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E C G E D U C AT I O N
BRADYCARDIA AND HEART BLOCK
Part three of an educational series on ECG analysis and arrhythmia diagnosis
DA Milne — Cardiologist in Private Practice,
Vincent Pallotti Hospital, Pinelands, Cape Town
Bradycardia is usually defined as a ventricular heart rate
below 60 beats per minute. There are two potential mechanisms for this slow heart rate. Either the impulse generation
(usually in the sinus node) has failed or the impulse, having
been generated normally, fails to conduct through the specialised conducting system within the heart.
The heart’s natural pacemaker is found in the sino-atrial
(SA) node situated at the junction of the right atrium and
the superior vena cava. The impulse is generated within
the SA node and travels through the atrium to reach the
atrioventricular (AV) node. The AV node is situated at the
tricuspid valve at the base of the inter-atrial septum. The
AV node slows conduction to the ventricle and is responsible for the normal delay in AV conduction, i.e. the PR interval. The impulse then enters the His bundle, which divides
into the right bundle branch (RBB) and the left bundle
branch (LBB). Both bundle branches further subdivide into
the distal His Purkinje system.
Conduction disorders may occur anywhere along this pathway.
First-degree AV block (Fig. 1): This is a misnomer as
the atrial impulse is excessively delayed in the AV node
resulting in a prolonged PR interval. However each P wave
is conducted to the ventricles. The PR interval is greater
than 0.20 seconds and although this delay may occur in
the His Purkinje system, the block is usually in the AV node.
Common causes include acute inferior myocardial infarction, digoxin toxicity, increased vagal tone, myocarditis,
other drugs (beta blockers, calcium antagonists) and ageing
of the AV node.
Fig.1
Second-degree AV block: In second degree AV block,
not all the atrial impulses are conducted to the ventricles.
Therefore some P waves will not be followed by a QRS
complex. Two types of second-degree AV block are recognised.
CLINICAL SYMPTOMS
Failure of sino-atrial impulses to reach the ventricles results
in a slow heart rate with symptoms of effort intolerance,
fatigue, breathlessness, angina and cardiac failure. When
the conduction disturbances are intermittent, the bradycardia is likely to precipitate disturbances of consciousness
with faintness, dizziness or syncope. The abrupt onset of
syncope with rapid full recovery is well recognised as the
Stokes-Adams attack consequent on intermittent complete
heart block. Usually the symptoms are easy to elicit and
the association with a slow heart rate (particularly below
50 beats per minute) warrants further evaluation.
• Mobitz type I (or AV Wenckebach block)
(Fig. 2). The P waves are normal in rate, rhythm, size
and axis. The PR interval lengthens progressively until a
QRS complex is dropped. Mobitz type I block usually
occurs at the level of the AV node. The causes are the
same as for first-degree AV block.
ATRIOVENTRICULAR BLOCK
Atrioventricular block (or ‘heart block’) implies failure of
transmission of the depolarising wave from the atrium via
the AV node and His Purkinje system to the ventricles. This
dysfunction can be classified according to its severity, and
can occur at different sites.
282 CME May 2004 Vol.22 No.5
Fig.2
Chronic AV Wenckebach heart block may be benign,
and if the patient is asymptomatic with no underlying
heart disease, no treatment is necessary. Progression to
complete heart block is uncommon and should it occur,
the escape rhythm is usually stable with a narrow QRS.
E C G E D U C AT I O N
However, in the presence of structural heart disease,
Mobitz I heart block is associated with an increased mortality and insertion of a permanent pacemaker should be
considered.
• Mobitz Type II AV Block (Fig. 3): Here the P waves
of sinus origin abruptly fail to reach the ventricle, without
any preceding prolongation of the PR interval in earlier
conducted beats. In the conducted beats the QRS may
be widened tending to support that the site of Mobitz II
block is in the His Purkinje system. If however, every
second P wave fails to conduct to the ventricle (2:1
block), Mobitz I block cannot be excluded (as every second QRS may be dropped after ‘hidden’ PR prolongation).
Fig.3
MANAGEMENT
Untreated symptomatic second- and third-degree AV block
have a very poor prognosis with mortality rates in excess of
50% within the first year. With pacemaker implantation
survival rates approach those of age-matched controls without AV block.
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• Symptoms of effort intolerance, dizzy spells and syncope should alert the clinician to the possibility of
bradycardia, confirmed by a slow pulse rate.
Examination may however be normal.
• ECG confirmation of the suspected arrhythmia is vital.
Normally each p wave is followed by a narrow QRS
complex.
• A conducted wide QRS complex indicates cardiac conduction system disease.
• AV block is present if a p wave is not followed by a
QRS complex.
• Reversible causes, particularly drugs, need to be
excluded.
• Referral for further evaluation is necessary to decide on
the need for permanent cardiac pacing.
In contrast to Mobitz type I AV block ( AV node), Mobitz
type II is consequent on intrinsic conduction system damage, usually from aging, but uncommonly associated with
myocardial infarction and myocarditis. There is a high
likelihood of progressing to complete AV block with an
unreliable escape rhythm and therefore prophylactic pacing is indicated.
Third-degree AV block (complete) (Fig. 4): There is
complete dissociation between P waves and QRS complexes. The site of block may occur in the AV node, usually
associated with a narrow QRS junctional escape rhythm of
between 40 and 50 per minute. If the block is below the
AV node, the QRS escape is slower (30 - 40 per minute),
wider and less reliable. Failure of the escape rhythm results
in Stokes-Adams attacks.
Fig.4
The commonest cause of complete AV block is senile degeneration of the conduction tissue (Lev’s or Lenegre’s disease).
Other causes include drugs, acute infarction, myocarditis
and hyperkalaemia. Congenital complete AV block is well
recognised in association with a narrow QRS junctional
escape rhythm. A high incidence of systemic lupus erythematosus has been seen in mothers of affected infants.
A SPECIAL INTEREST GROUP OF SA HEART ASSOCIATION
P.O. Box 2826, Benoni, 1500
E-mail: [email protected]
Fax: 021-448 7062
This article is sponsored by Johnson & Johnson Medical,
in the interest of continued medical education
For more information and referrals, please send your
request to [email protected]
May 2004 Vol.22 No.5 CME
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