In what type of heart block do atrial impulses fail to reach the ventricles at the correct time?

In second-degree heart block, there is an interruption in the conduction of electrical impulses from the atria to the ventricles. This results in some of the atrial impulses failing to lead to ventricular depolarization at the correct time, causing a discrepancy in the rhythm of the heart. There are typically two types of second-degree heart block: Type I (Wenckebach or Mobitz Type I) and Type II (Mobitz Type II), each characterized by different patterns of impulse conduction. In Type I, the progressively lengthening PR interval leads to a dropped QRS complex, while in Type II, there is a consistent PR interval with occasional dropped beats.

In contrast, first-degree heart block does not involve dropped beats; instead, there is a prolonged PR interval that indicates a delay but still allows for every atrial impulse to reach the ventricles. Third-degree heart block, also known as complete heart block, is characterized by a complete dissociation between atrial and ventricular depolarization, meaning that the atria and ventricles beat independently of each other. Complete heart block can lead to significant bradycardia, but it does not maintain a regular pattern of some impulses getting through like second-degree heart block.

Lastly, complete heart