Bradyarrhythmias and Heart Block
Failure or delay of impulse generation or conduction anywhere from the sinus node to the ventricles slows or interrupts the heart's electrical cascade, so the exact pattern of block on the ECG predicts how much of the atrial rhythm still reaches the ventricles, and therefore how dangerous it is.
First principles
The normal cascade and where it can fail
Normally the sinoatrial node generates an impulse that spreads across the atria, is deliberately delayed at the AV node to allow ventricular filling, and is then rapidly distributed to the ventricles via the His-Purkinje system. A bradyarrhythmia arises when generation fails (sinus node dysfunction) or conduction fails at any point along this pathway, most often from ischaemia, fibrosis, drugs or high vagal tone.
Educational content pending clinical review. Not medical advice.