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:
3.
4.
●
Especially for cardiovascular disease and for cardiovascular risk factors.
●
The results of any cardiac investigations, such as angiography.
Note any symptoms of cardiac failure:
●
Postural hypotension
●
Presyncope or syncope (Stokes-Adams attacks)
●
Dyspnea
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 Lee
Dr J. Hayes
July 2012