Atrioventricular Block

AV Block

In AV block,  conduction is altered between the atrium and the ventricles. It may manifest as prolonged PR interval, a dropped P waves or a P waves without a QRS after it.

First Degree AV Block

ECG Recognition:

• First degree AV block manifest with a PR interval greater than 0.20 sec that remains constant. 
• The P wave is normal in morphology. 
• The QRS is normal in duration or the wide if there is an existing bundle branch block. 

The lengthening of the PRI is due to the conduction delay in the atrium, AV node, or the His-Purkinje system.

First degree AV block. The rhythm is sinus at about 83 bpm. The PRI is prolonged at 0.36 sec (9 small squares).

First degree AV block. The PRI at about 0.40 sec (10 small squares) and the P wave is seen merging with the T wave.

Sinus rhythm, first degree AV block with bundle branch block. This is a regular wide QRS rhythm (0.14 sec) with prolonged PRI (~0.26 sec).

Sinus rhythm, first degree AV block (long PRI). This is regular narrow QRS rhythm with a rate of about 80 bpm with a PRI of about .40 sec. 

Second Degree AV Block

Second Degree AV Block Type I  (Mobitz I / Wenckebach)

ECG Recognition (Typical Type I) :

• Second degree AV block type I is characterized by  a normal P wave. 
• The PRI progressively lengthens until a P wave is not followed by a QRS. 
• As the PRI lengthens, there is shortening of the RR interval. 
• The RR interval containing the dropped P wave is less than 2x  of the shortest RR interval. 
• The PRI (may be normal or prolonged) of the first conducted P wave is shorter than the last conducted PRI. 
• The largest increment in the PRI is usually on the second conducted P wave. 
• There is "group-beating" on the ECG.

Sinus rhythm, first degree AV block, second degree AV block type I. There is a regular sinus P wave at a rate of about 68 bpm. There are 2 non-conducted P waves (P waves # 5 and 8). 

Sinus rhythm, second degree AV block type I, bundle branch block. There is a regular sinus P wave at a rate of about 75 bpm (1500/20 small squares). Focusing on the middle of the ECG strip, There is a non-conducted P wave that is "sitting" or on top of the T wave. You have to use a caliper to march the P wave up to that point. The second clue that there is a P wave on top of that T wave is the distorted shape of the T wave. Having determined that there is non-conducted P wave, you can then compare the PRI surrounding that non-conducted P wave. In this case, it is 0.28 sec vs .16 sec. 

Sinus rhythm with second degree AV block type I. There is regular sinus P wave at a  rate of about 75 bpm. Another way of looking Type I second degree AV block is the "group-beating" it creates. In this ECG strip, there is a group of 3, 2 and 1QRS complexes. This is because of the ratio created by 4:3, 3:2 and 2:1 which means that there are 4 P waves and 3 QRS and 3 P waves and 2 QRS complexes and 2 P waves and 1 QRS. So, you can also label this as sinus rhythm with 4:3, 3:2 and 2:1 AV Wenckebach.

ECG Recognition (Atypical Wenckebach):

• When the conduction ration exceeds 6:5 (6P and 5 QRS) or 7:6, the PR interval increment becomes unpredictable. 
• The PRI may remain the same (prolonged), then increase, and then the dropped beat.

Sinus rhythm with atypical Wenckebach - There is progression in PRI prolongation but at times the PRI remained the same and then increased. So, the increment becomes unpredictable. The number is the PR interval measured in millisecond.

Second Degree AV Block Type II (Mobitz II)

ECG Recognition:

• There is constant PR interval (normal or prolonged) before a P wave is dropped. 
• The QRS is usually widened because the location of the block is often infranodal. 
• The QRS complex maybe narrow indicating a more proximal location of the block (AV node).

Sinus rhythm, first degree AV block, second degree AV block type II, bundle branch block. There is  regular sinus P wave at a rate of about 65 bpm. P waves # 5,8 and 10 are not conducted. The PRI surrounding the non-conducted P waves are the same. The baseline PRI is prolonged (~.24 sec). There is also a wide QRS (0.12 sec). This patient was admitted due to a fall. The patient eventually got a pacemaker. 

Sinus rhythm, second degree AV block type II, bundle branch block. There is a regular sinus P wave at a rate of about 100 bpm with a PRI of 0.16 sec and a wide QRS. Every third P wave is not conducted. The PRI surrounding the non-conducted P wave is the same (0.16 sec). This can also be called second degree AV block type II with 3:2 AV conduction (3 P : 2 QRS). 

Second Degree AV Block 2:1

ECG Recognition:

• During sinus rhythm, 2 to 1 AV block manifest as 2 P waves followed by 1 QRS. 
• This is a subtype of second degree AV block. 
• A 2:1 AV block could either be second degree AV block type I or type II.
• A long strip is needed to capture the mechanism or true nature of a 2:1 AV block. 

Sinus rhythm, first degree AV block with second degree AV block 2:1. There is a regular sinus P wave at a rate of about 88 bpm. Every other P wave is not conducted or in a 2:1 pattern. There is prolonged PRI (0.24 sec) and normal QRS duration

Sinus rhythm, first degree AV block, second degree AVB 2 to 1. There is a regular sinus P wave at a rate of about 94 bpm with a narrow QRS complexes. Every other P wave is conducted or with a 2:1 pattern.

The previous 2:1 rhythm is revealed on this strips as due to second degree AV block type I. Unlike in static ECG where you randomly capture the cardiac activity, in telemetry settings you can review the saved strips and know the true nature of the arrhythmia. In this strip, there is gradual prolongation of the PRI then the dropped beat (red arrow). So, this case is second degree AV block 2:1 due to Wenckebach (type I) mechanism.

High-grade or Advance AV Block

ECG Recognition:

• During sinus rhythm, when 2 or more P waves are not conducted the term given is advanced or high-grade AV block. 
• The QRS may be wide or narrow. 
• This is a clinically concerning variant of Mobitz II and often implies advanced conduction disease and may progress to complete heart block.

Sinus rhythm, advanced or high-degree AV block, right bundle branch block. There is a regular sinus P wave at a rate of ~100bpm and a wide QRS with a qR pattern in V1 indicating right bundle branch block with a rate of ~33 bpm (1500/45 small squares) . Some of the P waves are "buried" in the QRS or hidden from view.  A caliper is needed to march-out the P wave. For every 3 P waves, there is 1 QRS or there is 3:1 pattern of conduction. 

Sinus tachycardia, advanced or high-degree AV block, bundle branch block. This ECG strip from the same patient in Figure 48.  There is a regular sinus P wave at a rate of about 107 bpm. The QRS is wide with an irregular ventricular rhythm. You need a caliper to "march-out" the P waves in this case. Some of the P waves are "partly hidden from view" or some of the P waves are merging with the descending component of the T waves. The partially hidden P waves are marked with red arrows. The P wave distortion can also be seen on V1 (red arrows). In this case, mostly 2 P waves are not conducted and only one P wave is conducted or followed by a QRS. The pattern is that of 2:1 and 3:1.  Most likely you will not see these kinds of strips in basic ECG test. If you see this, others would label it as second degree AV block type II (Mobitz II). 

Third Degree or Complete Heart Block

Third degree AV block is also called complete heart block (CHB). The supraventricular impulse is totally blocked from reaching the ventricles. The atria and ventricles beats independently. 

ECG Recognition:

• In sinus rhythm with complete AV block, the PP and RR intervals are regular but the P wave has no relationship with the R wave. 
• The PR interval varies because there is really no P and QRS relationship. 
• The ventricular rate is usually 40-60 bpm and narrow when it is driven by a junctional pacemaker (AV node). 
• The QRS is wide and less the 40 bpm when an infra-Hisian pacemaker takes over. 

Sinus tachycardia, complete heart block with idioventricular escape rhythm. There is a regular sinus P wave at a rate of about 107 bpm. There is regular wide QRS rhythm of about 38 bpm. The PRI varies. Hence, the P wave has no relationship to the QRS. A ladder diagram or laddergram is shown on the left to illustrate sinus impulse from the A or atrial tier/level is blocked in AV or atrioventricular junction tier/level. The laddergram also shows the ventricles are depolarized from the ventricular or V tier (infra-Hisian pacemaker). The reason for a wide QRS in ventricular escape rhythm is because muscle to muscle conduction is slow.

Sinus tachycardia, complete heart block with junctional escape rhythm.  There is a regular sinus P wave at a rate of about 125 bpm and regular narrow QRS complex rhythm of about 55 bpm. There is no relationship between the P and QRS which showed variable PRI. The reason for a narrow QRS in junctional escape rhythm is because of faster conduction via the normal conduction pathway (AV junction to the His-purkinje system).

Sinus tachycardia, complete heart block, accelerated idioventricular escape rhythm (AIVR).  There is a regular sinus P wave at a rate of about 125 bpm and a regular wide QRS complex rhythm at a rate of about 46 bpm. There is no relationship between the P and QRS.

Paroxysmal AV Block/Ventricular Standstill

ECG Recognition:

• It is characterized by an abrupt and persistent AV block ( multiple P waves with no QRS) in the presence of otherwise normal AV conduction.
• It may be initiated by a conducted or blocked PAC or PVC, acceleration or slowing of sinus rhythm. 
• Once the block is initiated, the block will persist until terminated by an escape, usually ventricular, with a predictable relationship of the escape to the following P wave. 

Continuous lead II strip of Paroxysmal AV block or ventricular standstill

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