Why ECG Interpretation Is Essential for Your OSCE
ECG interpretation appears in almost every UK medical school finals — as a standalone data interpretation station, as an examiner follow-up after a history, or as part of an emergency management scenario. Students who panic in front of an ECG do so because they have no system. A systematic approach means you will never panic again.
The goal is not to memorise every pattern. The goal is to have a consistent structure that finds the critical abnormality in under 60 seconds.
The 8-Step Systematic Approach
Always read ECGs in the same order, every time, without exception.
🧠 Mnemonic
RRAPQRST — the 8-step ECG system:
- Rate
- Rhythm
- Axis
- P waves
- PR interval
- QRS complex
- ST segment
- T waves
Step 1: Rate
For regular rhythms — the 300 rule:
300 ÷ number of large squares between two R waves
| Large squares between R waves | Heart rate |
|---|---|
| 1 | 300 bpm |
| 2 | 150 bpm |
| 3 | 100 bpm |
| 4 | 75 bpm |
| 5 | 60 bpm |
| 6 | 50 bpm |
For irregular rhythms: Count the QRS complexes in a 10-second strip and multiply by 6.
💡 Tip
Memorise the sequence 300, 150, 100, 75, 60, 50. This covers every clinically relevant heart rate. You will never need a calculator in an OSCE.
Step 2: Rhythm
Ask three questions:
- 1Are the R-R intervals regular?
- 2Is there a P wave before every QRS?
- 3Are all P waves the same shape?
| Finding | Diagnosis |
|---|---|
| Regular, P before every QRS, normal P morphology | Sinus rhythm |
| Irregularly irregular, no P waves, variable baseline | Atrial fibrillation |
| Saw-tooth baseline at 300 bpm, QRS at 150 bpm | Atrial flutter with 2:1 block |
| P waves present but unrelated to QRS | Complete (3rd degree) heart block |
| PR > 200ms (> 5 small squares), 1:1 relationship | 1st degree heart block |
| Progressively lengthening PR then dropped beat | Mobitz Type I (Wenckebach) |
| Fixed PR, intermittently dropped QRS | Mobitz Type II |
⚠️ Red Flag
Complete (3rd degree) heart block is a medical emergency. The atria and ventricles beat independently at their own intrinsic rates. Ventricular rate is typically 30–40 bpm. The patient may be haemodynamically compromised and may require transcutaneous pacing. This must be recognised and escalated immediately.
Step 3: Axis
Quick method:
- Lead I upright + aVF upright = Normal axis (–30° to +90°)
- Lead I upright + aVF downward = Left axis deviation (LAD)
- Lead I downward + aVF upright = Right axis deviation (RAD)
| Axis deviation | Common causes |
|---|---|
| Left axis deviation | Left anterior fascicular block, inferior MI, LVH |
| Right axis deviation | RVH, pulmonary hypertension, PE, lateral MI, RBBB |
Step 4: P Waves
Normal: upright in I, II, aVF; inverted in aVR
| Abnormality | Interpretation |
|---|---|
| Absent P waves | AF, junctional rhythm |
| Bifid P wave (M-shaped, especially in II) | Left atrial enlargement (P mitrale) — mitral stenosis |
| Tall peaked P wave (> 2.5mm in II) | Right atrial enlargement (P pulmonale) — cor pulmonale, pulmonary hypertension |
| P waves before QRS but inverted | Ectopic atrial or junctional rhythm |
Step 5: PR Interval
Normal: 120–200ms (3–5 small squares)
- Short PR (< 120ms): WPW syndrome (look for delta wave), LGL syndrome
- Long PR (> 200ms): 1st degree AV block (document, usually benign in isolation)
Step 6: QRS Complex
Width
- Normal: < 120ms (< 3 small squares)
- Broad QRS (≥ 120ms): bundle branch block, ventricular ectopic, hyperkalaemia, sodium channel blocker toxicity
Bundle Branch Block
💎 Clinical Pearl
The BBB quick rule — WiLLiaM MaRRoW:
- W shape in V1 + M shape in V6 = LBBB
- M shape in V1 + W shape in V6 = RBBB
RBBB: RSR' ("M" pattern) in V1; broad S wave in V5/V6
LBBB: deep QS or rS in V1; broad monophasic R in V5/V6
⚠️ Red Flag
New LBBB in the context of chest pain = STEMI equivalent. LBBB obscures ST changes and is treated as a STEMI regardless of other ECG features. This is a direct mark-scheme item and a patient safety point — get immediate senior help and cardiology review.
Pathological Q Waves
- Q wave > 1 small square wide OR > 25% of the R wave height
- Indicates prior full-thickness (transmural) MI in that vascular territory
Step 7: ST Segment
This is the most safety-critical step. Normal: isoelectric (level with the baseline)
| Finding | Interpretation |
|---|---|
| ST elevation ≥ 1mm in ≥ 2 contiguous limb leads | STEMI |
| ST elevation ≥ 2mm in ≥ 2 contiguous chest leads | STEMI |
| Concave ("saddle-shaped") ST elevation across all leads | Pericarditis |
| ST depression (horizontal or downsloping) | NSTEMI, unstable angina, posterior MI, digoxin effect |
| ST elevation + PR depression (widespread) | Acute pericarditis |
STEMI Territories — Must Know
| Leads affected | Territory | Culprit vessel |
|---|---|---|
| II, III, aVF | Inferior | RCA |
| I, aVL, V5–V6 | Lateral | LCx |
| V1–V4 | Anterior | LAD |
| V1–V6 + I, aVL | Extensive anterior | Proximal LAD |
| V1–V2 (ST depression + tall R) | Posterior | RCA or LCx |
⚠️ Red Flag
Posterior MI is the most commonly missed STEMI. It presents as ST depression in V1–V3 — the mirror image of posterior ST elevation. A tall R wave in V1 with ST depression should immediately make you think posterior MI. Confirm with posterior leads V7–V9 and escalate urgently.
Step 8: T Waves
Normal: upright in I, II, V3–V6; inverted in aVR; variable in V1
| Finding | Interpretation |
|---|---|
| Tall, symmetrically peaked T waves | Hyperkalaemia (early sign) |
| Flat T waves | Hypokalaemia, digoxin effect |
| Deep symmetrical T wave inversion in V2–V3 | Wellens syndrome (proximal LAD stenosis — high-risk pre-infarction pattern) |
| New T wave inversion in context of symptoms | ACS, PE |
| S1Q3T3 pattern + right axis + RBBB | Pulmonary embolism |
Hyperkalaemia — ECG Progression
🧠 Mnemonic
Hyperkalaemia ECG changes in order of severity:
- 1Tall peaked T waves (K+ > 5.5 mmol/L)
- 2PR prolongation + flattened P waves (K+ > 6.5)
- 3Widened QRS (K+ > 7)
- 4Sine wave pattern (K+ > 8)
- 5VF / asystole (K+ > 9)
Early recognition and treatment (calcium gluconate, insulin/dextrose, salbutamol) is time-critical.
Atrial Fibrillation — What to Say
- Irregularly irregular rhythm
- No discernible P waves — chaotic fibrillatory baseline
- Normal QRS width (unless aberrant conduction or pre-existing BBB)
In the OSCE: always comment on rate (is it rate-controlled?), assess haemodynamic stability, and mention stroke risk:
"I would calculate a CHA₂DS₂-VASc score to assess stroke risk and consider anticoagulation."
Presenting an ECG in the OSCE
Structure your presentation every single time:
"This ECG is from a [age]-year-old [sex]. The rate is [X] bpm and the rhythm is [regular/irregularly irregular]. [P waves are/are not] present. The axis is [normal/deviated]. The PR interval is [normal/prolonged/short]. The QRS width is [normal/broad]. [Describe any ST changes] in leads [X, Y, Z]. The T waves are [normal/inverted/peaked]. My overall interpretation is [X]. In the clinical context of [symptoms], my primary concern is [diagnosis] and I would [immediate action]."
💡 Tip
The final sentence — "my primary concern is X and I would do Y" — is what separates a descriptive answer from a clinical one. Examiners want to know you can act on the information, not just read it.
Common Examiner Follow-Up Questions
"What's your immediate management of this STEMI?"
"I would give aspirin 300mg, ticagrelor 180mg, and call the catheter lab for primary PCI. I would also give oxygen if sats are below 94%, establish IV access, take bloods including troponin, and keep the patient nil by mouth."
"The troponin comes back borderline — what now?"
"A borderline troponin doesn't exclude ACS. I would repeat at 3 hours, maintain continuous monitoring, keep the patient on the ACS pathway, and discuss with cardiology."
"You see a broad complex tachycardia — how do you approach it?"
"A broad complex tachycardia should be treated as ventricular tachycardia until proven otherwise. I would follow the ALS algorithm: assess haemodynamic stability, call for senior help, and if the patient is compromised, prepare for DC cardioversion."