Second-degree AV block Type II

If you are an ECG instructor, you probably carefully choose ECGs to illustrate the topic you are teaching. One of the reasons for the existence of the ECG Guru website is our desire to provide lots of such illustrations for you to choose from.

Sometimes, though, an ECG does not clearly illustrate one specific dysrhythmia well, because the interpretation of the ECG depends on so many other factors.  In order to get it “right”, we would need to know information about the patient’s history, presentation, lab results, or previous ECGs. We might need to see the ECG done immediately before or after the one we are looking at.  Some ECG findings must ultimately be confirmed by an electrophysiology study before we can know for sure what is going on.

For those of us who are “ECG nerds”, it can be fun to debate our opinions and even more fun to hear from wiser, more advanced practitioners about their interpretations.

My belief, as a clinical instructor, is that we must teach strategies for treating the patient who has a “controversial” ECG that take into account the level of the practitioner, the care setting, and the patient’s hemodynamic status.  In some settings, it might be absolutely forbidden for a first-responder to cardiovert atrial fibrillation, for example.  But atrial fib is routinely cardioverted under controlled conditions in hospitals.  The general rule followed by emergency providers that “all wide-complex tachycardias are v tach until proven otherwise” has no doubt prevented deaths in situations where care providers did not agree on the origin of the tachycardia.

The ECG:    We do not have much patient information to go with this ECG, just that it is from a 71-year-old woman who developed severe hypotension and lost consciousness, but was revived with transcutaneous pacing.   Here is what we do know about this ECG:

·        There are regular P waves, at a rate of about 39 bpm (sinus bradycardia).

This ECG is taken from an elderly woman who complains of feeling weak and tired. We have no other clinical information, unfortunately.

There is an obvious bradycardia, with more P waves than QRS complexes.  Here is what we see:

*  Atrial rate is around 115/min. and P waves are regular and all alike.

*  Ventricular rate is around 35/min. and QRS complexes are regular and all alike.

*  PR intervals, when they occur, are all the same at 162 ms.

*  QRS duration is wide at 122 ms.

*  QTc interval is prolonged at 549 ms.

What does this mean?  There is sinus tachycardia with second-degree AV block because the atrial rate is over 100/min, but not all P waves are conducted.  The AV block looks like a Type II (Mobitz II) block because the PR intervals are all the same.  This is a reliable indicator of conduction. (Not third-degree AVB).  The wide QRS complexes are due to right bundle branch block.  The ECG signs of RBBB are: 1) wide QRS; 2) supraventricular rhythm; and 3) rSR’ pattern in V1 and Rs, with a wide little s wave, in Leads I and V6.

Just like other subjects we are taught in school, ECG interpretation is usually taught in a very basic, simplistic way.  As we add to our knowledge, we are able to determine the mechanisms of more complex rhythms. 

When I took my first basic ECG rhythm monitoring course, I memorized all the “rules”, and at the end of the course, I thought I could read ANY strip correctly.  Then, in real life, I found that some rhythms can’t be interpreted from one lead, or even from one 12-lead ECG. 

This strip offers advanced readers to challenge themselves, and it offers teachers a chance to show students an “exception to the rules” if it is appropriate for those students.  We all learn the classification of second-degree AV blocks:  Both Type I and Type II show an underlying sinus rhythm with some P waves conducted and some not.  Type I has progressively prolonging PR intervals until a P wave is non-conducted.  The cycle restarts after the dropped QRS.  Type II has PR intervals that are all the same, and may be prolonged or normal. 

In this ECG, you will be able to “march out” a normal sinus rhythm at a rate of 80 bpm.  The P waves are marked with small dots at the bottom.  Two of every three P waves are followed by QRS complexes.  Is it Type I?  No – the PR intervals are not prolonging.  Is it Type II?  The PR intervals are not the same!  What is happening? 

There is also left bundle branch block, which is a sub-Hisian block.  Blocks occurring in the intraventricular conduction system include bundle branch blocks, second-degree AVB Type II,  and third-degree AVB with ventricular escape.  This group of blocks tends to be more threatening than the blocks that occur in the AV node (second-degree type I and third-degree with junctional escape). 

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.