💡 ECGs for the MLA Medical students should be able to recognise key ECG patterns in acute and chronic disease, linking them to underlying pathology and urgent management steps.

1. Basics of ECG

An ECG (electrocardiogram) records the heart's electrical activity. It uses 10 electrodes to produce 12 leads, each giving a different perspective of the heart.

📌 Key Points:

Leads I, II, III → Limb leads (frontal plane)
aVR, aVL, aVF → Augmented limb leads
V1–V6 → Chest leads (horizontal plane)
Each lead “views” the heart from a different angle

2. Systematic ECG Approach

Step 1: Rate 🏃‍♂️

Normal sinus rhythm: 60–100 bpm. Quick method: 300 ÷ number of large squares between R waves.

Step 2: Rhythm 🧭

Check if rhythm is regular or irregular
Look for P waves: Are they present and consistent?
PR interval: Normal 120–200 ms (3–5 small squares)

Step 3: Axis 📐

Determine QRS axis in frontal leads:

Normal: -30° to +90°
Left axis deviation: -30° to -90°
Right axis deviation: +90° to +180°

Step 4: Intervals ⏱️

PR interval: 120–200 ms
QRS duration: < 120 ms
QT interval: Rate-corrected using Bazett's formula (QTc)

Step 5: Waveform Morphology 🔍

P wave → Atrial depolarization
QRS complex → Ventricular depolarization
T wave → Ventricular repolarization
ST segment → Look for elevation/depression (ischemia or infarction)
U wave → Often seen in hypokalemia

3. Common ECG Patterns

Condition	ECG Features	Teaching Tip
Sinus Bradycardia	Rate < 60 bpm, normal P-QRS-T	Usually benign unless symptomatic
Sinus Tachycardia	Rate > 100 bpm, normal morphology	Look for underlying cause: fever, hypovolemia, pain
AF (Atrial Fibrillation)	Irregularly irregular, no distinct P waves, fibrillatory baseline	Check for thromboembolic risk (CHA₂DS₂-VASc)
STEMI	ST elevation in contiguous leads, reciprocal changes	Correlate with clinical chest pain → urgent reperfusion
Bundle Branch Block	QRS ≥120 ms, specific V1/V6 patterns for RBBB/LBBB	Can mask MI; recognise morphology carefully

4. Practical Tips 🧠

Always correlate ECG with clinical context
Check patient identity, calibration, and lead placement
Compare with previous ECGs for changes
Look for electrolyte disturbances and drug effects

🫀 Introduction to ECGs

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. 🔬

A normal ECG shows: - Regular sinus rhythm - Appropriate rate, axis, intervals, voltages, and morphology - No pathological Q waves, ST/T changes, conduction defects, hypertrophy, or arrhythmia Normal ranges are for resting adults (≥16 y); variations exist in children, athletes, pregnancy, ethnicity, body habitus.

Systematic Interpretation Checklist

Rate: 60–100 bpm
Rhythm: Sinus (P before every QRS, consistent PR, upright P in II)
Axis: –30° to +90° (normal); physiological left axis –30° to –90° common
Intervals: PR 120–200 ms, QRS ≤110 ms (≤120 ms acceptable), QTc ≤440 ms (men), ≤460 ms (women)
P waves: Upright I/II/aVF, ≤2.5 mm amplitude, ≤120 ms duration
QRS voltage & morphology: Normal progression (small R V1 → large R V5/V6), no pathological Q waves
ST segment: Isoelectric or minimal elevation/depression (<1 mm)
T waves: Concordant with QRS, upright I/II/V3–V6, inverted aVR
U waves: Absent or small (<1 mm) in V2–V3

Detailed Normal Values & Features

Parameter	Normal Range (Adults)	Notes / Variations
Rate	60–100 bpm	Sinus bradycardia <60 (common in athletes); tachycardia >100
Rhythm	Sinus rhythm	P wave before QRS, PR constant, P positive in II
P wave	Duration ≤120 ms Amplitude ≤2.5 mm (limb leads)	Upright I, II, aVF; biphasic V1 (positive then negative); no notching
PR interval	120–200 ms	Shorter in young/athletes; longer in elderly (up to 220 ms still normal)
QRS duration	≤110 ms (≤120 ms acceptable)	Narrow complex; no BBB, WPW (delta wave), or fascicular block
QRS axis	–30° to +90°	Left axis deviation –30° to –90° physiological (obesity, pregnancy, ascites)
QRS voltage	Limb: R ≤20 mm, S ≤15 mm Precordial: R V5/V6 ≤27 mm, S V1/V2 ≤30 mm	No low voltage (<5 mm limb leads) or high voltage (LVH criteria)
R-wave progression	Small R V1 → transition V3–V4 → tall R V5/V6	Poor progression can be normal in obesity/COPD
ST segment	Isoelectric (±0.5 mm)	Early repolarisation: concave upward elevation (1–3 mm) in V2–V5 common in young males
T wave	Upright I, II, V3–V6; inverted aVR	Concordant with QRS; amplitude <5 mm limb, <10 mm precordial; no deep inversion
QTc (Bazett)	≤440 ms men, ≤460 ms women	Upper limit ~450–470 ms; shorter in hypercalcaemia, longer in drugs/hypokalaemia
U wave	Absent or small (<1 mm) after T in V2–V3	Prominent U waves abnormal (hypokalaemia, drugs)

Normal Variants (Not Abnormal)

Early repolarisation: Concave ST elevation (1–4 mm) in V2–V5, notched/slurred J point, tall T waves – common in young, black males, athletes
Sinus arrhythmia: Rate variation with respiration (increases on inspiration) – normal in young/athletes
Persistent juvenile T-wave pattern: T inversion V1–V3 (sometimes V4) – normal in young adults, especially women
Wandering atrial pacemaker: Slight P-wave morphology change with rate variation – benign
Borderline axis: –30° to –45° or +90° to +110° – often normal

Common Pitfalls & When to Call Abnormal

Pathological Q waves: ≥0.04 s wide or ≥25% R-wave depth in ≥2 contiguous leads
ST elevation: ≥1 mm limb leads or ≥2 mm precordial (men <40 y), ≥1.5 mm women – consider STEMI/early repolarisation
ST depression: ≥0.5 mm horizontal/downsloping – ischaemia
Left axis deviation < –30° or right > +90° – consider conduction defect
Prolonged QTc >470–500 ms – risk of TdP
Low voltage + electrical alternans – pericardial effusion

Clinical Pearls (Exam/OSCE/MCQ)

Report systematically: "Rate, rhythm, axis, intervals, ST/T changes, voltage, progression"
Always compare to previous ECGs if available
Normal ECG does NOT exclude acute ischaemia (up to 20–30% NSTEMI have normal ECG)
Athletes often have sinus bradycardia, early repolarisation, high voltages – physiological
Ethnic variations: Higher voltage and early repolarisation more common in black individuals
Age: PR/QRS slightly longer in elderly; T inversion V1–V2 more common in women

Sources (2025–2026)

AHA/ACC/HRS: ECG Standardization & Interpretation (2009, reaffirmed 2025)
ESC Guidelines: Acute Coronary Syndromes (2023–2025)
Life in the Fast Lane ECG Library (updated 2025)
Marriott's Practical Electrocardiography (13th ed.)
UpToDate: ECG Interpretation (2026)

NORMAL

🧩 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).

❤️ Acute Coronary Syndromes

STEMI: Localised ST elevation in ≥2 contiguous leads ± reciprocal ST depression.
NSTEMI/Unstable angina: ST depression, T-wave inversion, or normal ECG with symptoms.
Hyperacute T waves: Earliest sign of MI, tall peaked T-waves before ST elevation.
Old MI: Pathological Q waves (deep, >40 ms wide).

⚡ Arrhythmias

Atrial fibrillation (AF): Irregularly irregular rhythm, absent P waves, fibrillatory baseline.
Atrial flutter: Saw-tooth flutter waves, often with 2:1 block (ventricular rate ~150).
Supraventricular tachycardia (SVT): Narrow complex tachycardia, regular, often no visible P waves.
Ventricular tachycardia (VT): Broad complex tachycardia, regular, “capture/fusion” beats.
Ventricular fibrillation (VF): Chaotic irregular waveform, no organised complexes - cardiac arrest rhythm.
Torsades de pointes: Polymorphic VT with twisting QRS axis; associated with prolonged QT.

⏱️ Conduction Blocks

First-degree AV block: PR >200 ms, every P followed by QRS.
Second-degree AV block Mobitz I (Wenckebach): Progressive PR lengthening → dropped beat.
Second-degree AV block Mobitz II: Intermittent dropped QRS with fixed PR - risk of complete block.
Third-degree (complete) heart block: No relation between P waves and QRS; escape rhythm.
Bundle branch blocks: – LBBB: “WiLLiaM” - W in V1, M in V6. – RBBB: “MaRRoW” - M in V1, W in V6.

🧪 Electrolyte & Drug Effects

Hyperkalaemia: Tall tented T waves, flattened P waves, widened QRS → sine wave (pre-terminal).
Hypokalaemia: U waves, flattened T waves, prolonged QT.
Digoxin effect: Down-sloping ST depression (“reverse tick”), flattened T waves.

🩺 Other Must-Know Patterns

Pericarditis: Widespread concave ST elevation + PR depression.
STEMI mimics: LBBB, LVH strain pattern, pericarditis.
Wolff–Parkinson–White (WPW): Short PR, delta wave, widened QRS.
Long QT syndrome: QTc >440 ms (men) / >460 ms (women).
Pulmonary embolism: S1Q3T3 pattern (rare, non-specific); sinus tachycardia most common finding.

⚠️ Red Flag ECGs in Exams

STEMI → immediate reperfusion (PCI/thrombolysis).
VT/VF → cardiac arrest protocols (defibrillation).
Complete heart block with syncope → pacing needed.
Hyperkalaemia with ECG changes → IV calcium, urgent treatment.
Torsades de pointes → IV magnesium, stop QT-prolonging drugs.

📚 Teaching Pearls

Always read ECGs systematically: Rate, Rhythm, Axis, Intervals, Morphology, ST/T changes.
In MLA exams, you’re often asked: “What is the diagnosis?” or “What is the next step in management?”.
Focus on life-threatening and common patterns, not rare syndromes.

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ECG Reading & Interpretation MLA and Exams 🫀