Mobitz 1 progression to Mobitz 2?

Hi all,
Does anyone know the mechanism for Mobitz 1 (Wenchebach) developing into Mobitz 2? I understand that if the Wenche'block' is in the AV node, which it is in 70% of presentation, that progression to Mobitz 2 is highly unlikely but that if the block presents in His-Perkinje, then it may. Does anyone know how this works? And for that matter, does anyone know exactly why beats are blocked? I understand that there may be a slow pathway or a lack of conduction but why do beats not make it through? I'm guessing it's a chemical issue that leads to a mispolarization. But it just seems to me like tissue or pathway should either conduct or not conduct and I don't understand the in between. In Mobitz 1, the phase of P-Q lengthen but why? This is never covered and even my med school friends can't explain it to me.

[ireneo]

Interesting question. I looked up the two type of heart blocks and their causes are quite similar (listed were certain heart meds, an MI, ischemia). I suppose if Type 1 is there and the conditions that caused it are still in place, it might progress to type 2. Sometimes we don't know the "why" of a situation. I have various arrhythmias but why did they form there? Who knows. I just have to deal with it.

Whether the lack of conduction is a chemical nature or tissue damage from a heart attack would have to be determined in each individual. I'm not sure they care about the cause as much as taking care of the problem. Sounds like it's going to be hard to find the answer. But still, it's a good question.