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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, Type II

This rhythm strip was obtained from a man who was suffering an acute inferior wall M.I.  There are ST elevation and hyperacute T waves.  The rhythm is SINUS ARRHYTHMIA WITH SECOND-DEGREE AV BLOCK, TYPE II.    There is also first-degree AV block.

There are more P waves than QRS complexes, with a 3:2 ratio.  The atrial rate varies between 55 -68 beats per minute.  The sinus rate speeds slightly after the dropped QRS in each group. The ventricular rate is about 40 bpm, with grouped beating. (Regularly irregular.)

The PR intervals are steady at 226 ms (slightly prolonged).

Dawn's picture

Second-degree AV Block, Type II

The patient:  Unfortunately, we no longer have information on this patient, other than the fact that she went to the OR for a permanent pacemaker implantation.

The ECG:  The atrial rate (P waves) is 99 beats per minute. The P waves are regular and all alike (NSR). The ventricular rate (QRS complexes) is 33 bpm, and the QRS complexes are regular and all alike. The PR intervals, when A-V conduction occurs, are 162 ms (.16 seconds) and all alike. The QRS complexes are wide, at 122 ms (.12 seconds). There is right bundle branch block, but no left hemiblock, as the frontal plane axis is normal. The QTc is prolonged at 549 ms.  Many ST segments on this ECG have a “flat” appearance, rather than the normal concave up shape.

The failure of 2 out of every 3 P waves to conduct indicates a second-degree AV block. Type I is a block of the AV node, with progressive prolongation of the PR interval until ONE P wave fails to conduct. Type II AV block is a block of the intraventricular conduction system. Clues that a second-degree AV block is Type II include:

·        The PR intervals are all alike.

·        More than one consecutive P wave is not conducted.

·        A P wave that is NOT in the refractory period of the preceding beat is not conducted.

Dawn's picture

ECG Basics: 2:1 AV Block

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

This is an original illustration by Dawn Altman. You may use it free of charge to enhance your presentations or student handouts. Click on image, or right click and SAVE image. For permission and charges for use in publications or for marketing uses, please contact the artist at [email protected]  

Dawn's picture

Second-degree AV Block, Type II?

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.

Dawn's picture

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

 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. 

Dawn's picture

Complete AV Block With Junctional Escape Rhythm

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. 

Dawn's picture

AV Block With Changing PR Intervals

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). 

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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