Long but not lost

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A 60’s y.o. man with vomiting

So, week 2 of our EKG rounds.  This one proved trickier than I expected.

We’ll start again from the beginning, first looking at the rate.  There is some irregularity (which we’ll come back to) as well as a PVC, but if you look at the rhythm strips, there are about 5 consecutive narrow complex QRS’s with a rate of just over 70.

Next up is rhythm which is where we’ll spend the bulk of our time today.  A most basic definition of Sinus Rhythm is a P before ever QRS and a QRS after every P.  Let’s focus just on the rhythm strip using lead II:

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Remember, the 3rd beat is a PVC so we’ll throw it out.  Looking at the next 5 QRS’s, we see 2 small bumps between each one.  The first thing we have to assume is that we’re looking at the T wave and the P wave.  If so, then the ST segment looks fairly ordinary but the PR segment is quite long.  Let’s look more closely at those PR segments, starting with the wave centered on a heavy gridline under the red dot.

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You’ll notice it’s 9 little boxes from the peak of the P wave to the beginning of the QRS.  A lot of people read this as one of the Mobitz rhythms, but if you count the little boxes, there are 9 little boxes between the peak of each P wave and the onset of the QRS.  It can’t therefore be a Mobitz I (Wenkebach) rhythm.  I think it’s probably the purple dotted beat especially that led some people to suspect Mobitz II.


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There isn’t an obvious P wave before this beat, but let’s look at that section of the EKG more closely:

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Next I’m dotting the previous T wave in blue and the previous P wave in yellow.  Note that both seem to be about one little box tall.  The mystery bump in green is 2 little boxes tall.

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What has happened is that the patient took a breath, blood flow to the heart changed a little due to the negative pressure exerted by inspiration, and the heart rate increased in response.  This is known as Sinus Arrhythmia, a terrible name if you ask me, but convention is what it is.  The P wave then fell on the T wave, and so the electricity was sensed simultaneously leading to a double-sized wave.  Notice again that the QRS starts 9 little boxes after the peak of the P wave.  The First Degree AV Block is enough to cause the P waves to be hidden in the ventricular repolarization when the heart rate exceeds about 90 in this patient.  Patients who don’t have as impressive of a First Degree Block will need a faster rate to hide their P waves, but can do it nonetheless (which will come up again at some point in the future).

One more look at the effect of adding one wave atop another, let’s look at the V1 rhythm strip atop II:

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Again looking at the green-dotted combo wave, note how its shape is a hybrid of the blue-dotted (somewhat short) T wave as well as the yellow-dotted biphasic P wave.  This causes the green-dotted apparent T wave to drop below the baseline.  Remember the T wave represents re-polarization.  The heart doesn’t again depolarize during repolarization so when you see extra inflection points in the T wave you have to ask if you’re actually seeing little lost P waves that aren’t transmitting through.  In this case, the P waves aren’t lost, they’re just taking a long time to transmit.

There’s one more place on the EKG that I suspect made some suspect Mobitz II, here black-dotted:

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In most of the leads, the P wave isn’t apparent above or below the black dot.  However, for whatever reason the P wave shows up nicely in the V1 rhythm strip; remember to look at everything!

Looking at the rest of the EKG, the QRS complexes appear narrow and there is no ST depression or elevation.  The T wave looks a little peaked in V2, but I think this is probably more a product of the QRS also having decent voltage in that lead; they’re not peaked elsewhere.  A number of people speculated that the vomiting in the vignette had caused an electrolyte disturbance (such as hypokalemia), but the patient’s lytes turned out to be normal.

Others who participated in the quiz felt the P waves in this EKG were U waves (which admittedly would fit nicely with hypokalemia).  It would be very unusual to have U waves that are taller than the T waves (I’m not sure if it’s even possible).  It would also be unusual to have a biphasic U wave (though this apparently can happen in some ischemic states).

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