High-grade AV Block

This ECG shows a second-degree AV block, Mobitz Type II.  It is also called “high grade AV block” because there is a 3:1 ratio of P waves to QRS complexes and a resulting slow rate.

Right bundle branch block and left anterior fascicular block are also present, as is common with Type II blocks.  The underlying rhythm is sinus.  Second-degree AVB, Type II, usually represents an intermittent tri-fascicular block:  often right bundle branch block and left anterior fascicular block (hemiblock) are present, and the left posterior fascicle develops an intermittent block.  During times of tri-fascicular block, the P waves are not conducted.  When the posterior fascicle is conducting, a QRS occurs.

A differential diagnosis for this ECG is complete heart block with ventricular escape rhythm.  A longer strip would be needed to see the P waves eventually dissociate from the QRSs, if they are going to do so.  Clinically, there is really little difference in the treatment of a high-grade "second degree" block and a "third degree" block. Both are treated with emergency support of the slow rate, as needed, and then a permanent implanted pacemaker.

It is notable that, in this case, the interpretation given by the machine is completely incorrect, even including the intervals.  This is not common, but does occur.  The machine's interpretation should be considered, but not followed blindly.

This week's ECG of the WEEK was donated to us by Sebastian Garay. These two ECGs were obtained less than 30 seconds apart from an 84 year-old man who called fire-rescue because he felt dizzy and fell.  He was not injured in the fall, and his vital signs remained stable, with an adequate BP.  These two ECGs were obtained prior to arrival in the Emergency Dept.

The first one shows a sinus rhythm at about 110/min.  There is a complete heart block (third-degree AV block), and the escape rhythm is a wide-complex rhythm at a rate of about 54/min and slowing severely toward the end.  The second ECG was taken less than 30 seconds after the first, and shows a significantly slower escape rhythm rate at 27/min., while the sinus rate increases to 120/min.  The change is sinus rate is likely an attempt by the nervous system to compensate for the lower cardiac output as the ventricular rate slows. The escape rhythm is not only slower, but there are some changes in the QRS morphology from the first ECG.

For your basic students, this ECG serves to demonstrate the AV dissociation seen in complete heart block.  It is easy to "march out" the P waves, and see that some of them are "hiding" in the QRS comlexes.  It also shows how quickly a rhythm can change rates.

For your more advanced students, you will want to have a discussion about escape rhythms.  This one initially has a fairly fast rate, suggesting junctional origin. The QRS morphology is of the right bundle branch type, with left anterior fascicular block.  However, ventricular rhythms originating from the posterior fascicle region can have the "RBBB / LAFB" morphology.  If this escape rhythm is fascicular (ventricular) in origin, it is an accelerated idioventricular rhythm.  The second escape rhythm appears very similar to the first, with the very noticeable exceptions of QRS morphology, especially in V1 and V2, and the rate.

This patient was given Atropine in the ED, with no change to the rhythm.  We do not know what transpired after that, but suspect a pacemaker was in his future. 

We look forward to comments from our members about these two very interesting ECGs.

This ECG has always caused a lively conversation in ECG classes, both beginner classes and advanced. There is an obvious underlying sinus tachycardia, with clear P waves.

Some propose that the ECG shows a second-degree AVB, Type II, in that the PR intervals are constant, or nearly so. There is a slight discrepency if you compare the first PRI with the others. The QRS complexes, while very slow, have a pattern of right bundle branch block with left anterior fascicular block - not an unlikely finding in second-degree AV block, Type II, since that is a block in the fascicles of the interventricular conduction system. Type II blocks usually are accompanied by signs of bundle branch dysfunction.

Others strongly believe this is a third-degree, or complete, heart block. They argue that the PR intervals are not identical, and propose that a longer strip would uncover the discrepency. The wide QRS complexes have a strong left axis deviation, which could support the argument for idioventricular escape rhythm.

What do you think? Please comment below.