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Complete AV block

Inferior Wall M.I. With Wide QRS and Complete AV Block

Wed, 06/28/2017 - 14:13 -- Dawn

This ECG is from a 66-year-old woman who called 911 for a complaint of chest pain for the past four hours. She also complained of nausea, vomiting, and diarrhea for that time. She was pale and diaphoretic, and her BP was 77/43 sitting up, improving to 90/54 reclining. She denied “cardiac” history.  Her medications included:  aspirin, an SSRI, cilostazol, amlodipine, umeclidinium and vilanterol inhaler, atorvastatin, levothyroid, and metoprolol. We don’t have a previous ECG.  The EMS crew followed their chest pain protocol and delivered the patient to a facility with an interventional cath lab, but they did not designate a “STEMI Alert” because of the wide QRS.  It is their protocol to use the term “STEMI Alert” only when no M.I. mimics, such as left bundle branch block, are present. 

What does this ECG show?     There is an underlying sinus rhythm at 75 bpm.  There is AV dissociation, with regular, wide QRS complexes at a rate of 44 bpm.   This meets the criteria for complete heart block (third-degree AV block).  The morphology of the QRS complexes meets the criteria for left bundle branch block (wide, upright in Leads I and V6, negative in V1).  At a rate of 44 bpm, two options for this escape rhythm are possible:  1)  junctional escape with LBBB and 2) idioventricular escape rhythm. Because the LBBB criteria are met and the rate is over 40 bpm, we are voting for 1).  Also, in the presence of IWMI, AV node ischemia is very likely, resulting in AV blocks at the level of the AV node.  CHB at the AV node would result in junctional escape rhythm, and CHB below that, in the fascicles of the bundle branches, would result in idioventricular escape. The issue for this patient, and ANY patient, is cardiac output, and we see several reasons for cardiac output to be lower:

·         Wide QRS

Complete AV Block With Junctional Escape Rhythm

Thu, 10/27/2016 - 14:29 -- Dawn

This ECG is from a 78-year-old woman.  We do not know any clinical details.

 We break from our usual habit of removing the ECG machine’s interpretation of the ECG to serve as a reminder that the computer interpretation can be wrong.  ECGs should ALWAYS be interpreted by a knowledgeable person.  The machine interpretation can serve as a reminder, but should not take the place of human interpretation. 

Here is what we DO see:  There is a normal sinus rhythm present, as evidenced by the regular P waves that do not change their morphology.  Some of the P waves are “buried” behind QRS or T waves.  The atrial rate is 95 bpm. 

The ventricular rhythm, at 40 bpm, is also regular, but is separate from the atrial rhythm.  Even though some of the P waves LOOK like they have conducted to produce QRS complexes, they have not.  The PRIs are not all the same.  Neither do they “progressively prolong”.  There is no irregularity of the QRS rhythm or variation in QRS morphology.  We see the classic “AV DISSOCIATION” of complete heart block. 

When there is a third-degree AV block with a narrow-QRS escape rhythm, we can assume the block is in the AV node.  The junction is the escape focus, producing a narrow-complex rhythm between approximately 40-60 bpm.  In this case, the QRS is slightly wide at 112 ms (.11 sec), and the QRS complexes in several leads are fragmented.  Some might argue that there is an idioventricular escape mechanism.  But, with a normal frontal plane axis, borderline width,  and no T wave inversions, the rhythm looks more supraventricular.  The R wave progresson on the precordial leads shows a persistently negative QRS with late transition in V5.  The QRS complexes in V1 and V2 appear to have pathological Q waves.  When R wave progression is not normal, we should also consider electrode misplacement. 

Paced Rhythm Following AV Node Ablation

Sun, 12/27/2015 - 20:38 -- Dawn

This ECG is taken from a woman who had suffered for several years with intractable intermittent atrial fibrillation. She had tolerated medications poorly, and several attempts at electric cardioversion had resulted in only temporary relief. Ultimately, she chose to undergo AV node ablation.  In the electrophysiology lab, her AV node was destroyed, preventing the atrial fib impulses from penetrating into the ventricles.  This resulted in a “man-made” complete AV block.  A pacemaker was implanted in the EP lab.  When she is in atrial fibrillation, the fibrillatory waves of the atria INHIBIT the atrial pacing electrode from firing, so she has no paced P waves at that time.  The right ventricular pacing electrode functions without inhibition, and makes a wide QRS complex with a leftward axis deviation (normal for RV pacing). 

In this ECG, we see the patient WITHOUT atrial fib, and the pacemaker is pacing the atria AND the ventricles, in a sequential fashion.  The spikes are very hard to see, as this is a “bipolar” pacemaker, which makes much smaller spikes than a “unipolar” pacemaker.  Some ECG machines will automatically enhance the spikes, but this one did not.  We have marked a “sample” atrial spike in blue for you and one of the ventricular spikes in red.  Each beat on this ECG actually has appropriately-timed atrial and ventricular stimuli (spikes), and the patient has optimized cardiac output provided by the “atrial kick”. A P wave occurring just before a QRS indicates that the ventricles are filling from the forceful contraction of the atria.  This provides much better filling than when the atria are not beating or are fibrillating. 

This is a good ECG to use to show your students how we can recognize a paced rhythm without being sure of the spikes.  Of course, without other evidence (patient history and exam), we can’t know for sure that this is a paced rhythm, but the steady, normal rate, wide complexes, and left axis deviation are signs of RV pacing.  Look for negatively-deflected QRS complexes in II, III, and aVF and positive QRSs in aVL and aVR. 

When pacing only one ventricle, the impulse travels relatively slowly through the other ventricle, resulting in wide QRS complexes.  This then results in SECONDARY ST-T WAVE CHANGES.  Wherever the QRS is positive, you will normally see some ST depression and T wave inversion.  In leads with negative QRS complexes, the opposite is true, and you will see ST elevation and upright T waves.  This can make evaluation of ST segments for acute M.I. difficult.



ECG Basics: Third-degree AV Block, Complete Heart Block

Wed, 11/18/2015 - 12:31 -- Dawn

This rhythm strip shows third-degree AV block, also called complete heart block or complete AV block.  The P waves are from the sinus node, and are regular at a rate of about 120/min. (Sinus tachycardia). This is a good strip for showing your students how to "march out" the P waves to find the ones that are hidden behind QRS complexes or T waves. Knowing that the P waves are regular, it is easy to find the hidden ones.

The QRS complexes are wide at 0.14 seconds, and regular, with a rate of about 28/min.  On first glance, it APPEARS that there are PR intervals.  That is, it appears that some of the P waves are conducting. If you measure the PR intervals carefully, you will note that they are NOT equal.  There is no connection between the P waves and the QRS complexes - this strip has just caught them near each other.  If we ran the strip longer, we would see the PR intervals "come apart", proving they are not real.  The QRS complexes are coming from an IDIOVENTRICULAR ESCAPE RHYTHM.  They are regular, wide, have no P waves associated with them, and the rate is below 40 bpm.

Patients with CHB that results in a very slow heart rate sometimes need emergency treatment aimed at increasing the rate.  When the escape rhythm is idioventricular, it is assumed that the AV block is located below the AV node, and emergency temporary pacing is often the method of choice.  In fact, a permanent implanted pacemaker is almost always needed.  When the AV block is located in the AV node, the escape rhythm will be junctional (narrow QRS complexes, rate about 40-60 bpm).     


Complete AV Block With Junctional Escape Rhythm

Thu, 09/03/2015 - 23:02 -- Dawn

This ECG was taken from a 90-year-old woman.  We have no other history, unfortunately.  It is a good example of a sinus rhythm with complete AV block, also called third-degree AV block.

The defining characteristics of this rhythm include:   1) an underlying rhythm that is regular and with a physiological rate.  In other words, the P waves are not so fast that they would not be expected to conduct one-to-one.  2)  a second rhythm of regular QRS complexes that is unrelated to the P waves.

Occasionally, a P wave may occur before a QRS and appear to have a PR interval.  This is just a chance meeting, as both rhythms (P waves and QRS complexes) are regular AT DIFFERENT RATES, so we would expect them to occur near each other from time to time.  NONE of the P waves are being conducted to the ventricles to produce QRS complexes. This is a good ECG to demonstrate "marching out" the P waves to see that they are very regular, even though some are hidden in the QRS, ST segment, or T waves.

In this case, the "escape rhythm" occurs from the AV junction.  The AV junctional pacemakers are "set" at a rate of about 40 - 60 beats per minute.  Normally, the sinus rhythm arrives in the AV junction faster than that, depolarizing the junctional pacemakers and preventing them from firing spontaneously.  In complete AV block, the atrial impulse never arrives, so the junctional pacemaker is free to "escape" and become the primary pacemaker of the heart.  We recognize this rhythm as junctional because the QRS complexes are narrow, and the rate is around 40 bpm.  Knowing that the escape rhythm is from the junction tells us that the AV block is in the AV node.  The AV junction is the first available pacemaker below the block.  Had the complete AV block been lower, in the bundle branches, the QRS would have come from the ventricles and would have been wide and slower.

In very general terms, this "supra-Hisian" type of AV block is preferable to a "sub-Hisian" block.  The rate is faster, and the QRS complexes narrow, both conditions causing a better cardiac output than wide QRS complexes and extremely slow rates.  However, the effect of the block on the patient has a lot to do with the cause of the block and the symptoms the slow rate cause.  Emergency treatment of the rate may be necessary if it causes a drop in blood pressure and perfusion.  Some patients with this type of block will need a permanent implanted pacemaker, but not all. 


Complete AV Block

Tue, 09/16/2014 - 14:26 -- Dawn

This ECG is from an 84-year-old man who experienced dizziness and a fall.  He was not injured in the fall.  In this ECG, we can clearly see regular P waves at about 110 per minute.  We also see wide QRS complexes at about 52 per minute.  There is AV  dissociation - there are no regular PR intervals, or even progressively-prolonging PR intervals.  The atrial and the ventricles are beating to separate rhythms.  What is interesting about this rhythm is the origin of the escape rhythm.  The wide complex suggests a ventricular focus and the rate suggests supraventricular origin.  Near the end of the ECG, the escape rhythm either fails or slows significantly.  To see the next 12-lead ECG for this patient, go to this LINK.

The second ECG makes it more clear that this is an idioventricular escape rhythm, but the morphology of the QRS complexes suggested that, even in the first ECG when the rate was faster.  There are several clues that this is probably ventricular, including a very "backward" axis with aVR being upright and II, III, and aVF all being negative.  Also, V6 is negative, and there is nearly precordial concordance:  all except V1 are negative.  The morphology of the QRS does not fit a diagnosis of either left bundle branch block OR right bundle branch block.  The evidence points to a ventricular origin for this escape rhythm, and the patient quickly goes on to slow down severely.  Ventricular escape rhythm strongly suggests a sub-Hisian location for the block, and they tend to be more life-threatening than supra-Hisian blocks.

The take-home clinical lesson here is to BE PREPARED for worsening of the rate whenever AV block is present, especially high-grade AV block or sub-Hisian block.  This ECG is a very good one for teaching students to "march out" P waves, and find "hidden" P waves.  We have included a marked copy of this ECG to indicate those P waves.

Thanks to Sebastian Garay for donating these ECGs.


ECG Basics: Pacemaker Failure to Capture

Sun, 04/27/2014 - 17:29 -- Dawn

This ECG is taken from a patient with an implanted pacemaker who was experiencing near-syncope.  She was taken to the hospital by EMS, where the pacemaker was adjusted to obtain ventricular capture.  This ECG did not have a Lead II rhythm strip, so the 12-lead ECG is being presented.  The P waves have been marked with a "P", pacemaker spikes marked with an arrow, and the QRS complexes marked with a "J" because they are junctional.  Because we can see 12 leads, or viewpoints, the morphology of the P waves and QRS complexes changes each time the machine switches to a new lead.

The underlying rhythm is sinus, with nearly regular P waves occuring at a rate of about 72 beats per minute.  The QRS complexes are also regular, but they are dissociated from the P waves.  Because the rate is near or just under 40 bpm, and the QRS complexes are narrow, this represents a slow junctional rhythm.  Because both atrial and ventricular rhythms are regular, but not associated with each other, an interpretation of complete heart block (third-degree AV block) can be made.  This explains why the patient had a pacemaker implanted.

The pacer spikes, for the most part, track the P waves, which is how this pacemaker is programmed.  They are not followed by a paced QRS complex, however.  This is failure to capture.  The second and fourth P waves did not stimulate a pacer spike because of their proximity to the T wave of the junctional beat.  The mA (energy setting) was adjusted in the Emergency Dept., and the pacemaker did not require repositioning.  The patient regained a reliable paced rhythm.

This section of the ECG Guru is meant to be for your basic students.  Pacemakers now have become very complex, with many options and variable settings.  So complex, that I would not feel comfortable getting into any more detail than I have here (although visitors to the site are welcome to).  It is important that, if you deal with patients in an emergency setting, you do not tell the patient that "something is wrong with their pacemaker" until it has been evaluated by a qualified person who can electronically interrogate the device.  It can be very difficult to determine from an ECG how a pacemaker is programmed, and how it should be reacting.  Since this patient had symptoms related to the bradycardia, and since pacemaker spikes occurred free of any refractory period and did not produce QRS complexes, it is safe to say there needs to be an adjustment.

In an emergency, with serious symptoms present, a transcutaneous or transvenous temporary pacemaker can be used.  Medications such as Atropine, epinephrine, and norepinephrine are also used, depending upon the type of AV block and the resources available.


ECG Basics: Sinus Rhythm With Complete AV Block and Ventricular Escape Rhythm

Sun, 07/28/2013 - 13:36 -- Dawn

This rhythm strip shows a good example of complete (third-degree) AV block with ventricular escape rhythm.  It will be easy for your basic students to "march out" the P waves.  They are regular at a rate of about 88/min., and they are either visible, or are "hiding" in the QRS complex.  The ventricular rhythm is wide and very slow, and completely dissociated from the sinus rhythm.

For your more advanced students, you may want to discuss the likely origin or "level" of the block.  Blocks above the Bundle of His can have JUNCTIONAL escape rhythms, while blocks that occur below the Bundle of His generally have ventricular escape rhythms.  Ask your students which type of CHB they would prefer to have:  suprahisian or subhisian - and why?

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