At the ECG Guru website, our main goal is to provide quality teaching materials to those who teach ECG interpretation and other cardiac topics. This ECG offers teaching opportunities for those who teach any level of student.
The patient: This ECG was obtained in the Emergency Department from a 54-year-old man who was complaining of severe chest pain and nausea. His BP was 130/68.
The ECG: The rhythm is interesting and not uncommon in the setting of inferior wall M.I. The atrial rate is about 158 bpm and the P waves are regular. (Marked in red on accompanying rhythm strip). Some P waves are hidden in T waves or ST segments, but we can see fragments of them. The QRS complexes are narrow and mostly regular at a rate of about 56 bpm. The second QRS on the strip is early, but from the third QRS on, they are regular. The PR intervals are not consistent. Careful measurements will show that they get shorter and shorter as the recording progresses. There are more P waves than QRS complexes and there is NO association between the P waves and the QRS complexes, so there is AV DISSOCIATION. One might argue that the first two QRS complexes are conducted from the P waves, with a shorter PR interval with beat #1 and a longer one with beat #2. This would be a short episode of Wenckebach conduction if that is what is happening. Since we don’t have a strip preceding this one, we can’t be sure. Clinically, it is smart to address where the patient is now, and that is SINUS TACHYCARDIA WITH AV DISSOCIATION AND A JUNCTIONAL RHYTHM. Or, it would be fine, after the first two beats, to say the patient is now in COMPLETE AV BLOCK WITH JUNCTIONAL ESCAPE or THIRD-DEGREE AV BLOCK WITH JUNCTIONAL ESCAPE. We know that this conduction failure is occurring at the level of the AV node because the escape rhythm is junctional. Blocks at the AV node level are often temporary. A block lower than this, from the His Bundle through the common branch of the bundle branches, would produce a ventricular escape rhythm. The important thing, if you are the patient’s health care provider, is that you assess this rate for adequate perfusion. A heart rate of 57 is almost always sufficient for good perfusion, and is actually preferable for a patient with an ongoing M.I. Which would you rather have if you were having an M.I., a heart rate of 57 or 158 bpm? Could we say that this AV block has actually HELPED this particular patient? I will happily leave more advanced discussions of this arrhythmia to our commenters.
The ST segments are noticeably elevated in Leads II, III, and aVF. There is reciprocal ST depression in Leads I and aVL, and also in the anterior leads. Lead III has a deep, but not wide Q wave. This pushes the frontal axis a bit to the left (I and aVL are taller than II). The T waves in many leads are “hyperacute”. That is, they are taller than expected, which is a sign of ischemia. This patient was confirmed to have a INFERIOR WALL M.I. Since the RCA supplies the inferior wall and the AV node in the majority of people, it is not surprising that there is an AV block at the level of the AV node. Also, the posterior wall is supplied by the RCA in most people, and the ST depression in the anterior wall with prominent R waves in the septal leads point to posterior M.I. The much less prominent ST depression in V1, as compared to V2, indicates that the right ventricle is sending a signal to V1 to "elevate", while the posterior wall tells V1 to "depress". This results in an ST segment that is cancelled by opposing forces. This would be a good patient to perform V4Right, V7, V8, and V9 on (16-lead ECG).
This is a good ECG to demonstrate what ST segment elevation looks like in acute M.I., as opposed to other causes of STE. In M.I., we see certain characteristics:
· The STE is found localized in related leads
· The ST segments tend to be straight or curved upward, rather than the normal convex shape
· There is ST depression is leads that are opposite the elevation (See III and aVL)
· There are accompanying signs of M.I., such as T wave inversion, hyperacute T waves, and pathological Q waves.
This ECG can also serve to start a conversation with students about when it is appropriate to treat bradycardia, and when it is best left alone.
EDIT: Dr. Ken Grauer has written a very informative and thought-provoking comment below. If you would like to hear his thoughts on this ECG in more depth, go to his blog at https://ecg-interpretation.blogspot.com/2021/12/ecg-blog-268-76-mobitz-i-vs-complete-av.html?m=1
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