Know what normal looks like
Use the checklist: Don’t skip steps
Ask for help: When uncertain, consult cardiology

Resources

Clinicapedia ECG Study Cards: Visual recognition cards
LITFL ECG Library: Free online resource
ECG Wave-Maven: Case-based learning

Conclusion

ECG interpretation is a skill that improves with deliberate practice. Use the systematic approach outlined here, and you’ll develop confidence in identifying both common and rare findings.

Want more practice? Check out our ECG resources →