SECOND DEGREE HEART BLOCK

Introduction

Second degree atrioventricular (AV) block is defined as the intermittent failure of conduction from the atria to the ventricles.

This is seen on an ECG as a non-conducted P wave (i.e. a P wave that is not followed by a QRS complex).

There are 2 main variants described according to the ECG appearance:

● Mobitz Type I AV block (also known as Wenkebach AV block):

● Mobitz Type II AV block

Some classifications also add the specific situation of a 2:1 A-V block, (where the differentiation of Mobitz I from Mobitz II cannot readily be made).

It is clinically important to distinguish between Mobitz type I and II as the prognosis for each is very different.

● Mobitz type I is often temporary and generally benign.

● Mobitz type II is almost always permanent and pathological, and frequently progresses to complete heart block.

ECG Distinguishing Features of Mobitz I and Mobitz II Heart Block

Mobitz type I

The PR interval can be used to distinguish Mobitz type I and type II AV block.

In Mobitz type I AV block, there is:

● Progressive lengthening of the PR interval on consecutive beats

● Followed by a non-conducted P wave, (i.e. a "dropped QRS" complex), which results in a pause on the ECG, (known as the Wenckebach period)

● After the dropped beat, the PR interval then resets to normal and the cycle repeats itself.

The QRS complex is normal, unless there is in addition to the A-V nodal disease as associated pathology of the His-Purkinje system.

Rhythm strip showing Mobitz type I AV block. Note the PR intervals that lengthen gradually until a QRS complex is dropped. The arrow marks P wave without a following QRS complex.

Mobitz type II

In Mobitz type II AV block:

● The PR intervals remains unchanged and are identical. Although the PR interval may be normal or prolonged, it is always constant before and after the pause.

● With a non-conducted P wave occurring periodically.

The magnitude of AV block is expressed as a ratio of P waves to QRS complexes (e.g. 2:1, 3:1, 4:1 etc)

Rhythm strip showing Mobitz type II AV block. Note there are constant PR intervals preceding each QRS complex until a QRS complex is dropped in this rhythm strip. There are four P waves to every three QRS complexes, this is an example of a 4:3 block.

High grade Mobitz type II AV block is said to occur when two or more P waves are not conducted. This implies advanced conduction disease.

The QRS duration helps localize the level of block to above (narrow QRS) or below (broad QRS) the branching portion of the bundle of His. 1

A prolonged P-R interval with a broad QRS would suggest a block in the bundle branches and either the A-V node or the bundle of His. 1

MOBITZ TYPE I A-V BLOCK

Introduction

Mobitz type I is often temporary and generally benign.

Pathophysiology

Causes:

Mobitz type I heart block is usually due to a block occurring at the level of the A-V node.

It may be seen in normal subjects particularly in athletes.

It may be seen in patients taking AV nodal blocking drugs.

The causes of a Mobitz type I AV block include:

1. Physiological:

● Normal variant

● Athletes (increased vagal tone)

● Increased vagal tone in general (from any cause)

2. Age, (i.e. elderly)

3. Myocardial ischaemia:

● Particularly inferior MI and right ventricular infarction resulting in ischaemia of the AV node.

4. Myocarditis.

5. Cardiomyopathies, (of any cause)

6. Drugs:

Second degree heart block may be seen in the setting of acute drug overdose or chronic toxicity, involving drugs that block the AV node and so may include:

● Beta blockers

● Calcium channel blockers

● Digoxin

6. Following cardiac surgery

Clinical Features

Uncomplicated (i.e. without underlying structural heart disease) Mobitz type I AV block does not produce symptoms.

However, if the sinus rate is slow, or there are fewer conducted beats, there may be a significant reduction in cardiac output with subsequent symptoms of hypoperfusion or cardiac failure.

Investigation

Blood tests

None may be necessary, depending on the clinical scenario, however the following should be considered:

1. FBE

2. U&Es/ glucose

● In particular serum potassium

3. Serum digoxin level

4. Serum magnesium and calcium

5. Troponin:

● In the setting of actual or suspected cardiac disease

ECG

See above

CXR:

● Look for cardiomegaly and/ or signs of cardiac failure.

Management

Management of Mobitz type I AV block will depend on:

● The likely underlying cause

● The presence of clinical symptoms.

Asymptomatic patients with Mobitz type I AV block do not require any specific therapy.

In individuals without structural heart disease chronic block at the level of the A-V node is benign. However when the block is associated with structural heart disease the prognosis will be related more to the underlying heart disease.

Reversible causes should be always excluded, such as myocardial ischaemia and/or drug effects.

Type I block in the specific setting of ACS places the patient at somewhat greater risk, than those who do not develop block, however this risk is much less than patients who develop type II block in the setting of ACS. In most cases however the effect is transient and requires no specific treatment. 1

In symptomatic patients, atropine 0.6-1.2 mg IV may be effective by reversing the decrease in AV nodal conduction induced by vagal tone.

It is important where possible to avoid any medications that may impair AV conduction.

A cardiac pacemaker may be appropriate in selected high risk patients.

12 lead ECG of a 54 year old male with a Mobitz Type I A-V heart block. There is progressive lengthening of the P-R interval followed by a non-conducted P wave. The

cycle then repeats.

2:1 A-V Block

A 2:1 second degree block (i.e. two sinus P waves for every QRS complex) represents a special situation. 1

The distinction between Mobitz type I and Mobitz type II AV block cannot be made from the ECG when there is 2:1 block present, because there are not two conducted P waves in a row to examine.

In other words in this situation, there is no opportunity to observe for possible PR prolongation characteristic of Mobitz type I AV block.

Certain ECG features may help differentiate (such as broad QRS complexes more likely with Mobitz type II AV block) but these are not failsafe.

It is prudent to assume the worst - that the block is a Mobitz type II - until proven otherwise.

MOBITZ TYPE II A-V BLOCK

Introduction

Mobitz type II is almost always permanent and pathological, and frequently progresses to complete heart block.

Pathophysiology

In contrast to Mobitz type I AV block, Mobitz type II AV block almost always results from conduction system disease infranodally, i.e. below the level of the AV node, either at the bundle of His or in the bundle branches.

Many patients also have associated bundle branch block, bifascicular or trifascicular disease.

Causes:

The causes of a Mobitz type II AV block include:

1. Age-related fibrous degeneration of the conducting system

2. Acute coronary syndrome:

● Especially inferior ischaemia which may indicate an infarcted bundle of His

3. Myocarditis, (of any cause)

4. Cardiomyopathies, (of any cause)

5. Drugs

As with Mobitz type I AV block, drugs which block the AV node are most commonly implicated.

● Beta blockers

● Calcium channel blockers

● Digoxin

Clinical Features

Mobitz type II AV block may be asymptomatic.

However, if the sinus rate is slow, or there are fewer conducted beats, there may be a significant reduction in cardiac output resulting in symptoms of hypoperfusion or cardiac failure or syncopal episodes, (Stokes-Adams attacks).

Symptoms are more likely to be seen in patients with Mobitz type II blocks than in patients who have type I AV block

Important points of history:

1. Note any chest pain symptoms suggestive of ACS.

2. Patient’s past history:

● Especially for cardiovascular disease and for cardiovascular risk factors.

● The results of any cardiac investigations, such as angiography.

3. Note any symptoms of cardiac failure:

● Postural hypotension

● Presyncope or syncope (Stokes-Adams attacks)

● Dyspnea

4. Drugs:

In particular:

● Beta blockers.

● Calcium channel blockers

● Digoxin.

Important points of examination:

1. Vital signs, especially the blood pressure

2. Look for signs of cardiac failure or hypoperfusion

Investigation

Investigations should be directed at finding the underlying cause and looking for any complications.

Blood tests

This will depend on the clinical scenario and may include:

1. FBE

2. U&Es/ glucose

3. Troponin

4. Serum digoxin level

5. Serum magnesiuma nd calcium

ECG

See above

CXR:

● Look for cardiomegaly and/ or signs of cardiac failure.

Management

As with Mobitz type I AV block, management of Mobitz type II AV block is directed towards:

● Seeking and correcting any reversible causes.

● Avoidance of any AV nodal blocking drugs

Increases in heart rate due to exercise or atropine should be avoided, as this can worsen Mobitz type II AV block. Conversely, slowing the sinus rate through vagal manoeuvres may improve the block.

The development of a Mobitz type II block in the setting of antero-septal myocardial infarction is associated with a high risk of progression to CHB and/or death. In this setting a temporary pacemaker must be considered. A permanent pacemaker is not usually required.

Given the high likelihood of progression to more severe conduction disease, all patients, even those who are asymptomatic, should be referred to the Cardiology Unit for consideration of a cardiac pacemaker.

References

1. Conover M. B Understanding Electrocardiography, 8th ed 1996.

2. Ufberg JW, Clark JS. Bradydysrhythmias and Atrioventricular Conduction Blocks. Emerg Med Clin N Am 2006;24:1–9

3. Chan JC. ECG in Emergency Medicine and Acute Care 1st ed 2005.

Dr Louisa LeeDr J. HayesJuly 2012