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ECG Basics: 2:1 AV Block

Submitted by Dawn on Fri, 06/07/2019 - 15:48

Second-degree AV block can either be Type I (Wenckebach) or Type II.  In either case, some P waves are conducted to the ventricles, and some are not. Type I blocks usually occur in the AV node, and are often benign. Type II blocks are more often "sub-Hisian", or fascicular blocks, and are more likely to progress to higher levels of AV block and bradycardia.  When a second-degree AVB is conducted in a 2:1 ratio, it is difficult to differentiate Type I from Type II.  Features that favor the diagnosis of Type I are narrow QRS complex and the non-conducted P waves land on the previous T waves - during the refractory period of the ventricles.

Type II blocks are more likely to have a wide QRS with a bundle branch block morphology.  That is because Type II blocks often reflect serious fascicular disease.  A typical Type II block is a persistent bifascicular block (ex: RBBB and left anterior hemiblock)) with an intermittent block in the third fascicle.  Another way to think of it is an intermittent tri-fascicular block. If that one remaining fascicle stops conducting, the patient will be in complete heart block.

Signs of Type II blocks include the wide QRS and also two or more non-conducted P waves in a row.  Also, P waves that are "out in the open", away from the refractory period, but fail to conduct are an ominous sign.

One strategy for reacting to a 2:1 block is to first assess the ventricular rate (54 bpm in this example).  Determine if it is adequate for the patient's hemodynamic stability.  If not, act to increase the rate.  Otherwise, it may be prudent in the stable patient to watch the rhythm strips for a while.  Sometimes, two p waves in a row will conduct - unmasking either progressive prolongation of the PR interval (Type I) or stable PR intervals (Type II). 

The patient in this example was having an inferior wall M.I.  The ST elevation will not always show up on a monitor strip, as it does here.  A 12-lead ECG is the minimum standard for evaluating for coronary artery disease and acute M.I.  It is possible that the 2:1 block will disappear when the atrial rate (about 108 here) is slowed.

Two Types of Complete Heart Block

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Two Types of Complete Heart Block

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Second-degree AV Block, Type II?

Submitted by Dawn on Wed, 02/01/2017 - 23:09

This ECG is taken from an elderly woman who complains of feeling weak and tired. We have no other clinical information, unfortunately.

There is an obvious bradycardia, with more P waves than QRS complexes.  Here is what we see:

*  Atrial rate is around 115/min. and P waves are regular and all alike.

*  Ventricular rate is around 35/min. and QRS complexes are regular and all alike.

*  PR intervals, when they occur, are all the same at 162 ms.

*  QRS duration is wide at 122 ms.

*  QTc interval is prolonged at 549 ms.

What does this mean?  There is sinus tachycardia with second-degree AV block because the atrial rate is over 100/min, but not all P waves are conducted.  The AV block looks like a Type II (Mobitz II) block because the PR intervals are all the same.  This is a reliable indicator of conduction. (Not third-degree AVB).  The wide QRS complexes are due to right bundle branch block.  The ECG signs of RBBB are: 1) wide QRS; 2) supraventricular rhythm; and 3) rSR’ pattern in V1 and Rs, with a wide little s wave, in Leads I and V6.

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Left Bundle Branch Block With Second-Degree AV Block, Type II

Submitted by Dawn on Mon, 11/28/2016 - 18:44

 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. 

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Complete AV Block With Junctional Escape Rhythm

Submitted by Dawn on Thu, 10/27/2016 - 14:29

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. 

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AV Block With Changing PR Intervals

Submitted by Dawn on Wed, 09/28/2016 - 21:23

Just like other subjects we are taught in school, ECG interpretation is usually taught in a very basic, simplistic way.  As we add to our knowledge, we are able to determine the mechanisms of more complex rhythms. 

When I took my first basic ECG rhythm monitoring course, I memorized all the “rules”, and at the end of the course, I thought I could read ANY strip correctly.  Then, in real life, I found that some rhythms can’t be interpreted from one lead, or even from one 12-lead ECG. 

This strip offers advanced readers to challenge themselves, and it offers teachers a chance to show students an “exception to the rules” if it is appropriate for those students.  We all learn the classification of second-degree AV blocks:  Both Type I and Type II show an underlying sinus rhythm with some P waves conducted and some not.  Type I has progressively prolonging PR intervals until a P wave is non-conducted.  The cycle restarts after the dropped QRS.  Type II has PR intervals that are all the same, and may be prolonged or normal. 

In this ECG, you will be able to “march out” a normal sinus rhythm at a rate of 80 bpm.  The P waves are marked with small dots at the bottom.  Two of every three P waves are followed by QRS complexes.  Is it Type I?  No – the PR intervals are not prolonging.  Is it Type II?  The PR intervals are not the same!  What is happening? 

There is also left bundle branch block, which is a sub-Hisian block.  Blocks occurring in the intraventricular conduction system include bundle branch blocks, second-degree AVB Type II,  and third-degree AVB with ventricular escape.  This group of blocks tends to be more threatening than the blocks that occur in the AV node (second-degree type I and third-degree with junctional escape). 

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ECG Basics: Second-degree AV Block With Characteristics of Type I and Type II

Submitted by Dawn on Thu, 09/01/2016 - 11:51

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.

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Complete AV Block With Junctional Escape Rhythm

Submitted by Dawn on Thu, 09/03/2015 - 23:02

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. 

 

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Second-Degree AV Block, Type I

Submitted by Dawn on Sat, 06/13/2015 - 22:07

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.   

Dawn's picture

High-Grade AV Block, Second-degree AVB Type II

Submitted by Dawn on Wed, 06/03/2015 - 21:25

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.

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