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Ideal sources for Wikipedia's health content are defined in the guideline Wikipedia:Identifying reliable sources (medicine) and are typically review articles. Here are links to possibly useful sources of information about First-degree atrioventricular block.
If the impulse is not "blocked" then why are you refering to this as first degree AV block, instead of only "prolonged PR interval" The name first degree AV block is misleading. With this reasoning then there's no trifascicular block. It is reasonable to assume that block of all three fascicles would lead to total AV block
Well, while it might not seem logical, nonetheless, that's what it's called. We're referring to it that way because that's what everybody else refers to it as. It may help you to think of "prolongation" and "blocking" as synonyms in this instance, but even if it doesn't you'll have to get used to the idea that "prolonged PR interval" = "first degree heart block", because we're not just making it up. As an overview, it would be fair to say "1°": every atrial beat triggers a ventricular beat; "2°": some atrial beats trigger ventricular beats; and "3°" no atrial beats trigger ventricular beats. - Nunh-huh 23:59, 26 Dec 2004 (UTC)
You can also think of a 1st degree block as impeding the conduction of electrical impulses through the AV node, blocking in a temporary, imperfect way. You may decide to block a Walmart entry door on "black friday", but all those determined shoppers will push past you anyway. They all get through, but slower than they'd wanted to. Wordreader (talk) 18:49, 28 November 2011 (UTC)[reply]
By the way, the apparent purpose of the AV node is to delay, or temporarily block, electrical conductions between the artia and the ventricles, a normal function. It's when that delay exceeds 0.20 seconds that it's considered abnormal. — Preceding unsigned comment added by 68.197.49.241 (talk) 23:58, 28 November 2011 (UTC)[reply]
Since WP isn't designed for professionals to study from, but for the interested lay public, I think an explanation of "AV" (other than the title of the article) is in order before plunging into using "AV-this" and "AV-that" jargon throughout the article. I know what you're talking about, but the overwhelming number of people in the English-speaking world have no idea that they possess an AV node, what it does, or that it can go awry. Thanks, Wordreader (talk) 18:40, 28 November 2011 (UTC)[reply]