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Second-degree AV Block

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.

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 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.

Second-Degree AV Block, Type I

Sat, 06/13/2015 - 22:07 -- Dawn

This ECG is from an 80-year-old woman who had an acute inferior wall M.I. with a second-degree AV block.
 
Some people incorrectly call ALL second-degree AV blocks that are conducting 2:1 "Type II".  This is incorrect, as Mobitz Type I can also conduct with a 2:1 ratio.  The progressive prolongation of the PR interval will not be seen with a 2:1 conduction ratio, because there are not two PR intervals in a row.

This is a good example of a Type I, or Wenckebach, block which is initially conducting 2:1.  At the end of the ECG, two consecutive p waves conduct, showing the "progressively-prolonging PR interval" hallmark of a Type I block. Type I blocks are supraHisian - at the level of the AV node - and generally not life-threatening.  Blocks that are conducting 2:1 present a danger, however, in the effect they have on the rate.  Whatever the underlying rhythm is, the 2:1 block will cut the rate in half!  This patient has an underlying sinus tachycardia at 106, so her block has caused a rate of 53.  In light of her acute M.I., that rate is probably preferable to the sinus tach. This patient’s BP remained stable, and she did not require pacing. 

The ST signs of acute M.I. are rather subtle here. Note the "coving upward" shape in Lead III, and the reciprocal depressions in I, aVL, V1, and V2.  Type I blocks are common in inferior wall M.I., since the AV node and the inferior wall often share a blood supply - the right coronary artery. 

While the print quality of this ECG is not the best, it is a great teaching ECG because it starts out with 2:1 conduction, then at the end of the strip, proves itself to be a Wenckebach block.   

High-Grade AV Block

Wed, 06/03/2015 - 21:25 -- Dawn

This ECG shows a second-degree AV block, Mobitz Type II.  It is also called “high grade AV block” because there is a 3:1 ratio of P waves to QRS complexes and a resulting slow rate.

Right bundle branch block and left anterior fascicular block are also present, as is common with Type II blocks.  The underlying rhythm is sinus.  Second-degree AVB, Type II, usually represents an intermittent tri-fascicular block:  often right bundle branch block and left anterior fascicular block (hemiblock) are present, and the left posterior fascicle develops an intermittent block.  During times of tri-fascicular block, the P waves are not conducted.  When the posterior fascicle is conducting, a QRS occurs.

A differential diagnosis for this ECG is complete heart block with ventricular escape rhythm.  A longer strip would be needed to see the P waves eventually dissociate from the QRSs, if they are going to do so.  Clinically, there is really little difference in the treatment of a high-grade "second degree" block and a "third degree" block. Both are treated with emergency support of the slow rate, as needed, and then a permanent implanted pacemaker.

It is notable that, in this case, the interpretation given by the machine is completely incorrect, even including the intervals.  This is not common, but does occur.  The machine's interpretation should be considered, but not followed blindly.

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

Mon, 03/02/2015 - 23:19 -- Dawn

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.

Second-degree AV Block,Type I, With 2:1 Conduction

Fri, 04/04/2014 - 23:17 -- Dawn

This ECG is a follow-up to last week's ECG of the WEEK, which presented an AVB that was mostly conducted 2:1, and proved to be a "Type II" block when it conducted 3:2 with consistent PR intervals at the end.  We often just use the term, "2:1 AVB", rather than try to discern the Mobitz type, realizing that the most important feature of a 2:1 block may be that it automatically cuts the heartrate in half.  That is, there are half as many QRS complexes as P waves, which can result in a very slow pulse.  

However, some AV blocks occur at the AV node level, and some occur below the node.  The latter type tend to be more ominous, and less likely to self-correct.  Type II (or sub-Hisian) blocks nearly ALWAYS need to be paced.  So, it is beneficial to make the correct diagnosis when possible.  One helpful sign is that Type II blocks frequently show signs of bundle branch pathology, and have wide QRS complexes.  It is possible for Type I blocks to have wide complexes, or bundle branch block, but it is also more likely that they will have narrow QRS complexes, indicating normal conduction through the ventricles.

The BEST way to determine the type of block from the  ECG is to get lucky and find a moment of 3:2 or better conduction.  Then, the PR intervals can be assessed for progressive prolongation (Type I) or no change (Type II).   This ECG, while not of the best print quality, clearly shows the progressive prolongation of the PR intervals in the last two QRS complexes, unmasking this rhythm as Type I, Wenckebach. 

ECG BASICS: Second-degree AV Block, Type II

Sat, 05/25/2013 - 13:12 -- Dawn

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. 

Second-degree AV Block, Type I

Sat, 02/16/2013 - 18:52 -- Dawn

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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