What describes a sudden block in P wave with no change in PR interval?

The situation described involves a sudden block in P waves without any alteration in the PR interval. This scenario aligns with third-degree heart block, also known as complete heart block. In third-degree block, there is a complete dissociation between the atria and the ventricles, meaning that the electrical signals from the atria fail to reach the ventricles at all. This results in some P waves being completely blocked from conducting to the ventricles.

Despite this block, the PR interval remains unaffected for the beats that are conducted since it's defined as the time from the start of atrial depolarization to the start of ventricular depolarization. However, in third-degree heart block, many P waves are simply not transmitted to the ventricles, leading to instances where P waves appear in the rhythm strip but do not result in QRS complexes.

In contrast, first-degree heart block would show a consistently prolonged PR interval, while Wenckebach (Mobitz type I) describes a progressive lengthening of the PR interval before a P wave is dropped. Mobitz type II involves some P waves being conducted to the ventricles without the progressive lengthening but does not exhibit sudden block of P waves like that seen in third-degree block.