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Electrocardiograms Quiz
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Which structure in the cardiac conduction system is primarily responsible for delaying the electrical impulse before it passes to the ventricles?
- A Purkinje fibres You picked ✕
- B Bundle of His You picked ✕
- C Sinoatrial node You picked ✕
- D Right bundle branch You picked ✕
- E Atrioventricular node You picked ✓
Correct!
Incorrect
The answer is E: Atrioventricular node
Explanation
Why E is correct
See The Conduction System. Option E is correct. The impulse starts in the sinoatrial node, in the wall of the right atrium, spreads across the atria and causes them to contract, and then reaches the atrioventricular (AV) node, which lies in the atrioventricular septum near the opening of the coronary sinus. The AV node conducts slowly, and the impulse is delayed there by about 120 ms. The pause allows the atria to finish ejecting blood into the ventricles before ventricular systole (contraction) begins, so the ventricles fill fully and pump efficiently. On the ECG this delay is the PR interval. After the AV node, the impulse passes into the bundle of His, down the interventricular septum and along the Purkinje fibres, which are fast conductors, so the ventricles activate almost together. Memory aid: The AV node is the slow traffic light between atria and ventricles.
- Site: The atrioventricular node, near the opening of the coronary sinus.
- Conduction: It conducts slowly, delaying the impulse by about 120 ms.
- Purpose: The atria empty into the ventricles before ventricular contraction begins.
Why the other options are incorrect
- A. Purkinje fibres: Purkinje fibres spread the impulse rapidly through the ventricular muscle, so that the ventricles contract together. They are fast-conducting tissue and do not cause a delay. Your answer
- B. Bundle of His: The bundle of His carries the impulse quickly from the atrioventricular node into the ventricular septum. It is a fast conducting pathway and so speeds the impulse, not delays it. Your answer
- C. Sinoatrial node: The sinoatrial node is the pacemaker that starts the impulse, not the structure that delays it. It sets the rate of the heart and the delay happens further down the pathway. Your answer
- D. Right bundle branch: The right bundle branch is part of the fast conducting system in the ventricles. A block here widens the QRS complex instead of adding a physiological delay. Your answer
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More questions from this set. Start the quiz to answer them and see each explanation.
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1
At an ECG paper speed of 25 mm/s, which method is most appropriate for estimating the average ventricular rate when the rhythm is irregular?
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2
A patient's ECG shows a QRS complex that is wider than normal. Which mechanism best explains the widening?
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