confused.. Non-conducted PAC vs Type II AV block?

ChaseZ · Oct 21, 2012

A blocked, or non-conducted PAC is what the name implies... A premature atrial beat that is not conducted to the ventricles; P wave but no QRS. The key is the fact that it is early. So take your calipers and measure out the P-P interval then measure form the last normal p wave and look where the calipers land. Does the non conducted P wave come sooner than where your calipers land? If so then it is a blocked PAC. They commonly are buried in the preceding T wave so look closely for changes in T wave moroplogy. There may be an extra hump or taller peak. Now with Second degree type 2 there is not a premature beat, there is an intermittent failure of conduction to the ventricles. So same as before take your calipers, get the P-P and measure out from the last normal P. Is there a non conducted P wave exactly where your calipers land? That is second degree hesrt block. Slide your calipers from that non conducted P wave and they should land exactly on another P wave, this time with an associated QRS. This is what people refer to as "marching out". With a second degree type 1 (Wenckeback) there is no marching out of p waves. Each consecutive PR Interval increases until there is a non conducted p wave. Longer, longer, dropped then the cycle repeats. So fixed PRI for the beats preceding a dropped best is not Wenckeback, most likely a PAC or 2nd degree type 2. Hope that helps, I typed this on my IPad so sorry for the format and typos

nooneelse · Oct 21, 2012

ahh.. I get it now!! thanks so much ChaseZ!

Bertpr8 · Feb 18, 2016

Everything that ChaseZ mentioned is on point. The only thing I should say is that on the 2nd degree Type I the P waves do march. They all come from the SA node, but the elongation of the PRI happened due to the delay on the AV node. I was trying to add an example, but I couldn't add it.

MsBruiser · Jun 18, 2016

Good question and good answer.

Csizi · Feb 2, 2018

What if the non-conducting P wave comes later than it should be? Like 8ms-10ms-6ms. Maybe a blocked atrial escape rythm that came late?

2210485 · Feb 7, 2018

What if the non-conducting P wave comes later than it should be? Like 8ms-10ms-6ms. Maybe a blocked atrial escape rythm that came late?

Well, of course 8- 10- 8ms would be virtually impossible to determine in a 12 lead or even an intracardiac electrogram. Even a diagnostic ep catheter has a margin of error and at least some degree of limitation.

If however you see like.. 80ms, 100ms, 80 ms variations:

It could be escape if the morphology (shape) of the wave is different. Possibly 2nd degree avb, sinus arrest with non conducting escape. That's a possibility.

It could also be a disease superior to the AV Node and/or SSS. Conduction Disease can occur just about anywhere along the conduction pathway.

Marching out the P waves as you did is a good plan. If you have a longer strip or a long term monitor watching for patterns in the P waves can help make sense of it sometimes.

If you have a PAC or a pathway you can have concealed conduction creating the phenomenon as well. So gathering information about potential interactions between ectopics or abberency is always useful, as is a thorough understanding of patient history.

Unfortunately with situations such as these theres just so many options that it's pretty difficult to develop a concise algorithm.

Once you get the basics down and you've seen enough if these recordings to know what things look like:

The only way to advanceg with this stuff, imo, is to stop thinking about EKG criteria, and a simple block of 3 components (pri, qrs, tu) and start thinking in terms of Anatomy and Physiology and electricity. Instead of seeing a P wave and associating it with 'atria' bear in mind that the atria can be futher subdivided into inconceivably small units and that each of those units plays a role in generating an ekg complex. Not sure if that helps or not, just my 2 cents on it.