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ECG Masterclass: Axis, Rhythms, and Blocks — MRCP Part 1

TL;DR

This clinician-written ECG masterclass distils axis determination, rhythm analysis, and conduction blocks into exam-ready rules for MRCP Part 1. It focuses on what is repeatedly tested, highlights common traps, and finishes with a worked mini-MCQ and a practical revision checklist to improve accuracy and speed.


Why ECGs matter in MRCP Part 1

ECG interpretation is one of the highest-yield, most predictable scoring areas in MRCP Part 1. Questions are usually short, visually driven, and reward candidates who follow a structured, rule-based approach rather than overthinking. Mastery of axis, rhythms, and blocks can reliably convert into marks because the diagnostic criteria are fixed and well defined.

For an overview of how ECGs fit into the wider syllabus, see the official MRCP Part 1 guide from the MRCP(UK):https://www.mrcpuk.org/mrcpuk-examinations/part-1


Scope of this masterclass

This article covers:

  • Rapid axis determination

  • Systematic rhythm analysis

  • AV blocks and bundle branch blocks

  • The 5 most tested ECG subtopics

  • The 5 most common exam traps

  • One worked MCQ-style case

  • A practical ECG revision checklist


The 5 most tested ECG subtopics

1. Cardiac axis (fast and reliable)

Axis questions are common because they are quick to test and easy to mark.

Two-lead method (exam favourite):

  • Lead I positive, aVF positive → Normal axis

  • Lead I positive, aVF negative → Left axis deviation

  • Lead I negative, aVF positive → Right axis deviation

  • Lead I negative, aVF negative → Extreme axis deviation

High-yield associations

  • Left axis deviation: left anterior fascicular block, inferior MI

  • Right axis deviation: right ventricular hypertrophy, pulmonary embolism

2. Sinus rhythm vs atrial arrhythmias

You must identify rhythm before interpreting anything else.

  • Sinus rhythm:

    • P wave before every QRS

    • Constant PR interval

    • Upright P waves in lead II

  • Atrial fibrillation:

    • No discrete P waves

    • Irregularly irregular rhythm

  • Atrial flutter:

    • Saw-tooth flutter waves (best in inferior leads)

    • Often regular ventricular rate at ~150 bpm due to 2:1 block

3. Narrow vs broad complex tachycardia

This distinction is heavily tested.

  • Narrow complex (<120 ms):

    • Supraventricular tachycardia

    • Atrial fibrillation with normal conduction

  • Broad complex (≥120 ms):

    • Ventricular tachycardia until proven otherwise

    • SVT with bundle branch block

    • Pre-excitation syndromes

In MRCP exams, the safest assumption is VT unless clear evidence suggests otherwise.

4. Atrioventricular (AV) block

PR interval interpretation is crucial.

  • First-degree AV block:

    • PR > 200 ms

  • Second-degree Mobitz I (Wenckebach):

    • Progressive PR prolongation → dropped beat

  • Second-degree Mobitz II:

    • Fixed PR interval with dropped beats

    • High risk of progression to complete block

  • Third-degree (complete) heart block:

    • AV dissociation

    • Independent atrial and ventricular rates

5. Bundle branch blocks

Recognising classic patterns saves time.

  • Right bundle branch block (RBBB):

    • rSR′ pattern in V1

    • Broad S wave in leads I and V6

  • Left bundle branch block (LBBB):

    • Broad, notched R waves in I, aVL, V5–V6

    • Deep S waves in V1

    • ST-T discordance

LBBB is particularly important because it obscures myocardial infarction.


Online ECG practice questions for MRCP Part 1 exam preparation

High-yield ECG rules to memorise (numbered)

  1. Always assess rate, rhythm, axis, intervals, QRS, ST-T — in that order.

  2. Axis can be determined using Lead I and aVF alone.

  3. Irregularly irregular rhythm = atrial fibrillation until proven otherwise.

  4. Broad complex tachycardia = ventricular tachycardia in exams.

  5. PR interval >200 ms defines first-degree AV block.

  6. Mobitz II is dangerous and often requires pacing.

  7. LBBB masks MI; rely on concordant ST changes, not elevation alone.

  8. RBBB does not hide ST elevation.

  9. 2:1 AV block cannot be reliably subtyped without context.

  10. Compare with previous ECGs when provided.


Mini-case (MCQ style)

Question A 72-year-old man presents with syncope. ECG shows:

  • Regular P waves at 90 bpm

  • Broad QRS complexes at 35 bpm

  • No fixed relationship between P waves and QRS complexes

What is the most likely diagnosis?

A. Sinus bradycardiaB. Mobitz I AV blockC. Mobitz II AV blockD. Complete heart blockE. Atrial flutter with variable block

Correct answer: D — Complete heart block

Explanation: AV dissociation with independent atrial and ventricular rates defines third-degree heart block. The broad QRS suggests a ventricular escape rhythm, explaining the low ventricular rate and syncope.


Common ECG traps in MRCP Part 1

  • Calling atrial fibrillation “irregular SVT”

  • Missing axis deviation by skipping Lead I/aVF

  • Misdiagnosing VT as SVT with aberrancy

  • Assuming LBBB rules out myocardial infarction

  • Forgetting that Mobitz II implies infranodal disease


Practical ECG revision checklist

  • Practise 20 ECGs/day using a fixed interpretation order

  • Drill axis determination until it is automatic

  • Memorise PR and QRS cut-offs

  • Use timed ECG question banks such ashttps://passmedicine.comandhttps://www.mrcpquestions.co.uk

  • Sit full mock exams under exam conditions

  • Keep a notebook of recurrent mistakes

For structured revision and ECG lectures, see:https://www.crackmedicine.com/lectures/and targeted practice questions at:https://www.crackmedicine.com/qbank/


Frequently Asked Questions

How do I quickly calculate ECG axis for MRCP Part 1?

Use Lead I and aVF polarity only. This method is fast, reliable, and sufficient for exam questions.

Is ventricular tachycardia more common than SVT with aberrancy in exams?Yes. In MRCP-style questions, broad complex tachycardia should be assumed to be VT unless proven otherwise.

Which AV block is most dangerous?

Mobitz II, because it reflects infranodal conduction disease and may progress to complete heart block.

Does left bundle branch block rule out MI?

No. MI can coexist with LBBB; look for concordant ST changes and clinical context.


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