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Escape rhythm

ECG Rhythms: AV Block

Wed, 11/22/2017 - 16:48 -- Dawn

This strip was obtained from a woman who presented to her doctor’s office with hypertension. While there is some artifact in the baseline, it is possible to determine the presence of P waves, thanks in part to having two leads to assess.  We have provided an unmarked version of the strip for you to use, and also a marked version for the sake of this discussion.

The underlying rhythm is sinus bradycardia, at about 60 bpm, but with some slight variation in the P to P intervals (about 920 ms to 1040 ms). Because of the artifact, it is difficult to determine the exact P to P intervals, and the exact morphology of the P waves. So, we can’t say for sure that the P waves are all alike.

The AV block occurs at a 3:1 ratio.  That is, for every three P waves, one is conducted and produces a QRS complex.  When the P waves are not conducted, an escape rhythm occurs.

The escape rhythm occurs at an escape interval of about 1720 ms.  In other words, when a QRS does not occur by that time, the escape beat is produced.  It appears to be from the AV junction, in spite of the slow rate, because the escape QRSs look like the sinus conducted QRSs.  Both sinus and junctional rhythms are conducted along the bundle branches and produce the same QRS morphology.  The QRS complexes are approximately .08-.10 seconds wide.  Note that QRS complexes numbered 3, 5, and 7 have a P wave fused to the beginning of the QRS, making the QRS look wide when it is not.   A junctional escape rhythm results from AV block in the AV node, as the junction is the first available pacemaker below the AVN. 

This patient was scheduled for a treadmill stress test in her doctor’s office, which was cancelled. She had no cardiac symptoms at the time of the ECG, except the above-noted hypertension.  Unexplained bradycardia, especially when accompanied by AV node blocks, should trigger an assessment for inferior wall M.I., since the inferior wall of the LV shares a blood supply with the SA and AV nodes in the majority of people.

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.


High-grade AV Block

Thu, 03/13/2014 - 14:42 -- Dawn

To continue on a topic started by Jason Roediger in his February ECG Challenge -

This series of two ECGs was taken from a 71-year-old man who complained of dizziness and near-syncope the day before these ECGs were done.  He was seen in an Emergency Dept., and advised to follow up with a neurologist. On the day of these ECGs, still feeling dizzy and like he would pass out, he called EMS again.  He denied chest pain.  We do not know his past medical history.  The first ECG was taken at 10:22 am.  His BP was 177/76 and SpO2 99%.  It shows a regular sinus rhythm (p waves marked by small asterisks) at a rate of about 75 / min.  There is a high-grade AV block, meaning that some P waves are conducted (beats 2, 4, 7), but most are not.  In addition, he has an escape rhythm, probably ventricular, at a rate of just over 40 / min.  The overall effect of the escape rhythm is to keep the heart rate above 40 beats per minute.

Fifteen minutes later, at 10:37 am, another ECG is taken.  The patient's BP is 154/86.   This ECG shows the high-grade AV block quite well, but this time, most of the QRS complexes on the strip are conducted from P waves.  It is difficult to see all the P waves in every lead, but if you remember that all three channels are run simultaneously, you will find evidence of the P waves in at least one of the three leads represented at any given time.  (Example:  V1, V2, and V3 - V3 shows the P waves well).  The next-to-last QRS on the page is interesting, as it has a different PRI than the normally conducting beats.  Is this a fusion beat or an aberrantly-conducted one?   It probably does not matter to the outcome of the patient. 

The slowing of the rate in the second strip gives us a clue as to why the patient felt dizzy, but the blood pressures recorded did not catch hypotension.  Possibly if the patient had been standing instead of lying on a stretcher, we would have seen more hemodynamic changes.

Unfortunately, we do not know the outcome of this patient, but it seems he is a candidate for an implanted pacemaker.

Complete Heart Block

Sat, 04/27/2013 - 21:38 -- Dawn

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.


Third-degree AV Block and Junctional Escape Rhythm With Right Bundle Branch Block and Prolonged QTc Interval

Sat, 01/26/2013 - 13:48 -- Dawn

This ECG is from a 70 year old woman for which we have, unfortunately, no clinical information.  It shows a sinus rhythm with a rate of about 72 bpm (NSR) with AV dissociation caused by third-degree heart block.  The escape rhythm is junctional at a rate of 38 bpm.  There appears to be a right bundle branch block, based on the QRS duration of 132 ms, and a wide S wave in Leads I and V6.  The precordial leads do not show the usual RBBB pattern of rSR' in V1 and V2, and the r wave progression is poor (non-existent).  This is felt to be due to poor lead placement (a good teaching point).  Of interest, the ECG machine has reported a "severe right axis deviation" based on the tall upright R wave in aVR and the deep S in avF.  In RBBB, the first part of the QRS represents left ventricular depolarization, and the terminal wave represents the delayed right ventricle.  In effect, the two ventricles have their own electrical axes, which we can see because the ventricles are not depolarizing simultaneously.  The axis of the LV appears to be normal in this tracing.

In addition to the above, this patient has a very prolonged QT interval.  The QT is longer in bradycardic rhythms, but when corrected to a standard of 60 bpm (QTc), this patient's QT interval is still prolonged at QTc: 552 ms.  Without clinical data, we cannot speculate  as to why this patient's QTc is prolonged, but it can be a very dangerous situation.  Follow the links for more information on QT prolongation and Torsades de Pointes and Long QT Syndrome.

As always, we welcome comments from our members adding insight to this interesting ECG, and also questions you would like to ask our Guru members.

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