How to Read an ECG: A Systematic Approach for Students and Residents
A systematic approach to ECG reading takes 60 seconds and catches everything. Students who skip the system and pattern-match get burned on the unusual presentation. Here's the method that holds up in every context.
Reviewed for clinical accuracy
Progress Note Editorial · Evidence-based
Updated July 2026
Aligned to current board blueprints
★★★★★4.8(244)
·9.1k students studied this·EM residents love this
The 60-second version
Rate: 300 divided by number of large boxes between R waves
Irregularly irregular = atrial fibrillation until proven otherwise
Atrial flutter typically presents at 150 bpm (300/min with 2:1 block)
Wellens syndrome (deep V2–V3 T inversions) = critical LAD disease — no stress test
Peaked T waves → wide QRS → sine wave = hyperkalemia emergency
1. Rate: count R waves in 6 seconds × 10, or 300/number of large boxes between R waves. 2. Rhythm: regular or irregular? 3. P waves: present, one before each QRS, upright in I and II? 4. PR interval: normal 120–200ms (3–5 small boxes). 5. QRS: normal <120ms. 6. Axis: normal −30° to +90°; left axis if aVF negative + I positive. 7. ST/T waves: elevation, depression, inversions. 8. QTc: corrected QT, normal <440ms men, <460ms women.
Rate and Rhythm Essentials
Regular tachycardia >100: sinus tach, SVT, atrial flutter (often 150 bpm with 2:1 block — look for flutter waves). Irregularly irregular: atrial fibrillation (no P waves, variable QRS intervals). Regular bradycardia <60: sinus brady, junctional rhythm, complete heart block (AV dissociation). Regularly irregular: second-degree AV block (Mobitz I = progressive PR lengthening; Mobitz II = fixed PR, dropped beats).
ST Changes — The High-Stakes Findings
ST elevation: STEMI (convex/tombstone elevation), pericarditis (diffuse saddle-shaped elevation, PR depression), early repolarization (concave elevation, notching at J point). ST depression: subendocardial ischemia, NSTEMI, posterior MI (ST depression in V1–V3 = look for posterior MI). T wave inversions: ischemia, LVH strain pattern, Wellens syndrome (deep symmetric T inversions in V2–V3 = critical LAD stenosis — do NOT stress test).
Classic Patterns Worth Memorizing
LBBB: wide QRS, 'WiLLiaM' pattern (W in V1, M in V5–V6). RBBB: wide QRS, 'MaRRoW' pattern (M in V1, W in V5–V6), rSR' in V1. LVH: Sokolow-Lyon criteria — S in V1 + R in V5 or V6 >35mm. Hyperkalemia: peaked T waves → PR prolongation → wide QRS → sine wave → VFib. Hypokalemia: flattened T waves, prominent U waves.
Test yourself
Quick check — 3 questions
1. An ECG shows an irregularly irregular rhythm with no discernible P waves and a ventricular rate of 110 bpm. Most likely diagnosis?
Explanation. Irregularly irregular rhythm with absent P waves = atrial fibrillation. AFib generates chaotic atrial activity at 350–600 per minute; only some impulses conduct through the AV node, producing the irregular ventricular response.
2. A patient has deep, symmetric T-wave inversions in V2–V3 without chest pain. This ECG pattern warrants:
Explanation. Deep symmetric T-wave inversions in V2–V3 (Wellens syndrome) indicate critical proximal LAD stenosis in a pain-free phase. Stress testing can precipitate STEMI. These patients need urgent cardiology evaluation and typically catheterization.
3. A regular rhythm at 150 bpm. P waves are not clearly visible. The most important next step in ECG analysis is:
Explanation. Atrial flutter classically presents at 150 bpm because the atria fire at ~300/min and 2:1 AV block lets every other beat through. Look carefully in II, III, aVF for the sawtooth flutter waves. Vagal maneuvers or adenosine can temporarily slow conduction to reveal the flutter pattern.
Frequently asked questions
What is the fastest way to estimate heart rate on an ECG?
For regular rhythms: count the number of large boxes between two R waves and divide 300 by that number. (1 box=300, 2=150, 3=100, 4=75, 5=60, 6=50.) For irregular rhythms (like AFib): count the number of QRS complexes in a 10-second strip (30 large boxes) and multiply by 6.
How do you differentiate STEMI from pericarditis on ECG?
Pericarditis: diffuse ST elevation in most leads (not just one territory), saddle-shaped (concave up) morphology, PR segment depression (especially in II and aVF), and reciprocal PR elevation in aVR. STEMI: focal territorial elevation (anterior, inferior, lateral), convex/tombstone morphology, reciprocal ST depression in the opposite territory.
When does a wide QRS require urgent action?
A wide QRS (>120ms) with hemodynamic instability is a medical emergency. Wide complex tachycardia should be treated as VTach until proven otherwise — never adenosine in WCT without confirming SVT with aberrancy. A widening QRS at rest can indicate hyperkalemia, sodium channel blocker toxicity, or LBBB/RBBB — each requiring different management.
Practice makes permanent
Lock this into long-term memory
Progress Note's adaptive engine spaces your reviews automatically — so this stays with you through boards and onto the wards.