Ray Fowler, MD is Professor of EM / EMS at UTSW / Parkland. Edited by Alex Koyfman, MD.
A 38 year-old female presents about a month after having had epigastric and chest pain that was quite severe for an entire day about a month ago. She took some Zantac and Maalox, felt better, and went to bed. The next day she was weakened, but she gradually felt better and went about her business.
In the last 24 hours she has noticed that she has had episodes of lightheadedness and occasional palpitations, so she comes to the ED. Your nurse hands you her ECG. What is your interpretation?
The rate is slow, and the rhythm is irregular.
There is a P wave for every QRS, and there is actually MORE than one P wave for every QRS. On the beats that are conducted, the PR interval progressively prolongs until a P wave is dropped. Notice that on the first two QRS complexes on the ECG, in leads I, II, and III, that there is only a single conducted beat before a P wave is dropped.This is a second degree block. “Degree” means that ALL, SOME, or NONE of the P waves are conducted. “Type” means the location of the block. AV Block only occurs at one of two places: The AV node OR the bundle branches. The blood supply for the AV node is the right coronary artery in 90% of patients. The blood supply for the bundle branches is the left anterior descending artery from the left coronary artery. The AV node produces the PR interval. The bundle branches control the QRS complex. “Type 1” block occurs at the AV node, which means that the PR interval is affected, and on the conducted beats, the QRS complex is narrow. “Type 2” block occurs in the bundle branches, so the PR interval is “fixed” in length (usually normal), and there is usually (though not always) a bundle branch block present on the conducted beats.In this ECG, we see progressive prolongation of the PR interval in the aVR, aVL, and aVF leads, implying that the AV node is sick, and the QRS is narrow. This is a “Type 1” block, also known as Mobitz 1, and also known as “Wenckebach” (pronounced appropriately “ven-keh-BAK’ “ in Dutch). But, since four of the five beats on the strip have a ratio of 2:1, this makes this a “high grade” block. Most commonly, high grade blocks are seen with Type 2 block (Mobitz 2, which doesn’t have a nickname). So, this ECG is unusual to be a Wenckebach with a high grade block!
A 36 year old male calls EMS due to chest pain and palpitations. Medics come and pick him up, and find him to be having severe chest pain with a systolic of 90. The medics call into BioTel (the online medical control) requesting instructions. They tell me that the man has a history of SVT. I asked them to transmit the ECG, and they sent this:
They are still on-scene, and their ETA will be about 10 minutes once en route. What would YOU do??
Well, what I did, given that this strip was suggestive of VTach AND that the patient was in shock AND that he was having chest pain, was to give strong consideration to cardioverting the patient, which I like to avoid when I can in the back of an ambulance. And, he has a history of SVT. Buying time for a few minutes, I asked them to start an IV, give him a bolus of saline, go en route, and send me another ECG in five minutes. They do as I asked, and five minutes later this ECG comes over:
So, this is odd. This guy first had a wide complex tach of 186 that I felt might be VTach, and now he is clearly in what I felt is SVT with a rate of 226. What’s going on?
There is a problem-solving principle called Occam’s Razor where the “simplest diagnosis is usually right”. Or, “do you give the patient one disease that fits all the facts, or two diseases?”
In this case, this man had a wide complex tachycardia FOLLOWED by a narrow complex SVT. Does he have two diseases: VTach that converted then leading to SVT, two unrelated issues? Or, does he have ONE issue causing both problems? So, what did we do? He came into the ED, we gave adenosine 12 mg IV push, and he converted promptly to this:
So, using the principle of Occam’s Razor, what was this case? This man has WPW. He has had palpitations for over 25 years, and was seen over a year ago at our hospital with the identical narrow complex SVT. This is AVRT, or atrioventricular re-entrant tachycardia (a bypass tract) as opposed to AVNRT, or AV nodal re-entrant tachycardia, which is an AV node issue. The first strip is WPW with “antidromic” conduction, going down the bypass tract and producing a wide complex and back up the bundle of His and AV node to the atrium. Then, suddenly, the guy flips over into “orthodromic” conduction, or conduction down through the AV node producing a narrow SVT complex and traveling back up the bypass tract only to be headed back down the AV node. In his normal resting ECG, his bypass tract is concealed, so you don’t see a short PR plus a delta wave. Very interesting! This patient was discharged on a beta blocker the next day to followup with the EP folk!