Monday, April 16, 2018

Is this just right bundle branch block?

This ECG was texted to me with the text:

"There is a history of RBBB but we do not have an image to compare this with.  I interpret as RBBB with atrial fib.  Anything else?"
What was my response?

The QRS duration is long: the computer measured it at 212 ms.  I measured it at approximately 180 ms.  Either way, it is is too long for simple RBBB.

My response:  "It is a pretty wide RBBB.  So think about high potassium."
The minimal ST segment shifts seen throughout are within normal limits, so there is no ischemia here.

The response was: K = 6.3 mEq/L.

The patient was treated for hyperkalemia.  Unfortunately, no post-treatment ECG was recorded.

QRS duration in RBBB and LBBB

RBBB by definition has a long QRS (at least 120 ms).  But very few are greater than 190 ms.  Literature on this is somewhat hard to find, but in this study of patients with RBBB and Acute MI, only 2% of patients with pre-existing RBBB had a QRS duration greater than 200 ms.  This study only reported durations in 10 ms intervals up to 150 ms, but one might extrapolate from it that approximately 10% of patients with baseline RBBB have a QRS duration greater than 160 ms.  194 ms would be quite unusual.

The point of this is that if you see BBB with a very long QRS, you must suspect hyperkalemia.  Then of course the peaked T-waves should tip you off.   Unless a patient has severe hypercalcemia (this should be evident by a short QT on the ECG as seen at the bottom of this post), or severe hyperphosphatemia (which is very unusual), treatment with calcium is harmless if you read an ECG falsely positive for hyperkalemia.

So don't wait for the laboratory K or you might be resuscitating a cardiac arrest (see the case with ECGs #3 and #4 of this post).

How about LBBB?

In this study of consecutive patients with LBBB who were hospitalized and had an echocardiogram, 13% had a QRS duration greater than 170 ms, and only 1% had a duration greater than 190 ms. 

See this case.

Is This a Simple Right Bundle Branch Block?

Here is a case of RBBB with a K of 7.9 and QRS duration of 194 ms.

This was recorded next day at a K of 3.3:
The QRS duration is 149 ms, much more appropriate for simple RBBB


  1. In our HEART FAILURE CLINIC,we have seen multiple cases of Non Ischemic CMP with severe LV Systolic Dysfunction and badly damaged,scarred ,dilated and globally hypokinetic hearts with QRSd > 200 mSec in cRBBB. It is interesting to know their exact percentage in a study. Thought Provoking ! Thanks Dr Smith for putting up the Case !

  2. NICE case! In addition to the important point that any excessively wide QRS rhythm should prompt consideration of hyperkalemia — I would add the Key Learning Point that you have NO idea what a hyperkalemic patient’s ECG will look like until you correct serum K+. This concept is wonderfully illustrated by the 2nd and 3rd ECGs — in which the RBBB remains after the serum K+ came down from an initial value of 7.9 mEq/L — and along with it, not only does QRS duration return to a reasonable level for RBBB — but diffuse T wave peaking completely resolves. As a result, we are able to say that with a serum K+ = 3.3 mEq/L (as in the 3rd ECG) — the ECG now shows no sign of primary ST-T wave change. The same might not be true for the 1st ECG. T wave peaking is really ONLY seen in lead V2 in this 1st ECG. We have NO idea if the subtle-but real ST segment flattening and slight depression in leads V3-thru-V6 (more than should be seen with “simple” RBBB) — or the upright T in V1 (which should be negative in V1 with “simple” RBBB) — or the Q in V1 (No Q is normally seen in V1 with “simple” RBBB) — or the relatively small and rounded T waves in V5,V6 (which typically should be peaked in hyperkalemia) — might reveal concerning ischemic changes once serum K+ is normalized. The T wave peaking typically seen with a serum K+ = 6.3 mEq/L may be altering this initial ECG by opposing forces that blunt what otherwise might reflect a RBBB with acute cardiac changes. We simply have NO idea what a hyperkalemic patient’s ECG will look like until serum K+ is corrected … and unfortunately, no follow-up tracing is available for this 1st ECG in this blog post. THANKS to Dr. Smith for presenting this case!

  3. possibly only Ken would pick up the abnormal Q waves in V1 in that first ekgs...
    i hope you got snow in florida this weekend.
    guys , what say you about the rhythm?
    i'm not convinced its a-fib as initially interpreted. and hyperK can distort the p wave conduction, no?
    thank you.

  4. I have a question.
    Say for example, we have a resulted potassium of 6.9 mmol.
    And at the same time, we have the results of that patient's calcium levels at 3.0mmol.
    Would it still be hazardous to give calcium to treat the hyperkalaemia ?

    1. That is a low calcium. Did you mean to ask about a high calcium? If so, you might be more careful. But if there is any ventricular dysrhythmias, then you can't give too much calcium.

    2. Yes. I was thinking about those with concomitant severe hypercalcemia which is malignancy related. Calcium levels could go up to 3.5-4.0mmol. These patients may very well have AKI and their K could be at 6.0-6.9mmol.


DEAR READER: I have loved receiving your comments, but I am no longer able to moderate them. Since the vast majority are SPAM, I need to moderate them all. Therefore, comments will rarely be published any more. So Sorry.

Recommended Resources