Block in conduction between the atria and ventricles (AV block) mainly affects which interval?

• A. Primarily affects the PR interval, may eventually widen QRS in severe cases.
• B. Primarily affects the QT interval.
• C. Only affects P wave amplitude.
• D. Heart rate increases.

Answer: (A)

Block in conduction between the atria and ventricles slows the passage of the atrial impulse to the ventricles, so the timing from the start of atrial depolarization to ventricular depolarization changes. That timing is shown on the ECG as the PR interval, so AV block mainly elongates the PR interval. In milder blocks the PR interval simply lengthens; in more severe blocks some impulses may not reach the ventricles at all, and the ventricles may be driven by a slower escape rhythm, which can make QRS complexes wider. The QT interval reflects ventricular depolarization and repolarization, not AV conduction, and P wave amplitude isn’t the measure of block. Heart rate changes can occur with different escape rhythms but aren’t the primary interval affected.