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Rate-related Left Bundle Branch Block

Submitted by Dawn on Fri, 01/14/2022 - 16:58

The patient:  This ECG is from an 87-year-old man who was transported to the Emergency Department by paramedics. His chief complaint, as reported by caregivers, was lethargy, fever, and a declining mental status.  He appeared tired and slightly confused, and was normotensive.

The ECG:  There are a rhythm strip with two leads, II and III, and also a standard 12-lead ECG. The RHYTHM STRIP shows a tachycardiac rhythm that slows very slightly toward the end.  The rate is around 107 bpm, with an R to R interval of approximately 543 ms in the earlier, regular portion.  There are regular P waves present, all followed by QRS complexes.  Most of the QRS complexes are normal width, but the 2nd, 5th, and 8th are slightly wide at 130 ms, or .13 seconds.

These wider QRS complexes represent aberrant conduction with LBBB occurring intermittently.  Aberrant conduction often occurs due to a faster heart rate, but the only clue here is the intermittent conduction disturbance seems to disappear when the rate slows very slightly.  It is hard to determine mechanism of aberrant conduction when we have only a ten-second rhythm strip.

The 12-LEAD ECG  has essentially the same rate and rhythm, except all the beats in the first ¾ of the ECG are conducted aberrantly, in a LEFT BUNDLE BRANCH BLOCK pattern.  This indicates that the LBB is refractory at this time.  Beat No. 15 is premature (PAC).  The pause after the PAC allows the left bundle branch to repolarize, conducting one single beat normally.

Dawn's picture

ECG Basics: Paroxysmal Supraventricular Tachycardia Converting to Sinus With PACs

Submitted by Dawn on Fri, 10/29/2021 - 15:20

This strip shows a supraventricular tachycardia, rate 196 bpm, after adenosine was administered to the patient.  The PSVT breaks, and an irregular rhythm composed of sinus beats and premature atrial contractions ensues.  This is common after medical cardioversion. The patient later settled into a normal sinus rhythm.  The abrupt change from a fast, regular rhythm to a slower, irregular rhythm is evidence that the tachycardia was due to a reentrant circuit, and not sinus tachycardia.

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ECG Basics: Sinus Rhythm With Non-Conducted PACs

Submitted by Dawn on Fri, 10/01/2021 - 15:07

This is a good strip to demonstrate the change in the appearance of a T wave when a premature P wave occurs on the preceding T wave.  The PACs found the atria ready to depolarize and produced a P wave that landed on top of the preceding T wave, making it appear taller than the others.  The PACs also reset the sinus node, causing a slight delay before the next sinus discharge.  The PACs occurred while the ventricles were still refractory, so no QRS complexes followed.

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Sinus Rhythm With Atrial Bigeminy

Submitted by Dawn on Tue, 07/07/2015 - 15:56

This ECG is from an 88-year-old man with congestive heart failure.  No other clinical information is known.  It shows an underlying sinus rhythm with atrial bigeminy - every other beat is a premature atrial contraction.  There is very little, if any, difference in the morphology of the sinus P waves and the ectopic P waves, indicating that the ectopic focus is in the vicinity of the sinus node.  There is no "compensatory" pause, because PACs penetrate the sinus node, resetting it.  So, the underlying sinus rate here is about 72 beats per minute.  There are several mechanisms for bigeminy to occur, but ectopic bigeminy is the most common.

Dawn's picture

ECG Basics: Sinus Bradycardia With A Premature Atrial Contraction

Submitted by Dawn on Mon, 04/13/2015 - 21:45

This strip shows an underlying sinus bradycardia with a rate less than 40/min.  There is one "premature" beat, which can be considered to be ectopic, because it interrupts an otherwise regular rhythm.  The interesting thing is that the premature beat is not terribly early - it is about 740 ms from the previous beat.  If all the beats were spaced like this, the heart rate would be about 84/min.  There is probably an element of "escape" here, in that the ectopic beat is able to express itself due to the slow rate.  A faster sinus rate would override this ectopic focus.  So, we could view this early beat as a help, rather than a problem.  The most important consideration here is to address the cause of the bradycardia, and treat appropriately. 

Dawn's picture

ECG Basics: Sinus Rhythm With A Premature Beat

Submitted by Dawn on Thu, 03/19/2015 - 14:11

This strip offers something interesting for both your basic-level students and for your more advanced students.  First, it is a good example of sinus rhythm with a premature beat.  The PR interval was measured by the machine at .21 sec (218 ms).    The premature beat is supraventricular - that is, it is not a PVC.  Because of the slightly long PRI in this strip, it's P wave COULD be buried in the preceding T wave.  That would make this a premature atrial contraction (PAC).  

For discussion with your more advanced students, the P wave could, instead, be retrograde, and occurring during the QRS or slightly after it.  That would make the premature beat junctional, or an atrial echo beat. The origin of the premature beat is mostly academic - there is likely no clinical need to determine the origin.  

In looking for clues as to the origin of the premature beat, we would scrutinize the premature beats for "hidden" P waves.  Upright and before the premature beat would indicate a PAC.  Negative P waves before, during, or after the premature QRS would indicate PJCs.  In this strip, the T waves just before the premature beats are slightly deeper than the other T waves.  This could indicate atrial "echo", or reciprocal beats, which requires the presence of dual junctional pathways, in which the impulse turns around, reenters the atria, and causes a new beat.  It can be helpful to look at multiple leads (the more the better) in your search for P waves.  For a look at this patient's 12-lead ECG, go to this link.  

The P wave of a premature beat often penetrates the SA node and "resets" it, causing the next normal beat to occur after a "normal" R-to-R interval from the premature beat. This fact can help us find "hidden" P waves, as well.

Another interesting feature of this strip for your students who are interpreting 12-Lead ECGs, is that this ECG shows the criteria for left ventricular hypertrophy.  See the link above for the 12-lead and discussion.

 

 

 

 

 

 

Dawn's picture

Left Bundle Branch Block

Submitted by Dawn on Thu, 08/07/2014 - 21:57

This is a good example of sinus rhythm with left bundle branch block.  There is some irregularity due to a PAC at the beginning.  The QRS is wide at 144 ms (.14 seconds).  There is also first-degree AV block, with a prolonged PR interval of 228 ms.  The criteria for diagnosis of left BBB are:  wide QRS, supraventricular rhythm, and a negatively-deflected QRS in V1 with a positive QRS in Leads I and V6.  

Left bundle branch block can be associated with many forms of heart disease, including CHF.  It can be permanent, transient, intermittent, or rate-related.  The wide QRS of LBBB significantly decreases cardiac output, causing poor perfusion symptoms in some people.

This ECG is a good one for your students who are just transitioning from reading rhythm strips to reading 12-lead ECGs.  It shows the value of multi-lead assessment of rhythms. You will notice that P waves are difficult to see in some leads.  Armed with the knowledge that the four channels on this ECG are run simultaneously, you can show the students how finding P waves in one lead will allow you to find them in the leads that are above and below that lead. 

Similarly, it can be difficult to see the QRS width in some leads.  The leads in the same vertical column can help you see the QRS's true width, even if part of the QRS is "flat" in the isoelectric baseline.

Dawn's picture

Normal Sinus Rhythm With Aberrantly-Conducted PACs

Submitted by Dawn on Tue, 04/02/2013 - 22:00

This is a normal 12-Lead ECG with two PACs that are aberrantly conducted in a right bundle branch block pattern. (Sixth and ninth beats). In the PACs, the QRS is slightly wider than the normal beats.  The aberrantly-conducted beats have an rsR' pattern in V1, and a wide little S wave in aVL. No PACs are seen in Lead I to demonstrate the wide S wave.  This represents a right bundle branch block pattern, which is a common form of aberrancy, and is rate-related.  That is, the PAC occurs early in the cycle, catching the right bundle branch is a refractory state and unable to depolarize.  Slower beats are easily acommodated by the right bundle branch.

Dawn's picture

PACs With Left Anterior Fascicular Block Aberrancy

Submitted by Dawn on Mon, 10/08/2012 - 19:08

A good example of aberrantly conducted premature beats (PACs or possibly PJCs) that are conducted with a left anterior fascicular block.  The underlying rhythm is sinus at about 80/min.  The timing of the premature beats is best seen in the Lead II rhythm strip at the bottom, as this ECG machine does not print the 12 leads in an uninterrupted manner.  You will see interruptions each time the leads change.

The first beat on the ECG is one of the premature beats.  You can observe the left axis deviation without pathological Q waves.  Lead I shows the premature beats with an Rs pattern, and Leads II and III have rS.  The early beats have caught the anterior fascicle of the left bundle branch refractory from the preceding beat.  It recovers for the normally-timed sinus beats.

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