ECG Quiz: Practice ECG Interpretation

Fifteen questions: rhythm strips and a 12-lead from past Arrhythmix puzzles, plus the basics every ECG reader needs. Tap an answer to see if you’re right, and the key features light up on the trace.

Standard paper: 25 mm/s and 10 mm/mV, so one small square is 40 ms and 0.1 mV.

  1. Question 1 of 15

    A 30-year-old woman at a routine check-up. Pulse 72 and regular. Lead II rhythm strip:

    What is this rhythm?

    The ECG trace is drawn here by your browser (it needs JavaScript).

    1. Sinus bradycardia
    2. Normal sinus rhythm
    3. Junctional rhythm
    4. First-degree AV block
    Show answer

    Answer: Normal sinus rhythm. A P wave before every QRS, a normal PR interval, narrow QRS complexes and a regular rate between 60 and 100.

  2. Question 2 of 15

    A 58-year-old man on a beta blocker feels well. Pulse 44. Lead II rhythm strip:

    What is this rhythm?

    The ECG trace is drawn here by your browser (it needs JavaScript).

    1. Sinus bradycardia
    2. Junctional rhythm
    3. Normal sinus rhythm
    4. Complete heart block
    Show answer

    Answer: Sinus bradycardia. Every QRS has its own P wave and a normal PR interval, but the rate is under 60. Beta blockers, fitness and sleep are common causes.

  3. Question 3 of 15

    A 19-year-old woman felt her heart start pounding suddenly while bending to tie her shoelaces. She is otherwise well. Lead II rhythm strip:

    What is this rhythm?

    The ECG trace is drawn here by your browser (it needs JavaScript).

    From Arrhythmix #1. Computer-generated trace, fictional case.

    1. Ventricular tachycardia
    2. Sinus tachycardia
    3. Supraventricular tachycardia (AVNRT)
    4. Atrial fibrillation
    Show answer

    Answer: Supraventricular tachycardia (AVNRT). Regular, narrow and about 185 a minute with no visible P waves, starting suddenly: AV nodal re-entrant tachycardia, the commonest regular SVT.

  4. Question 4 of 15

    A 74-year-old woman with high blood pressure and type 2 diabetes says her smartwatch keeps flagging an irregular pulse. Lead II rhythm strip:

    What is this rhythm?

    The ECG trace is drawn here by your browser (it needs JavaScript).

    From Arrhythmix #4. Computer-generated trace, fictional case.

    1. Multifocal atrial tachycardia
    2. Sinus arrhythmia
    3. Atrial flutter with variable block
    4. Atrial fibrillation
    Show answer

    Answer: Atrial fibrillation. Irregularly irregular, no P waves and a wavy baseline. AF raises stroke risk about five-fold, so her stroke risk needs assessing.

  5. Question 5 of 15

    A 63-year-old man who had a heart attack two years ago has palpitations and feels light-headed. BP 92/60. Lead II rhythm strip:

    What is this rhythm?

    The ECG trace is drawn here by your browser (it needs JavaScript).

    From Arrhythmix #3. Computer-generated trace, fictional case.

    1. Sinus tachycardia
    2. Atrial flutter with 2:1 block
    3. Ventricular tachycardia
    4. Supraventricular tachycardia (AVNRT)
    Show answer

    Answer: Ventricular tachycardia. Regular, broad and fast, with P waves marching through on their own. After a heart attack, a broad-complex tachycardia is VT until proven otherwise.

  6. Question 6 of 15

    An 82-year-old woman feels exhausted and dizzy. Pulse 38 and regular. Lead II rhythm strip:

    What is this rhythm?

    The ECG trace is drawn here by your browser (it needs JavaScript).

    From Arrhythmix #2. Computer-generated trace, fictional case.

    1. Sinus bradycardia
    2. Second-degree AV block, Mobitz II
    3. Junctional rhythm
    4. Complete heart block
    Show answer

    Answer: Complete heart block. P waves at about 88 a minute and broad QRS complexes at about 38, with no link between them: complete (third-degree) heart block.

  7. Question 7 of 15

    A 34-year-old woman ran a marathon yesterday and is having a routine check. She feels well; her resting pulse is slow and a little irregular. Lead II rhythm strip:

    What is this rhythm?

    The ECG trace is drawn here by your browser (it needs JavaScript).

    From Arrhythmix #5. Computer-generated trace, fictional case.

    1. Second-degree AV block, Mobitz I (Wenckebach)
    2. Second-degree AV block, Mobitz II
    3. First-degree AV block
    4. Sinus arrhythmia
    Show answer

    Answer: Second-degree AV block, Mobitz I (Wenckebach). The PR interval gets longer with each beat until a P wave doesn’t conduct, then resets. Often harmless in fit people with high vagal tone.

  8. Question 8 of 15

    A 71-year-old man has had two fainting spells this week. Lead II rhythm strip:

    What is this rhythm?

    The ECG trace is drawn here by your browser (it needs JavaScript).

    1. Complete heart block
    2. Second-degree AV block, Mobitz II
    3. Sinus arrhythmia
    4. Second-degree AV block, Mobitz I (Wenckebach)
    Show answer

    Answer: Second-degree AV block, Mobitz II. The PR interval stays the same, then a P wave suddenly fails to conduct. Mobitz II can progress to complete heart block and usually needs a pacemaker.

  9. Question 9 of 15

    A 66-year-old man collapses in a shopping centre. He is unresponsive and not breathing normally. The defibrillator shows:

    What is this rhythm?

    The ECG trace is drawn here by your browser (it needs JavaScript).

    1. Atrial fibrillation
    2. Ventricular tachycardia
    3. Ventricular fibrillation
    4. Torsades de pointes
    Show answer

    Answer: Ventricular fibrillation. Chaotic, disorganised activity with no recognisable QRS complexes. This is cardiac arrest: start CPR and defibrillate as soon as possible.

  10. Question 10 of 15

    A 63-year-old woman has had 40 minutes of heavy central chest pain spreading to her jaw. She is sweaty and nauseated. 12-lead ECG (scroll sideways inside the box on a phone):

    What does this 12-lead ECG show?

    The ECG trace is drawn here by your browser (it needs JavaScript).

    From Arrhythmix #6. Computer-generated trace, fictional case.

    1. Posterior MI
    2. Anterior STEMI
    3. Acute pericarditis
    4. Inferior STEMI
    Show answer

    Answer: Inferior STEMI. ST elevation in II, III and aVF with reciprocal ST depression in aVL: an inferior STEMI, usually from the right coronary artery. Call for urgent reperfusion.

  11. Question 11 of 15

    On standard ECG paper (25 mm/s), how long is one small square?

    1. 0.04 seconds (40 ms)
    2. 0.1 seconds (100 ms)
    3. 0.02 seconds (20 ms)
    4. 0.2 seconds (200 ms)
    Show answer

    Answer: 0.04 seconds (40 ms). One small square is 40 ms and one big square (five small) is 200 ms, so five big squares make one second.

  12. Question 12 of 15

    What is a normal PR interval?

    1. Over 300 ms (more than 7.5 small squares)
    2. 120 to 200 ms (3 to 5 small squares)
    3. 40 to 80 ms (1 to 2 small squares)
    4. 200 to 300 ms (5 to 7.5 small squares)
    Show answer

    Answer: 120 to 200 ms (3 to 5 small squares). Longer than 200 ms (one big square) is first-degree AV block. Shorter than 120 ms raises the possibility of a pre-excitation pathway such as WPW.

  13. Question 13 of 15

    The rhythm is regular with 4 big squares between R waves. What is the heart rate?

    1. 100 a minute
    2. 60 a minute
    3. 50 a minute
    4. 75 a minute
    Show answer

    Answer: 75 a minute. For a regular rhythm, rate = 300 ÷ big squares between R waves. 300 ÷ 4 = 75. For irregular rhythms, count QRS complexes in 10 seconds and multiply by 6.

  14. Question 14 of 15

    A QRS complex counts as broad when it is longer than:

    1. 40 ms (1 small square)
    2. 120 ms (3 small squares)
    3. 80 ms (2 small squares)
    4. 200 ms (5 small squares)
    Show answer

    Answer: 120 ms (3 small squares). Normal QRS is under about 120 ms. Broad complexes mean the impulse is travelling the slow way: bundle branch block, a ventricular rhythm, pacing or hyperkalaemia.

  15. Question 15 of 15

    Which leads look at the inferior wall of the heart?

    1. II, III and aVF
    2. I, aVL, V5 and V6
    3. V1 and V2
    4. V3 and V4
    Show answer

    Answer: II, III and aVF. II, III and aVF are inferior; I, aVL, V5 and V6 are lateral; V1 to V4 cover the septum and anterior wall.

Want a new ECG every week? Play Arrhythmix: one case, one ECG, six diagnoses, three tries. Every past ECG and its answer is in the Arrhythmix archive.

Common questions

How do you read an ECG?

Use the same order every time: check the patient details and calibration, then the rate, the rhythm (regular or not, P waves before every QRS?), the axis, the PR interval, the QRS width, the ST segments and T waves, and the QT interval. Finish by comparing with an old ECG if there is one.

How do you work out the heart rate from an ECG?

For a regular rhythm, divide 300 by the number of big squares between two R waves (or 1,500 by the number of small squares). For an irregular rhythm, count the QRS complexes on a 10-second strip and multiply by 6.

What does a normal ECG look like?

Sinus rhythm at 60 to 100 a minute, a P wave before every QRS, a PR interval of 120 to 200 ms, a narrow QRS under about 120 ms, no significant ST elevation or depression, upright T waves in most leads and a normal QTc.

Are these real ECGs?

No. The traces are computer-generated from parameters to show the textbook features, and the cases are fictional. They’re for learning, not for interpreting a real person’s ECG.

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Reviewed 8 November 2026 by a Malady Wise GP. ECG traces are computer-generated to show textbook features; cases are fictional. General education, not personal medical advice.