Snow Shoveling is Bad

The two-day blizzard that your town endured has just ended.  The area received about a foot of snow, but now it is sunny out.  You and your partner are assigned to the report of chest pain at a little house in the southeast corner of your jurisdiction.  When you arrive, the residential street is unplowed and it doesn’t look like your ambulance will be able to make it to the patient’s driveway.  So you park on the main street and walk four houses up to the address with your gear.  The patient’s house’s driveway is half-shoveled, so you have an idea of what is going on before even entering the house.

Inside, you find a 60-year old male patient complaining of 10 out of 10 chest pain that he describes as squeezing.  The pain radiates to his left shoulder, and he states that it feels like he isn’t getting enough air.  The pain started about an hour ago, while he was… wait for it… shoveling snow.  He is warm, pink, and dry.  His wife reports a history of hypertension and hyperlipidemia, for which he takes lisinopril and a statin that they can’t name.  The patient had a “heart attack” 15 years ago, but this pain is not similar to that episode – as he remembers it, the prior episode was much less severe compared to now.

The patient’s blood pressure is 110/70, heart rate of 40, respiratory rate of 20, and a pulse ox reading of 98% on room air.  You ask your partner to place the patient on oxygen via nasal cannula and run a 12-lead ECG.  Click to enlarge:

Courtesy DogMan41 via Wikimedia Commons, with permission.

 What is your interpretation of this ECG?  What is your plan?

This is a Second Degree AV Block with Fixed 2:1 Conduction.  The atrial rate is approximately 80 and the ventricular rate is approximately 40.  The PR interval is normal at 0.12 and the QRS duration is narrow – 0.08.  The axis is normal and there are no signs of chamber enlargement.  There are Q waves from V1-V4, the zone of transition is late, and there is ST elevation in the inferior leads with ST depression everywhere else.  The T waves are pretty large in comparison to the QRSs.

This brings us to what I wanted to talk about: Second Degree AV Blocks.  There are three kinds of 2° AV blocks.  They are Type I (Wenckebach), Type II, and 2:1 conductors.  All have the same issue going on: A P wave that doesn’t cause ventricular firing when it should.  This means that the P wave fell at a time when the ventricles can be assumed to be repolarized – the P wave falls well after the T wave.  This is why blocked PACs, for example, aren’t 2° AV blocks.  PACs can fall so prematurely that the ventricles are not yet repolarized.  This isn’t the case in 2° AV blocks.

A 2° AV Block Type I has a progressively lengthening PR interval, a non-conducted P wave (making it a 2° AV block), then the PR interval pattern resets.  The QRS duration is usually narrow, and if rate assistance is needed the rhythm usually responds well to atropine.  Type Is are more commonly associated with inferior MIs, but the MI is not specifically necessary.

A 2° AV Block Type II is an extremely rare rhythm.  These have consistent PR intervals with occasional non-conducted P waves.  In contrast to Type Is, a Type II can have multiple non-conducted P waves in a row.  The damage to the myocardium that results in this rhythm is more extensive, so the QRS duration is almost always wide.  Type IIs do not usually respond to atropine, and pacing is almost always required.  Type IIs are more commonly associated with anterior MIs, and can progress to Complete Heart Blocks or ventricular asystole (no QRSs, just P waves marching along) suddenly.  These rhythms cause medics to go pale and sweat – these are dangerous.

So you can see that the difference between a Type I and a Type II is whether the PR interval is lengthening or constant.  Thus, you need at least two PR intervals consecutively to compare before a dropped beat: You need a PR interval ‘A’ to compare to PR interval ‘B’.  You have to compare the first PR interval to the second PR interval.  You can’t compare two first PR intervals.  Since the non-conducted P wave can reset the cycle, you need the comparison PR intervals to be between two dropped Ps: You need at least 3:2 AV conduction to be going on, but 4:3 or 20:19 or whatever works just as well.  However it works, you have to compare the first PR interval after the non-conducted P to the second PR interval.  You can’t compare two first PR intervals. 

What if you have 2:1 conduction?  You can’t technically tell if it is a Type I or a Type II.  So you should call it what you know it is: Second Degree AV Block with Fixed 2:1 Conduction.  Look at the ECG above, with the P waves noted.  Two to one atrial-ventricular conduction:

We can surmise that it is probably a Type I with 2:1 conduction going on for a couple of reasons.  First, there is an inferior MI, which is more commonly associated with Type Is.  Second, the QRS duration is narrow, which would be unusual in a Type II.  But when you look back above at the definitions, the words usually appear.  So there is no way to be technically positive, 100% sure.  The Q waves and late transition in the V-leads are probably an old anterior MI – maybe the one from 15 years ago, but again there is no way for us to be positive in a dude’s living room.

What’s your plan for this gent?

Hopefully it involves titrating his O2 delivery to maintain saturations of 100%, aspirin, and fentanyl, with rapid transport and early hospital notification.  Is this a STEMI alert?

I should say so.  This fellow is almost certainly having an acute inferior myocardial infarction.  Would you give him nitroglycerin?

Well, that’s a discussion for another day.  This post is long enough as it is.  My preview answer is that I would, if he met a couple of criteria.   See you next week for that discussion, and we’ll see if I can convince you too.