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Case #38

You are on call covering the entire hospital. Your pager goes off and you answer the call. It's a patient you don't know, but are covering for whose heart rate shot up to 150's and whose blood pressure suddenly dropped. You tell the nurse you will be right there.

You arrive on the scene and quickly eye ball the patient, he looks stable but uncomfortable. You ask the nurse for the latest vitals, which are a pulse of 148, BP of 82/69, 20 and 97% on RA. You take a look at the vital flow sheet and notice he usually runs in the 140's/70's and his pulse was in the 70's most of his hospital stay until 20 minutes ago when the heart rate shot up and the blood pressure dropped. He was admitted a week ago for new left sided weakness and aphasia. He has PMHx significant for an old CVA, HTN. On physical exam he does not appear to be dry, has left sided upper and lower paralysis, is aphasic and not following commands, which is how he presented. He does not appear to be in distress. His exam is essentially normal besides that and notably you feel the pulse to be rapid and regular. You grab the EKG machine and try to figure out why he is going so fast? Below is the EKG obtained at that time. What is the rhythm causing him to go 150? (the answer is present on the EKG)

Click here for better quality image of EKG

 

This is a typical EKG for a Supra-Ventricular Tachycardia (SVT). What is even more interesting about this EKG, is that we can figure out what type of SVT is present because of subtle hints on the EKG.

It is important to approach the tachycardia in a systematic approach. Like any EKG the first step is to try and figure out the rhythm. In this case there are no obvious p waves so the next step is to determine if it is regular or irregular. This rhythm is obviously regular. The next step is to determine if the origin is in the ventricles, or above the ventricles. By having the presence of a narrow QRS complex, this tells us where ever the rhythm is originating from it is using the usual conduction system, i.e. AV node, bundle of his, perkinje fibers. So this means the rhythm can be originating from the AV node and above. So in this case it is a Regular Supra-ventricular Tachycardia (SVT).

SVT is a generic term when the specific rhythm can not be determined on the EKG and it has it's own differential. SVT encompasses 5 different rhythms.1) Sinus Tachycardia 2)AV-Nodal Reentry Tachycardia 3)Atrio-Ventricular Reentry Tachycardia i.e. WPW 4)Atrial Flutter 5) Atrial Tachycardias. In this EKG we can be more specific than saying SVT because there are subtle clues to the underlying rhythm.

No p waves are seen preceding the normal appearing QRS complexes. This does not exclude sinus tach as they may be buried in the t wave because the rate is so fast. The rate is 150 which also usually makes you think of atrial flutter. But there are no obvious signs of "p waves marching through" to go along with that.

What is present are these notches in the ST segment in II, III, AVF, AVL, and AvR as seen in figure 1. The notches are actually retrograde p waves. Notice looking back in the EKG that the notches are downward in II, III and AvF and up in AVR. Exactly the opposite of a sinus p wave. This makes the diagnosis of Typical AV-Nodal Reentry tachycardia. This is caused by the presence of both a slow and fast pathway with in the AV node which if a PAC occurs at just the right moment will start the re-entry circuit and cause this rhythm to occur. Usually the p waves will be buried in the QRS, but sometimes, they will occur right after the QRS and be retrograde.

Figure 1.

The next step is to confirm or disprove the diagnosis. Remember, the patient is still going at 150 so we have to slow him down. When you have a narrow SVT, the first step is a vagal maneuver (carotid massage or bearing down) to slow AV nodal conduction down to differentiate what type of SVT it is. If that doesn't work the next step is adenosine. If we are hypothesizing by the EKG that this is AVNRT, adenosine should break the rhythm itself. And on the following EKG which is post adenosine, you will see not only did the rhythm break and his blood pressure returned to 137/70, but notice that the retrograde p waves are gone, and sinus rhythm is restored!!

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