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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.

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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.   

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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

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.


Dawn's picture

Ask The Expert

RP - PR Reciprocity, PR Intervals

QUESTION: How do you explain the changing PR intervals in the following series of Strips, and is the mechanism related to Wenckebach conduction?

Our expert today is our very own Jason Roediger, who is an ECG Guru blogger, and frequent contributer to the Ask the Expert page.  His blog, ECG Challenges, offers you a chance to try your skills and get feedback from Jason.  He has a special interest in complex arrhythmias, and is adept at constructing laddergrams that help clarify the mechanisms involved in the rhythms. His bio can be found on the "About Us" page, or on his previous contributions to this page.

Strips courtesy of R.O. from California, USA.



I opted to only construct a laddergram for the middle rhythm tracing rather that for all 3. The bottom strip didn't deserve one since it is only showing sinus rhythm with no ectopy. The top 2 are more-or-less identical in content. However, the middle one was of slightly better diagnostic quality.

This is a classic representation of "R-P/P-R reciprocity". There is a near-linear relationship between the P-R (or P'-R) intervals and their associated R-P (or R-P') intervals. Along the right margin of the tracing, I have compiled a list of the longest R-P interval at the top which is associated with the shortest P-R interval (i.e., 81 / 29). Conversely, at the bottom of the list, the shorted R-P interval is associated with the longest P-R (i.e., 45 / 50). The two lines that I've circled are the only inconsistancy in the logic but are well within an acceptable margin of error. 





This patient definitely has AV nodal disease and at a faster atrial rate, they would undoubtedly develop Type I AV block (Wenckebach periodicity) and start "dropping" beats. Whenever you encounter longer P'-R intervals on the APBs, you always have to consider the possibility that conduction jumped over from the fast pathway to the slow pathway which is the initiating mechanism seen in AVNRT. I think that all of the atrial impulses are conducting via the fast pathway (FP) here but the FP is exhibiting varying conduction delays depending on how short or long the preceding R-P interval is. As I've illustrated in the laddergram, you can see that the amount of time it takes for the sinus node to recover after an APB is essentially equal to the basic sinus cycle (e.g., 110 = 111). Dr. Charles Fisch graphically charted R-P/P-R relationships in his book "Electrocardiography of Clinical Arrhythmias" on pages 28 and 29. If you were to chart the R-P and P-R intervals on a graph, they would form a diagonal line that is about 45-degrees. If you don't have Dr. Fisch's book in your personal library, you should really buy a copy. I refer to it all the time!


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