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Dr A Röschl's picture


We see the EKG of an 81-year-old patient with a pacemaker; the PM was briefly deactivated to assess the patient's intrinsic heart rhythm. At the beginning of the EKG, there is already a prolonged AV conduction time, which progressively lengthens from beat to beat. The last conducted P-wave has a PR interval of nearly 800 ms (!). The next P-wave is blocked, but the subsequent displayed P-waves are conducted again, with the PR interval increasing from beat to beat. This indicates a classic second degree AVB Type Mobitz I (Wenckebach).

Dr A Röschl's picture

Why is this not second degree AVB Type II and no high grade AVB

(Image 1) Why is there no second-degree AVB  Mobitz type II and no high-grade AV block? To the first question: Basically, second-degree AV block Mobitz type II is rare. The two ECG patterns that can easily be confused with Type II Mobitz block are: blocked/non-conducted PACs and second-degree AVB Mobitz type I (Wenckebach). (Image 2) You have to compare the PR duration before the pause and after it. With the naked eye, the difference is often difficult to recognize, a pair of calipers does a good job here.

Dawn's picture

ECG Basics: Second-degree AV Block, Type I

This two-lead rhythm strip shows a normal sinus rhythm at about 63 bpm.  The P waves are regular. After the sixth P-QRS, there is a non-conducted P wave.  The normal rhythm then resumes.  The two most common reasons for a non-conducted P wave in the midst of a normal sinus rhythm are 1) non-conducted PAC, and 2) Wenckebach conduction. The first is easy to rule out.  The non-conducted P wave is not premature, so it is not a PAC.  The second one is a little harder when we only have a short strip to look at.  We are conditioned to look for progressively-prolonging PR intervals until a QRS is "dropped".  In this case, the progression is in very tiny increments that are hard to see unless you zoom in and measure.  But they ARE progressively prolonging.  An easy hack:  measure the last PRI before the dropped beat and the first one after the pause.  You will see that the cycle ends on a longer PRI (about .28 seconds) and the new cycle starts up with a PR interval of about .20 seconds.  Fortunately, this conduction ratio will have very little effect on the patient's heart rate.

Dawn's picture

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

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.

Dawn's picture

Supraventricular Tachycardia With Wenckebach Conduction

This ECG was obtained from a patient in a walk-in health clinic.  We do not have any other information on the patient.  We thank Joe Kelly for donating this interesting ECG to the GURU. 

IRREGULAR RHYTHM    If you march out the P waves, you will see that they are regular, at a rate of approximately 130 bpm.  But the QRS complexes are not regular, and there are fewer QRS complexes than P waves.  

WENCKEBACH CONDUCTION   Looking closely at the PR intervals, you will notice that they progressively prolong.  This “pushes” the QRS complexes progressively toward the right.  Eventually, the T wave – and the refractory period – will land on the next P wave.  That P wave will be unable to conduct to the wave, and no T wave of course, so the next P wave will conduct with a shorter PR interval.

We are including a short rhythm strip from this patient, with conduction marked with a laddergram. 

Dawn's picture

Second-Degree AV Block, Type I

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

Acute Inferior Wall M.I. With Right Ventricular M.I. and Atrial Fibrillation

This 31-year-old man presented to the Emergency Dept. complaining of chest pain, shortness of breath, and nausea. His heart rate on admission was 120 - 130 bpm and irregular, and the monitor showed atrial fibrillation. His rate slowed with the administration of diltiazem. His 12-lead ECG shows the classic ST elevation of inferior wall M.I. in Leads II, III, and aVF. This patient also had JVD, bibasilar rales, orthopnea, and exertional dyspnea, signs of CHF. He had no history of acute M.I., CHF, or atrial fibrillation. He offered no history of drug use or medications.

This ECG is very useful for the basic student, in that the ST elevations are readily seen, and the atrial fib is definitely irregularly-irregular. For the more advanced student, the ST depression in V2 indicates posterior wall injury, while the flat ST segment in V1 indicates a possible right ventricular M.I.  While the posterior wall is trying to depress the ST segment, the right ventricle is trying to elevate it, resulting in flattening. Also, Lead III has a greater STE than Lead II, which has been shown to be a reliable indicator of RV infarction.  This should be confirmed with a V4 right, or all chest leads done on the right side. Right ventricular injury has been shown to increase mortality, and it also requires different management of hemodynamics.

Dawn's picture

ECG Challenge: Grouped Beating - Double Tachycardia - ANSWER

This series of strips was donated by Arnel Carmona, and was taken from a patient admitted to the hospital for a urinary tract infection.  No other history is known.   On close examination of this rhythm what do we see?

Strip 1:   Narrow-complex tachycardia with NO apparent P waves.

Strip 2:   Some irregularity, with long regular groups and still NO P waves.

Strips 3 & 4:  Grouped beating.

Strip 5:   A narrow-complex rhythm that is approximately ½ the rate of Strip 1. 

When grouped beating is seen, one should always suspect Wenckebach conduction.  Wenckebach conduction (progressively longer conduction times through the A-V conduction system) can occur in rhythyms other than sinus rhythm.  Without P waves and PR intervals, GROUPED BEATING is our major clue to Wenckebach conduction. 

This patient has an underlying atrial fibrillation – hence no P waves.  Fine fibrillatory waves can be seen, but artifact can cause the same appearance.  So, why is there no irregular irregularity?  There is another rhythm at work here along with the atrial fibrillation.  Junctional tachycardia is seen in Strip 1.  When two tachycardias coexist, one from above the AV junction, and one from below, the rhythm can be called a “double tachycardia”.  This particular combination often happens in patients with digitalis toxicity. 

In some cases, a complete heart block at the level of the atrial conduction fibers or the AV node causes  two rhythms to operate independently.  Any supraventricular rhythm, including atrial fib, can occur with a complete heart block, in which case we would see an “escape” rhythm.  Escape rhythms are usually slow, either idiojunctional (40-60 bpm) or idioventricular (< 40 bpm).  

 Let’s look at each of the strips in detail.  We will begin with the hypothesis that this is atrial fibrillation with concurrent junctional tachycardia at around 150 bpm.  I will include laddergrams to illustrate my view of what is happening. 

Dawn's picture

ECG Challenge: Grouped Beating - Double Tachycardia

This very interesting set of strips was donated to the ECG Guru by Arnel Carmona, well-known to many of you as the Administrator of the blog, "ECG Rhythms" and the FB page by the same name.  He is a frequent contributer to the FB page, "EKG Club", and is an ECG Guru!  This set of strips was previously posted to his blog and to the EKG Club.  In case you haven't already seen it, we will withhold the interpretation for now to give everyone a chance to comment.  In one week, we will post the interpretation.



Dawn's picture

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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