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Right bundle branch block; Pathological Q waves; First-degree AV block; Previous M.I.

Instructors' Collection ECG: Right Bundle Branch Block With Probable Previous M.I.

Sat, 09/08/2018 - 14:35 -- Dawn

This ECG was obtained from an 87-year-old man with chest discomfort.  We have no other clinical information.

ECG Interpretation   The rhythm is regular and fast, with P waves, at 95 beats per minute. So, it is normal sinus rhythm, but the rate is probably not “normal” for this patient.  The P waves are small, and difficult to see.  We suggest Lead I to best view the P waves in this example. This is a good opportunity to teach the value of evaluating rhythm strips in more than one simultaneous lead, as subtle features may not show up well in all leads.  There is a first-degree AV block, with a PR interval of 232 ms.

We see the right bundle branch block (RBBB) pattern: rSR’ in the right precordial leads (with a tiny q wave in V1, which is not typical of  RBBB).  The QRS is wide at 148 ms (.148 seconds).  The R prime (R’) represents the right ventricle depolarizing slightly after the left ventricle.  This terminal delay widens the QRS without affecting the depolarization or contraction of the left ventricle.  This delay can be seen in every lead, but is especially easy to see in Leads I and V6, where there is a wide little s wave.  It is normal for the T waves to be in a direction opposite that of the terminal wave (inverted in Leads V1 and III, for example.)

There is left axis deviation.  The causes of LAD are many.  It is not unusual for people with RBBB to also have a left anterior hemiblock (LAH), also called left anterior fascicular block.  The left anterior fascicle has the same blood supply as the right bundle branch.   LAH causes a frontal plane axis shift – instead of Lead II having the tallest QRS of the limb leads, Leads I and aVL will be the tallest upright QRS complexes of the six limb leads.  Lead II will be very small, or flat, or negative. However, the probability of pathological Q waves in the inferior leads offers a more likely explanation for the leftward axis shift.  The M.I. that would have caused these Q waves is old, as there are no acute ST changes.  It would, of course, help to know this patient’s history.

Right bundle branch block can make evaluating for ST segment elevation a bit tricky.  Occasionally, the terminal delay – especially in Leads III and aVF – can be mistaken for ST elevation.  The J points in this ECG all appear to be at the baseline, with no overt STEMI.

 

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