Electrocardiogram Pathology Quiz

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Which ECG feature is most characteristic of a ventricular ectopic beat?

A Prolonged PR interval with otherwise normal P waves and QRS complexes
B Broad QRS complex occurring early without a preceding P wave ✓
C Inverted P wave that may be hidden within a normal QRS complex
D Abnormally shaped P wave appearing early followed by a normal QRS complex
E Irregularly irregular rhythm caused by multiple atrial foci

The answer is B: Broad QRS complex occurring early without a preceding P wave

Explanation

Why B is correct

See Ectopic Beats. Option B is correct. An ectopic beat is an extra beat that arises from a site other than the sinoatrial node, which is why it comes early, before the next normal beat is due. A ventricular ectopic starts in the ventricular muscle itself. Because it begins below the atria there is no P wave before it. The impulse cannot use the fast His-Purkinje system, so it spreads slowly from muscle cell to muscle cell across the ventricles, and the QRS complex is therefore broad and unusually shaped. The next normal beat usually follows after a pause. Ventricular ectopics are common, and patients may feel them as a missed or thumping beat. On their own, in people with a structurally normal heart, they are often harmless, but frequent ectopics, or ectopics with symptoms or heart disease, deserve further assessment. Memory aid: Ventricular ectopic: early, broad, no P wave.

Timing: The beat comes early, before the next normal beat is due.
P wave: Absent before the beat, because it starts below the atria.
Shape of the QRS: Broad and unusually shaped, because the impulse spreads slowly from muscle cell to muscle cell.

Why the other options are incorrect

A. Prolonged PR interval with otherwise normal P waves and QRS complexes: A prolonged PR interval with otherwise normal waves is first-degree atrioventricular block. It is a conduction delay and not an early beat.
C. Inverted P wave that may be hidden within a normal QRS complex: An inverted P wave that may be hidden within a normal QRS suggests a junctional ectopic. The impulse starts near the AV node and spreads backwards into the atria.
D. Abnormally shaped P wave appearing early followed by a normal QRS complex: An early, abnormally shaped P wave followed by a normal QRS is an atrial ectopic. The impulse starts in the atria and travels down the normal conduction system, so the QRS is narrow.
E. Irregularly irregular rhythm caused by multiple atrial foci: An irregularly irregular rhythm from multiple atrial foci describes atrial fibrillation. A single ectopic beat is a premature beat within an otherwise regular rhythm.

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1
Which electrocardiographic pattern best characterises Mobitz type I second-degree atrioventricular block?
2
Which electrocardiographic feature best characterises atrial fibrillation?

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Key concepts in this topic

Short revision pages: the key facts, a memory aid and the exam traps for each concept.

Early afterdepolarisations and long QT syndrome
An early afterdepolarisation is an extra upswing in a heart cell's voltage during prolonged repolarisation (return to resting voltage), as in long QT syndrome (ventricles slow to repolarise).
First-degree AV block
First-degree atrioventricular (AV) block is a delay rather than a true block: the AV node holds each electrical impulse back for longer than usual, yet every impulse still gets through.
Re-entry loops and unidirectional block
Re-entry is an impulse running round a circuit of two connected pathways instead of dying out, so it keeps re-triggering the heart and sustains a fast abnormal rhythm.
Third-degree (complete) AV block and escape rhythms
In third-degree (complete) atrioventricular (AV) block, no impulses pass from the atria to the ventricles, so a slow backup pacemaker, called an escape rhythm, drives the ventricles.

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