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Instructors' Collection ECG of the WEEK: Left Bundle Branch Block With Second-Degree AV Block, Type II

Mon, 11/28/2016 - 18:44 -- Dawn

 This ECG was obtained from an 84-year-old woman who was scheduled for surgery.  When the anesthesiologist did this ECG, the surgery was cancelled. It is a very good example of fascicular-level blocks. 

The underlying rhythm is a regular sinus rhythm at about 95 bpm.  There are some non-conducted P waves which are part of the sinus rhythm (not premature beats).  When the P waves DO conduct, the PR interval is steady at about .15 seconds (148 ms).

In addition, there is a LEFT BUNDLE BRANCH BLOCK.  The ECG criteria for LBBB are:  1) A supraventricular rhythm, 2) A wide QRS, and 3) A negative QRS in Lead V1 and a positive QRS in Leads I and V6.  The QRS duration in this ECG is 136 ms.

There are generally two fascicles (branches) in the left bundle branch, and one main fascicle in the right bundle branch.  So, a LBBB represents a “bi-fascicular block”.  That means that A-V conduction is proceeding down only one fascicle (the right bundle branch).  In that fascicle, there is an “intermittent” block.  When the RBB is not blocked, we see a QRS.  When it is blocked, we see none.  This is then termed an “intermittent tri-fascicular block” – otherwise known as SECOND-DEGREE AV BLOCK, TYPE II.  Type II blocks nearly always have a wide QRS due to the underlying bundle branch pathology.  You may see RBBB, LBBB, or RBBB with left anterior fascicular block (hemiblock).  Very rarely, the combination might include left posterior hemiblock.  The intermittent block in the “healthiest” fascicle(s) is what makes this a second-degree block, and not a complete heart block (third-degree AVB).

The clinical implications of this block are that the heart is operating on only one fascicle, and that fascicle is showing obvious signs of distress.  A third-degree AVB could be imminent.  In addition, LBBB causes a wide QRS, which decreases cardiac output.  Second-degree, Type II AVBs can result in very slow rates, and sometimes cause more hemodynamic instability that some third-degree AV blocks.

This patient was scheduled for pacemaker implantation instead of the originally-scheduled surgery. 

ECG Challenge: Changing QRS Morphology

Sat, 11/12/2016 - 21:01 -- Dawn

Today's ECG CHALLENGE offers a two-lead rhythm strip and a 12-lead ECG from an 87 year old man with a complaint of shortness of breath.  We do not know any other clinical information.  What is the underlying rhythm?  How can we explain the changing QRS morphology?  There are two distinctly different QRS complexes - one wide and one narrow.  Comment below and let us know what you think.  We will answer your questions as soon as we can,  and will give a final diagnosis in one week.

ANSWER:  Please see Dr. Grauer's comment below for the diagnosis.  The intermittent left bundle branch block that is seen in this strip appears in the 12-lead ECG to be rate-related.  That is, after a PAC, we see a pause, and then normal conduction for one beat.  As the rate increases, the left bundle branch block resumes.  The rhythm strip, which was recorded about 5 1/2 minutes before the 12-lead ECG, shows more intermittent normally-conducted beats, which don't seem to be rate-related.  We would need a longer rhythm strip to definitively prove rate-related left bundle branch block, but it is a very common cause of intermittent LBBB.

Ask the Expert

Thu, 10/27/2016 - 15:33 -- Dawn

Question:

 

Dr. Jones,

I am confused about the repolarization abnormalities that occur in conditions other than acute M.I. (Bundle branch block and hypertrophy, for example). I have been taught that the repolarization abnormalities should point opposite the MAIN part of the QRS, but also I have been told that they should point opposite the TERMINAL deflection of the QRS.  Which is right?

 

Today’s expert is Dr. Jerry W. Jones, MD, FACEP, FAAEM

Jerry W. Jones, MD FACEP FAAEM is a diplomate of the American Board of Emergency Medicine who has practiced internal medicine and emergency medicine for 35 years. Dr. Jones has been on the teaching faculties of the University of Oklahoma and The University of Texas Medical Branch in Galveston. He is a published author who has also been featured in the New York Times and the Annals of Emergency Medicine for his work in the developing field of telemedicine. He is also a Fellow of the American College of Emergency Physicians and a Fellow of the American Academy of Emergency Medicine and, in addition, a member of the European Society of Emergency Medicine. Dr. Jones is the CEO of Medicus of Houston and the principal instructor for the Advanced ECG Interpretation Boot Camp and the Advanced Dysrhythmia Boot Camp. 

 

Answer:

 

 

Which Direction Should the Repolarization Abnormality Point?

OK. You've got an abnormal QRS complex followed by a repolarization abnormality (RA). Which direction should the repolarization abnormality point? As a young resident, I was taught that the RA should point in the direction opposite the terminal deflection of the QRS complex. But years later, I see other physicians stating that the repolarization abnormality should point opposite the main deflection of the QRS complex. Which is correct?

The answer is both are correct. Why? How?

The reason is that the repolarization abnormality is connected to the ventricle in which the problem is located - not the QRS complex itself. To better understand this, let's look at some of the major causes of repolarization abnormalities (you can find examples in the illustration at the top of this page):

Right Bundle Branch Block (RBBB) - When you look at the QRS complex in V1, you see an R and an R'. The R represents left ventricular activation while the R' represents right ventricular activation. So, the problem lies in the right ventricle represented by the R'. The repolarization abnormality reflects the problem in the RV so it should be opposite the R' which is always the last deflection in V1 in the presence of RBBB. Therefore, in cases of RBBB, the repolarization abnormality is always opposite the terminal deflection of the QRS.

Left Bundle Branch Block (LBBB) - When you look at the QRS complex from V6 which has a LBBB, we see a relatively tall, upright monophasic QRS complex. Part of that QRS represents right ventricular depolarization and part represents left ventricular depolarization. But how much of which? We don't know, but all we need to know is that this is a monophasic complex and it is upright. Therefore, since the repolarization abnormality reflects the problem in the left ventricle, and the LV is represented somewhere in that monophasic R, the repolarization abnormality should be opposite the main deflection. Therefore, in cases of LBBB, the repolarization abnormality is always opposite the main deflection of the QRS.

Left Ventricular Hypertrophy (LVH) - When you look at the QRS complexes from V5 and V6, we see a relatively tall, upright monophasic QRS complex. Part of that QRS represents right ventricular depolarization and part represents left ventricular depolarization. But how much of which? Again, we don't know, but all we need to know is that this is a monophasic complex and it is upright. Therefore, since the repolarization abnormality reflects the problem in the left ventricle, and the LV is represented somewhere in that monophasic R, the repolarization abnormality should be opposite the main deflection. Therefore, in cases of LVH, the repolarization abnormality is always opposite the main deflection of the QRS.

Right Ventricular Hypertrophy (RVH) - The same concept discussed regarding LVH applies in cases of RVH. Therefore, in cases of RVH, the repolarization abnormality is always opposite the main deflection of the QRS.

Ventricular Pre-excitation - Most people reading ECGs don't realize that ventricular pre-excitation can also produce a repolarization abnormality. Just as repolarization abnormalities are not always present in cases of LVH and RVH, they are not always present in cases of ventricular pre-excitation, either. However, the repolarization abnormality IS present in some cases. The RA is connected to the ventricle containing the accessory pathway, but don't worry: you don't have to determine which ventricle that is. If a repolarization abnormality is present in a lead, it should be negative if the delta wave is positive and vice versa. Therefore, the repolarization abnormality points opposite to the direction of the delta wave.

 

So, the question really isn't whether the repolarization abnormality should be opposite the terminal or the main deflection of the QRS. It should be opposite the deflection that represents the involved ventricle.

Ventricular Systole

Click to open: 
Ventricular systole

This is an original illustration by Dawn Altman. It may be used free of charge for enhancement of classroom teaching materials. For commercial publication, please contact the artist at [email protected] 

ECG Basics: Second-degree AV Block With Characteristics of Type I and Type II

Thu, 09/01/2016 - 11:51 -- Dawn

This strip shows a second-degree AV block.  During most of the strip, 2:1 conduction is present.  At the beginning, however, two consecutive p waves are conducted, revealing progressive prolongation of the PR interval.  This usually represents a Type I , or nodal, block:  progressive refractoriness of the AV node.   However, the wide QRS ( possibly left bundle branch block), and the fact that the non-conducted p waves are "out in the open" where they should have conducted, points to Type II - an intermittant tri-fascicular block. Wenckebach periods in patients with LBBB can be caused by progressive conduction delay in the right bundle branch.

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1924:  Willem Einthoven wins the Nobel prize for inventing the electrocardiograph.

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