Dawn's picture

This rhythm strip was obtained from a man who was suffering an acute inferior wall M.I.  There are ST elevation and hyperacute T waves.  The rhythm is SINUS ARRHYTHMIA WITH SECOND-DEGREE AV BLOCK, TYPE II.    There is also first-degree AV block.

There are more P waves than QRS complexes, with a 3:2 ratio.  The atrial rate varies between 55 -68 beats per minute.  The sinus rate speeds slightly after the dropped QRS in each group. The ventricular rate is about 40 bpm, with grouped beating. (Regularly irregular.)

The PR intervals are steady at 226 ms (slightly prolonged).

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Jerry W. Jones MD FACEP's picture

With a Mobitz II AV block, the PR interval that immediately precedes the non-conducted P wave and the PR interval that immediately follows it should be equal.

Jerry W. Jones MD FACEP FAAEM
https://www.medicusofhouston.com
Twitter: @jwjmd

Dawn's picture

Thanks, Dr. Jones. 

Dawn Altman, Admin

fibrillate's picture

I have been an instructor for a Basic Cardiac Monitoring class for 17 years. Mobitz I & II + 3rd degree, are where the students struggle the most. I focus on the P to QRS relationship, and tell them over and over to use the calipers to ascertain the Atrial and Ventricular rates. 

I must say though, we don't provide any strips of Mobitz II that include Sinus Arrhythmia; that would throw them off completely. 

Pete Hupp

Jerry W. Jones MD FACEP's picture

Pete...

I agree with you and Dawn that those students at or just beyond the introductory level can be easily confused or even frustrated when "exceptions to the rules" appear. And AV blocks can be very confusing at times. I still find a lot of misinformation circulating on the web about AV blocks. I think that to remind the students that slight variations in the sinus rhythm will have NO effect on the PR interval might help. They can still use the PR interval to diagnose first degree AV block and Mobitz I, II and 2:1 second degree AV blocks.

Third degree AV block is difficult for many people because they are often taught to diagnose it by recognizing AV dissociation. Unfortunately, AV dissociation is NOT what DEFINES 3rd degree AV block. Of course, ALL cases of 3rd degree AV block manifest AV dissociation, but so do a lot of other dysrhythmias. What defines 3rd degree AV block is the fact that P waves appear in areas of diastole that SHOULD have allowed conduction... but they did not conduct!

I found very early in my years of teaching ECG interpretation that to mention all the "exceptions" in the beginning was overwhelming for many students. I think that at the introductory level it's best NOT to mention any exceptions. Introduce the exceptions slowly in a more advanced class or in a later session. The students will forgive you for not telling the "whole truth" in the beginning.

Best of luck with your teaching!

Jerry W. Jones, MD

Jerry W. Jones MD FACEP FAAEM
https://www.medicusofhouston.com
Twitter: @jwjmd

Dawn's picture

I agree. I think it is more difficult to teach beginning students, especially if they have very little A&P background. How to teach accurately without getting into the weeds???  And, they are bound to be confronted with rhythms that don't follow the "rules" they have learned. Teachers who teach more advanced students don't always know the struggle!  :-). 

Dawn Altman, Admin

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