Why ECGs matter: ECGs translate cardiac electricity into patterns you can learn. Start every trace the same way: calibration → rate → rhythm → axis → intervals → waveforms/segments → compare with old.

Normal ECG example 1 — Normal ECG (paper speed 25 mm/s, calibration 10 mm/mV). Always confirm speed & gain first. 📏

Normal ECG example 2 — Another normal trace showing good R-wave progression (V1→V6). 📈

✅ Normal ECGs

Normal ECG example 3 — Sinus rhythm, narrow QRS, normal intervals, concordant T waves in precordials.

Normal ECG example 4 — Axis within normal range (about 0° to +60°). Lead I and aVF both positive. ➕➕

Normal ECG example 5 — Small septal Q waves in I/aVL/V5–V6 (physiological), *not* pathologic. ✔️

Normal ECG example 6 — Nice R/S transition around V3–V4. “Transition zone” ≈ equal R & S. 🔄

Normal ECG example 7 — Sinus arrhythmia (respiratory) can be normal in young/fit patients. 🧘

Annotated normal ECG. Sinus rhythm 60–100 bpm (this one ≈82), normal PR/QRS/QT, isoelectric ST.

🧭 The 7-Step ECG Reading Method (teach & test)

Calibration & quality 📏 - Confirm 25 mm/s and 10 mm/mV; check artefact and lead placement (V1–V6 positions).
Rate ⏱️ - Regular rhythm: 300-150-100-75-60-50 (big-box method). Irregular: count complexes in 10 s strip ×6.
Rhythm 🔄 - P before every QRS? Constant PR? Narrow vs wide QRS? Sinus = upright P in I/II/aVF, negative in aVR.
Axis 🧭 - Normal ≈ −30° to +90°. Quick check: Lead I & aVF both positive → normal; I positive / aVF negative → left axis; I negative / aVF positive → right axis.
Intervals 📐 - PR 120–200 ms; QRS ≤120 ms; QTc <440 ms (men), <460 ms (women). Use Bazett (QT/√RR) or Fridericia (QT/∛RR) if brady/tachy.
Waves/segments 🌊 - P (morphology, P pulmonale/mitrale), QRS (hypertrophy, bundle blocks), ST (↑/↓), T (inversions, hyperacute), U waves (hypokalaemia).
Compare 🗂️ - With prior ECGs and clinic context (pain, K+, troponin, meds like digoxin). UK practice: document “ECG unchanged vs prior”.

📏 Normal Ranges (quick reference)

Feature	Normal	Notes
P wave	≤110 ms, ≤2.5 mm height	Upright I/II/aVF, negative aVR; V1 often biphasic.
PR interval	120–200 ms	Short PR: pre-excitation; long PR: AV block 1°.
QRS duration	≤120 ms	Look for RBBB/LBBB patterns, delta waves.
QRS voltage	≥5 mm limb lead OR ≥10 mm precordial	High voltage ≠ always LVH; check criteria + repolarisation.
QTc	<440 ms (M), <460 ms (F)	Correct for rate; avoid Bazett in extremes (consider Fridericia).
Axis	−30° to +90°	Age and body habitus influence axis.

🧪 Pattern recognition (must-know)

Hyperkalaemia 🔺K⁺ - Tall, peaked T; PR prolongation; QRS widening → sine-wave. Urgent calcium gluconate.
Hypokalaemia 🔻K⁺ - Flat T, U waves, prolonged QT → torsades risk.
Pericarditis ♨️ - Diffuse concave ST↑ with PR↓; reciprocal PR↑ in aVR; no localised reciprocal ST↓.
STEMI ❤️‍🔥 - J-point ST↑ in contiguous leads (limb ≥1 mm; V2–V3 sex/age cut-offs). Look for reciprocal ST↓. New LBBB with ischaemic symptoms → treat as STEMI.
Wellens’ ⚠️ - Deep/biphasic T in V2–V3 pain-free → critical LAD; avoid stress testing; cardiology review.
Brugada 🧬 - Coved ST↑ V1–V3 with RBBB-like pattern → refer; fever can unmask.
PE 🫁 - Sinus tachy, S1Q3T3, RBBB/Right axis; not sensitive-use clinical pre-test + D-dimer/CTPA.

🧠 AV & Bundle Branch Blocks (pattern drill)

Block	ECG	Pearl
1° AV	PR >200 ms, every P conducts	Often benign; watch with β-blockers, CCB, digoxin.
Mobitz I (Wenckebach)	PR progressively ↑ then drop	Usually AV-nodal; often transient (e.g., inferior MI).
Mobitz II	Fixed PR with dropped QRS	Infranodal; risk of complete heart block → pacing.
3° AV (complete)	AV dissociation; escape rhythm	Unstable → atropine/chronotrope; likely pacemaker.
RBBB	V1 rsR′, wide S in I/V6	May be normal variant; consider PE/ischaemia.
LBBB	Broad/notched R in I/V5–6, absent Q in lateral leads	Mask ischaemia; use clinical picture (Sgarbossa/Smith-mod).

🛠️ Quick how-to: calculate rate

Regular: 300/150/100/75/60/50 big-box method (R-R distance).
Irregular: Count QRS in 10-second strip ×6 (or 6-second hash marks ×10).

🩺 UK exam & practice tips

Document calibration, rate, rhythm, axis, intervals, abnormalities, and comparison: “ECG: 25 mm/s, 10 mm/mV; SR 78; normal axis; PR 160 ms; QRS 90 ms; QTc 420 ms; no ST-T changes; unchanged from 2023.”
STEMI criteria follow UK pathways (J-point thresholds + symptoms). Don’t miss posterior MI (V7–V9) if V1–V3 ST↓ with tall R. 🚩
Always interpret ECG in clinical context-ECG cannot “rule out” MI without symptoms/troponin. 🔬

🧩 Common pitfalls

Misplaced chest leads → fake septal infarct (poor R-wave progression). Re-attach V1 at 4th ICS right sternal edge, V4 at 5th ICS mid-clavicular, etc.
Calling early repolarisation “STEMI”: look for reciprocal changes, clinical pain, and evolving troponins.
Using Bazett at extremes of rate-prefer Fridericia for tachy/brady to avoid over- or under-correcting QT.

🧪 Mini-cheat card (normal values)

PR 120–200 ms • QRS ≤120 ms • QTc <440 ms (M), <460 ms (F) • Axis −30°→+90° • R-wave progression V1→V6 with transition V3–V4 • ST isoelectric (≤1 mm variation; see sex/lead cut-offs).

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ECG - Normal ECG basics

🫀 Introduction to ECGs