What is a third-degree AV block?

What is a third-degree AV block?

• A. Complete failure of all action potentials to reach the ventricles.
• B. Prolonged PR interval with intact conduction.
• C. Intermittent AV conduction with rare QRS.
• D. Normal conduction with ectopic atrial rhythm.

Answer: (A)

Third-degree AV block means the AV node and conducting pathways fail to transmit any atrial impulses to the ventricles. Because of this complete dissociation, the atria and ventricles beat independently: the atrial rate is set by the sinus node, while the ventricles rely on an escape pacemaker below the block, producing a slower, regular ventricular rhythm. On ECG you’d see P waves marching through at a normal atrial rate with QRS complexes occurring independently at a slower rate, with no fixed relationship between the two. This is exactly what “complete failure of all action potentials to reach the ventricles” describes. The other patterns reflect different conduction statuses: a prolonged PR interval with intact conduction points to a first-degree AV block, where every atrial impulse still reaches the ventricles but with delay. Intermittent AV conduction with rare QRS fits a second-degree block, where some atrial impulses fail to conduct. Normal conduction with ectopic atrial rhythm isn’t a block at all, since conduction from atria to ventricles remains intact despite an abnormal atrial origin.

Third-degree AV block means the AV node and conducting pathways fail to transmit any atrial impulses to the ventricles. Because of this complete dissociation, the atria and ventricles beat independently: the atrial rate is set by the sinus node, while the ventricles rely on an escape pacemaker below the block, producing a slower, regular ventricular rhythm. On ECG you’d see P waves marching through at a normal atrial rate with QRS complexes occurring independently at a slower rate, with no fixed relationship between the two. This is exactly what “complete failure of all action potentials to reach the ventricles” describes.

The other patterns reflect different conduction statuses: a prolonged PR interval with intact conduction points to a first-degree AV block, where every atrial impulse still reaches the ventricles but with delay. Intermittent AV conduction with rare QRS fits a second-degree block, where some atrial impulses fail to conduct. Normal conduction with ectopic atrial rhythm isn’t a block at all, since conduction from atria to ventricles remains intact despite an abnormal atrial origin.