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Arrhythmia Recognition Webcast Series
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Practice ECG 21
This is a great ECG! It is a tough one to interpret, so spend some extra time to examine it thoroughly and systematically.
The rate is below 60 BPM, so we know that we are dealing with a bradycardic rhythm. There are P waves before each complex, with a fixed PR interval. The rhythm, therefore, is sinus bradycardia. Now, let's look at the intervals. The PR interval is slightly over 0.20 seconds, so the patient has a first-degree heart block. The QRS intervals are wide, over 0.12 seconds to be exact. This means there are three possibilities: LBBB, RBBB, and IVCD. The presence of slurred S waves in leads I and V6, and the RSR' pattern in V1, make this an RBBB.
Next, calculate the axis. It is in the right quadrant, so this is a right axis deviation. Now, whenever we have an axis in the right quadrant we should immediately ask ourselves if this is a posterior fascicular block. Well, we don't see evidence of any excluding criteria, RAE or RVH, so it could be a LPH. Do we have an S wave in lead I and a small Q in III? Yes. Therefore, this patient also has an LPH. In fact, this is a bifascicular block with RBBB and LPH. This is a dangerous combination, usually present when a large amount of myocardium has been affected by some pathologic process, most often a myocardial infarction.
Every time you see a bundle branch block you should also look for concordance, right? Right! Are there any leads with concordance on this ECG? V2 and V3 are concordant. In addition, they have ST segment elevations. Normally in RBBB, the ST segments in V1 to V3 are at baseline or slightly depressed. ST elevation, especially with concordance, is a sign of a possible AMI in progress. The presence of a first-degree heart block and a bifascicular bundle block, RBBB and LPH, in a patient having an acute MI is an ominous sign. This is an indication for placing a transvenous pacemaker. You don't have to turn it on if you don't need it, but it is comforting to have in place in the event that the patient goes into complete block. It's easier to place a pacemaker in a stable patient than in one who is arresting.
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