Hi, Daisy,

Your link to Photobucket did not work for me.  We are working on a way for you to upload an image here.  For now, can you scan it and send it to [email protected]?   Or, post it on our Face Book page http://www.Facebook.com/#!/ECGGuru   

In general terms, if the p waves are regular, and one is not conducted, you have an AV block.   If a PREMATURE p wave (PAC) is not conducted, you have a blocked PAC.  Usually, it is easy to simply march out the p waves.

I was able to copy and paste your link into the address bar of a browser window and view it.  It looks like someone took a picture of a rhythm tracing with their camera phone in a dark room so the resolution is quite poor.  It's also somewhat warped in the fact that the graph paper lines are curving instead of being straight.  This makes things difficult to accurately measure with calipers but I'll take a stab at it anyway.  I enlarged the image in Photoshop and examined the rhythm's intervals in fine detail.  Bottom line:  Your initial interpretation is correct!  This is NOT an example of a nonconducted PAC.  There are a total of 5 conducted beats.  At the far left, the first fully conducted beat has a P-R interval of about 0.32s.  The next P-R interval is one millimeter longer at about 0.36s.  The next sinus impulse is "dropped".  To recap:  So we have consecutive sinus impulses conducting with progressively lengthening P-R intervals immediately before the "dropped" beat.  This meets all of the criteria for calling this Type I A-V block (Wenckebach periodicity).  Immediately following the "dropped" beat, the returning sinus impulse conducts with a shorter (yet still prolonged) P-R interval of about 0.23s.  From here on out, the process repeats itself much like it started at the beginning of the tracing with the next P-R interval lengthening again to 0.32s.  Finally, the last P-R interval progressively prolongs to a duration of 0.35s and the next sinus impulse is again blocked.  The P-waves, that are not followed by QRS complexes, are of sinus origin.  They are not premature and they appear to march out fairly regularly with the other P-waves.

I would probably go one step further and postulate that this is actually a variation of the A-V (nodal) block theme called vagotonic A-V block.  Vagotonic A-V block can be triggered by something as simple as pressing hard on your closed eyelid and eyeball with your thumb.  Below I've included a link to a page in one of Dr. Kenneth Ellenbogen's textbooks.  I'm referring to figure 1.7.  The image on Photobucket has a similar feature in the fact that the sinus rate slows down slightly before the sinus impulse is blocked which is somewhat paradoxical behaviour for your typical Wenckebach sequence.  

Vagotonic A-V block (hypervagotonia)

My only constructive criticism and advise is to revise your terminology.  Instead of using potentially confusing terms like "blocked PAC", it is far better to call them "nonconducted PACs" thereby avoiding use of the word "block" altogether.  "What's the difference?" you might say.  I'm going to quote the late-great Dr. Henry J. L. Marriott here:  The term "block" always implies abnormality whereas "nonconducted" does not necessarily.  There's nothing abnormal about an atrial impulse not conducting if it's perched up on the first portion of the ST segment or upstroke of the T-wave when the ventricles are normally in absolute refractory period and the tissue is physiologically nonresponsive.  However, if the sinus impulse lands well after the end of the T-wave (as they do in the Photobucket image), then I would attribute that to pathological block.  You basically want to divide conduction into two categories:  1.)  Abnormal, pathological failure of conduction (i.e., block); and . . . 2.)  Normal, physiological absence of conduction.  Type I A-V block (Wenckebach phenomenon) would qualify as # 1 and nonconducted PACs would qualify as # 2.  I know all too well, from personal experience, that casual use the term "block" is only going to invite others to misinterpret, overdiagnose, and mismanage the patient's cardiac rhythm.  Invariably, people are only capable of thinking of A-V block or heart block when the hear the word "block" come out of your mouth, even if it's followed by the acronym PAC after it.  If they think they patient is in an A-V block, then they are far more likely to start talking about needlessly implanting pacemakers at that point when there is no indication for one.  Sorry for belaboring the point, but I have no love for the term "blocked PAC". 

Hope this has been helpful and informative.  Keep the questions coming. The "Guru's" contributing to this website are always willing to answer your questions A.S.A.P.

I don't really understand the concept of this person's photobucket album.  There is just one image of a "grainy" rhythm tracing and the rest of the images are of Japanese Anime cartoon and comic book art.  Hardly a unifying theme!  I didn't see any interpretation for the one rhythm tracing though.

I don't have much to add to jer150's great analysis, but another neat feature which Mobitz I will have over a simple nonconducted PAC would be the shortening of the R-R (even if it is just between the first 3 complexes) along with the PRi prolongation. Sometimes the R-R shortening is more obvious than the PRi changes.

Moreover...don't read the automated interpretations. Those are there for comic effect only :)