How does 3rd degree AV block differ from 2nd degree AV block?

Third degree AV block, also known as complete heart block, is characterized by a complete dissociation between atrial and ventricular activities. This means that the atria can contract independently from the ventricles, leading to a situation where the ventricles may beat at their intrinsic rates, which is typically slower. In contrast, second degree AV block, which includes both Type I (Wenckebach) and Type II blocks, involves intermittent failure of conduction from the atria to the ventricles, but there is still some degree of communication between them.

The distance between RR intervals is a key distinguishing feature between the two types of blocks. In third degree AV block, the RR intervals are constant and consistent but are typically slower due to the independent rhythm of the ventricles. In second degree AV block, particularly in Type I, the RR intervals can progressively get shorter until a beat is dropped; in Type II, the RR intervals are more variable, reflecting the missing ventricular beats. Therefore, the consistent RR interval distance in third degree block is a clear indicator of its difference from the RR interval variability seen in second degree block.

This nuanced distinction helps in the assessment and management of patients, as the presence of total dissociation in third degree AV block often requires