Tracings 4.1

A 62 year-old man was conveyed to the  ED in Police Department custody complaining of pressure-like chest pain. The patient relates that his chest pain came on coincidentally with the stress brought on by his arrest approximately 24 hours prior. It has been “off and on” since then. The police had taken the patient to an outside hospital on the day prior to this ED visit for chest pain. The patient reports that he was told that he had “some type of heart block” and that they wanted to do a cardiac catheterization but he refused and left AMA.

The patient relates a history of chest pain dating back several years but he has never had a heart attack. The pain occurs twice a week, is precipitated by stress or exercise, does not radiate, is accompanied by “a little” shortness of breath and generally resolves after 20 minutes.  He relates that he “miserably failed” a stress test one year prior, but did not want to undergo a cardiac catheterization at the time.

PMH: Type 2 diabetes, hyperlipidemia.  Medications are glipizide 10 mg twice a day, nitroglycerin sublingually when necessary, simvastatin 20 mg daily, aspirin 81 mg daily. He denies alcohol, tobacco, and illicit drug use.

On exam, BP is 122/76, R 18, P 78 and irregular, T 36.0. there are no other significant findings.

This ECG is obtained. What is your interpretation?

The ECG reflects normal sinus rhythm at a rate of 75 bpm (P waves identified with arrows). Some of the P waves that fall on T waves look slightly more peaked than the others but this is an interaction with the T wave, not a second atrial focus. This fact is confirmed by the observation that the P waves all march out.

The long PRs and widely varying PR intervals might make one tempted initially to interpret the tracing as 3rd degree AV block.

However, a basic ECG principle is: be suspicious of complete HB as a diagnosis when the QRS rhythm is irregular. Why? Because you should expect an escape AV nodal or ventricular rhythm to be regular. And this rhythm is pretty irregular.

So….another look reveals that there are repeating PR intervals of differing durations, each identified on the tracing above as “A”, “B”, and “C”. This rules out A-V dissociation…and eliminates complete heart block as a diagnosis.

The cycles of increasing P-R intervals with some blocked  P waves (marked with “X”) are diagnostic of Mobitz Type I (Wenckebach) second degree heart block. One of the blocked beats (marked with a question mark) falls on a QRS complex and is therefore not visible.

There is generally 3:2 conduction present, that is, for each 3 P waves, 2 are conducted to the ventricles and produce QRS complexes. (Or you could also say 3:1 block; for each 3 P waves, 1 is blocked.) The exception is the third complete group of beats, for which there is only an “A” PR interval before a P wave is blocked, reflecting 2:1 conduction (or, if you prefer, block).

When second degree heart block presents with consistent 2:1 conduction, it is impossible to tell whether there is Type I or Type II second degree block. However, when there is 2:1 block with concurrent evidence of Wenckebach conduction, as we have here, we can assume that Wenckebach conduction  prevails.

Finally, there is also first degree heart block, as the shortest P-R interval measures 0.44 sec. Indeed, if this were not present, the diagnosis of Mobitz I would be considerably less problematic.

Case follow up: the patient ruled out for acute MI with 2 negative troponins. He was offered cardiac catheterization and/or further cardiology consultation but signed out against medical advice.

Tracings is a learning module involving actual cases of patients and their ECGs that present to the Emergency Department.  Topics are derived from the EM Model for Resident Education. Cases are prepard by Dr. William Berk.

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