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2:1 AV block

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Dawn's picture

ECG Basics: 2:1 AV Block

Fri, 06/07/2019 - 15:48 -- Dawn

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.

Dawn's picture

Inferior Wall M.I. With Right Ventricular M.I.

Sat, 10/28/2017 - 18:36 -- Dawn

This ECG was recorded from a 75-year-old man with substernal chest pain and diaphoresis.  It shows a pretty classic picture of acute inferior wall M.I. The second ECG is a repeat tracing with the V4 wire moved to the V4 Right position, and it is positive for right ventricular M.I.  The patient was found to have a 100% occlusion of the right coronary artery, which was opened and stented in the cath lab.

There are several other examples of IWMI with RVMI in our archives, so we will confine this commentary to the ECG signs that make these tracings so typical of right coronary artery occlusion. Once you are familiar with the typical pattern of IWMI / RVMI, it is easy to see, even when the ST elevation is subtle (as this one certainly is NOT).

Signs of IWMI in these ECGs are

·         ST elevation in inferior leads II, III and aVF.

·         Reciprocal ST depression in leads I and aVL. 

Signs of RVMI in these ECGs are:

·         ST elevation in V4 right.

·         ST elevation in V1 without ST elevation in V2.

Dawn's picture

Second-degree AV Block, Type II?

Wed, 02/01/2017 - 23:09 -- Dawn

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

Second-degree A-V Block, Type II

Thu, 03/27/2014 - 20:17 -- Dawn

Paramedic Erik Testerman has generously donated several excellent teaching ECGs to the Guru, and we will be featuring all of them soon.  This week, we show you the ECGs from a 59-year-old man with a blood glucose of 30 mg/dl.  He had no complaints .  After a bolus of Dextrose 10%, his blood glucose was 105 mg/dl.  He gave a past medical history of diabetes mellitus, hypertension, and left bundle branch block. Vitals were reported as normal and stable, except for the slow heart rate.

The first ECG, taken in the field, show a second-degree AV Block.  The conduction ratio is 2:1.  That is, there are two P waves for every QRS complex.  With this ratio, it is sometimes difficult to determine whether the patient has Type I (usually AV nodal) or Type II (Infranodal) AVB.  In order to diagnose Type I AVB (Wenckebach), we need to see TWO P waves in a row conducted, to see the prolongation of the PR interval.  It is not correct, however, to call ALL 2:1 AV blocks "Type II".  Often, simply taking a longer rhythm strip will expose a period of 3:2 conduction, showing progressive prolongation of the PRI.

This ECG, however, gives us some clues that it is probably TYPE II.  The patient has a left bundle branch block.  Type II AVBs are infranodal - that is, they affect the structures below the AV node:  the His Bundle and the Bundle Branches.  Type II AVBs represent INTERMITTENT TRI-FASCICULAR BLOCK, and that is common in the presence of RBBB  and LBBB (a bi-fascicular block).  This ECG probably represents an existing LBBB with an intermittent RBBB - When the right bundle is blocked, the patient has a tri-fascicular block, and no conduction to the ventricles.

Another clue that this is Type II is that the NON-CONDUCTED P waves fall CLEAR of the preceding T waves, meaning that they had ample opportunity to conduct, not being in the absolute refractory period.

Fortunately, the rhythm strip, taken one minute later, uncovers the diagnosis!  The sixth and seventh QRS complexes are conducted with a 3:2 ratio, showing PR intervals that stay the same, proving the rhythm is Type II. 

For this patient, the heart block and resulting bradycardia don't seem to be causing symptoms.  But infranodal blocks can easily progress to complete heart block and should be treated with implanted pacemakers.  The EMS crew in this case had transcutaneous pacer pads on the patient as a precaution, but he remained well-perfused and with a good BP the whole time. 


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