You are admitting your next patient to the medical wards. He is a 76 year old male admitted with profuse bloody diarrhea. He has a medical history significant for CAD, High Cholesterol, Diverticulosis, 2 PPD x 20 year smoking history. He states he woke up this morning and had a Bowel movement that was painless but when he looked in the toilet was full of blood. He denies any abdominal pain at all and tells you that this has happened before and that they told him he had "Sticks N Losis" in his GI tract. He denies any vomiting or nausea and he does admit to some dizziness when standing up. Otherwise he denies any other symptoms. Vitals are 130/70 / 98 / 18 / and 97% on RA. He is in mild discomfort and pale. Physical exam is essentially benign including the abdomen. You decide to be a stickler and make the medical student check his stool for guiac positivity, which of course is positive. You confirm that he does have a history of diverticulosis and previous hospitalizations for the same reason and are comfortable that that is why he is having his bloody diarrhea. On laboratory exam the only significant finding is a HGB of 8 with a trend with previous ones at around 11. The Chest X-Ray is normal. You look all over the ER for his EKG to complete his H&P and finally find it. What is the underlying rhythm?

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This is a Great EKG. The findings are very subtle but if you look hard enough, as usual, you will find it.

Starting off with the normal systematic approach to interpretation, the first step gets us puzzled. What the heck is the rhythm. At first glance it looks like a junctional rhythm. But before we "jump the gun" look a little closer.

Using Lead II as the example, we can see no p waves preceding the QRS complex. We also see a regular notch in the QRS-T complex occurring after each complex and when there is a long enough pause between qrs complexes, beat 5 and 14 we can see this notch on it's own. With out these pauses, it would be harder to figure out but having these notches on their own tell us that they must represent some kind of atrial activity. Taking your caliper you can see that all these notches, which I will now call p waves "March out" at a rate of 115. Looking at II, III and AVF and AVR this is the normal axis for a p wave so we can assume they are generating from the sinus node.

But they are coming after the QRS complex, how can that make any sense to have a sinus p after the complex?

The next step is to see the R-R relationship to each other. Does it hold up? Yes and NO. With calipers you can see that there is some regularity to the R-R intervals to each other except for beat 5 and 14. And that regular interval is at about 105 bpm. OK now what?

Now the key is, is the atrial activity (p-p intervals) related to the ventricular activity (R-R's)? A quick glance would make you think so but look closer. Take your caliper and see if there is a steady R to P relationship and you will notice you can never establish a steady one, it is always a little smaller or bigger than the previous one.

THE CLINCHER!! Since you can not establish a steady R to P interval, but the R-R interval is regular and the P-P interval is regular, this must be atrial and ventricular disassociation. Hence, third degree heart block.

This can explain why there is a normal axis p wave coming after the QRS. This is complete heart block going pretty fast and close to each other looking like a junctional rhythm. The Atrium going at 115 and ventricle at a close rate of 105. So close to each other and so subtle to see. With out what happens in beat 5 and 14 it would be a lot harder to catch.

So what happens in beat 5 and 14. Check out the p-r intervals. they are long (about .22 sec) and exactly the same. They both also seem to be on a "island of their own". These are also the only times that the R-R interval doesn't match up, further proving that this is AV disassociating. Immediately following these captures, the R-R returns to it's slower beat. So this must represent ventricular capture and what is even more revealing is that the notch is only part of the p wave, and that the beginning of the p wave is lost in most of the complexes, only the notch was enough to come out on the tracing.

So to summarize, we have a 3rd degree heart block with occasional ventricular capture proven by the p-p interval never changing and the R-R interval only changing when there is an "association" between the p and the qrs, than reverting to it's own slower R-R until the impulse from the sinus node arrives at the AV node at just the right time to conduct prolonged down the av node.