*In a type I SA block, the P-P interval shortens until one P wave is dropped.
*In a type II SA block, the P-P intervals are an exact multiple of the sinus cycle, and are regular before and after the dropped P wave.
This usually occurs transiently and produces no symptoms. It may occur in healthy patients with increased vagal tone. It may also be found with CAD, inferior MI, and digitalis toxicity.