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ECG Basics: Atrial Fibrillation With Complete AV Block

This patient has an underlying atrial fibrillation with complete heart block and an idioventricular escape rhythm.  She was treated successfully with a permanent implanted pacemaker.

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Second-degree AV Block with 2:1 Conduction and Right Bundle Branch Block

This interesting ECG is a great one for your more advanced students who are ready to discuss the anatomical and physiological differences between the AV blocks, as opposed to just measuring PR intervals.  It shows a sinus rhythm with an atrial rate of 72/minute.  Second-degree AV block causes every other p wave to be blocked, resulting in a pulse rate of 36 beats per minute.  In addition, the ECG shows right bundle branch block, as evidenced by the wide QRS (136 ms), rsR' pattern in V1, and the wide little S wave in Lead I.

When second-degree AVB conducts 2:1, it can sometimes be difficult to determine if the block is Type I (occuring above the Bundle of His), or Type II (occuring at or below the Bundle of His).  This is because two p waves must be conducted in a row to see the tell-tale progressive prolongation of the PR interval seen in Type I (Wenkebach).

Two clues that this block is Type II are:  1) the presence of right bundle branch block.  Type II blocks are sub-Hisian blocks, often in the fascicles, and the right bundle branch block is a fascicle block.  Many Type II AV blocks show signs of right bundle branch block;   2) The non-conducted p waves occur well clear of the refractory periods of the preceding beats.  In Type I blocks, the QRS is eventually dropped because the p wave occurs in the refractory beat of the preceding QRS. Only one beat is missed.  In Type II blocks, p waves that SHOULD have conducted, don't.  Sometimes, more than one p wave in a row will be non-conducted.

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Second-degree A-V Block, Type II

Paramedic Erik Testerman has generously donated several excellent teaching ECGs to the Guru, and we will be featuring all of them soon.  This week, we show you the ECGs from a 59-year-old man with a blood glucose of 30 mg/dl.  He had no complaints .  After a bolus of Dextrose 10%, his blood glucose was 105 mg/dl.  He gave a past medical history of diabetes mellitus, hypertension, and left bundle branch block. Vitals were reported as normal and stable, except for the slow heart rate.

The first ECG, taken in the field, show a second-degree AV Block.  The conduction ratio is 2:1.  That is, there are two P waves for every QRS complex.  With this ratio, it is sometimes difficult to determine whether the patient has Type I (usually AV nodal) or Type II (Infranodal) AVB.  In order to diagnose Type I AVB (Wenckebach), we need to see TWO P waves in a row conducted, to see the prolongation of the PR interval.  It is not correct, however, to call ALL 2:1 AV blocks "Type II".  Often, simply taking a longer rhythm strip will expose a period of 3:2 conduction, showing progressive prolongation of the PRI.

This ECG, however, gives us some clues that it is probably TYPE II.  The patient has a left bundle branch block.  Type II AVBs are infranodal - that is, they affect the structures below the AV node:  the His Bundle and the Bundle Branches.  Type II AVBs represent INTERMITTENT TRI-FASCICULAR BLOCK, and that is common in the presence of RBBB  and LBBB (a bi-fascicular block).  This ECG probably represents an existing LBBB with an intermittent RBBB - When the right bundle is blocked, the patient has a tri-fascicular block, and no conduction to the ventricles.

Another clue that this is Type II is that the NON-CONDUCTED P waves fall CLEAR of the preceding T waves, meaning that they had ample opportunity to conduct, not being in the absolute refractory period.

Fortunately, the rhythm strip, taken one minute later, uncovers the diagnosis!  The sixth and seventh QRS complexes are conducted with a 3:2 ratio, showing PR intervals that stay the same, proving the rhythm is Type II. 

For this patient, the heart block and resulting bradycardia don't seem to be causing symptoms.  But infranodal blocks can easily progress to complete heart block and should be treated with implanted pacemakers.  The EMS crew in this case had transcutaneous pacer pads on the patient as a precaution, but he remained well-perfused and with a good BP the whole time. 

 

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High-grade AV Block

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.

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ECG Basics: Idioventricular Escape Rhythm

This six-second monitor strip was from a patient who was designated "Do Not Resuscitate", and whose heart rhythm was slowing dramatically.  It shows an idioventricular escape rhythm, with very wide QRS complexes and only two complexes in six seconds. (The top arrows mark three-second segments.)  If you look closely at the points marked by the lower arrows, you will see small, uniform, regular P waves.  The mechanism leading to this agonal rhythm was complete heart block.  A longer strip would show the P waves as all alike, and fairly regular, but slowing.  

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

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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ECG BASICS: Second-degree AV Block, Type II

Today's basic rhythm strip illustrates second-degree AV block, Type II.  Even though there is fine baseline artifact present, it is easy to measure the P-to-P interval, and your students will be able to see that every third P wave falls in the T wave.  The PR intervals are constant and the atrial rate is about 110/min.  The ventricular rate results from a 3:1 conduction ratio, and is less than 30/min.  For your students who have learned about bundle branch block, this strip shows a right bundle branch block, which is very common in second-degree Type II blocks, as they usually represent "intermittent tri-fascicular block" - that is, two of the three fascicles in the bundle branches are blocked, and one is intermittently blocked.  Other combinations of complete block and intermittent block are possible, resulting in intermittent failure of conduction.  This strip can start a lively classroom discussion about treatment of bradycardias.  See comments below for discussion of terminology, second-degree AVB and high-grade AVB.  This strip can be used as a good example of high-grade AVB. 

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Second-degree AV Block, Type I

This 67 year old man is noted to have a slightly irregular pulse.  At the beginning of this ECG, he appears to be in NSR with a first-degree AV block.  Twice, P waves are non-conducted.  Careful measurement of the P to P interval shows that it is regular, there are no PACs noted.  The PR interval changes very subtly by lengthening just before the non-conducted P waves.  A hint when non-conducted P waves are noted, first check for non-conducted PACs.  If the sinus rhythm is regular, check the PR interval before the non-conducted beat, and the PR interval immediately after the non-conducted beat.  You will see the PRI preceding the non-conducted P is longer than the PRI after the NCP.

Wenckebach conduction is caused by RP/PR reciprocity.  In other words, the shorter the RP interval, the longer the PR interval.  So, as the PRI lengthens, the QRS "moves" to the right, eventually causing the next regular sinus P wave to fall into the refractory period and fail to conduct.  This results in a pause, or a long RP interval, which shortens the next PRI. 

 If you or your students would like to review AV Blocks, go to this LINK for Dr. Grauer's excellent, FREE, self-directed tutorial.

For a slightly more advanced discussion of RP/PR reciprocity, see Jason's Blog.

 

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

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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