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Chapter 2: Cardiac arrhythmias 59

of the right coronary artery, which supplies the AV Complications
node and bundle of His. It may also occur following Cardiac failure, Stokes–Adams attacks, asystole, sudden
a massive anterior myocardial infarction and is a sign cardiac death.
of poor prognosis.
Chronic complete heart block is most commonly due

Management
to fibrosis of both bundle branches in the elderly. Rare In acute complete heart block, intravenous isopre-
causes include drugs, post-surgery, rheumatic fever
naline or a temporary pacing wire may be used. Post-
and myocarditis.
MI it often resolves within a week.
Identification and removal of any cause.

Chronic complete AV block requires permanent pac-
Pathophysiology
ing even in asymptomatic cases as this reduces mor-
With AV dissociation, an ectopic ventricular pacemaker
tality.
is responsible for maintaining ventricular contractions.
Depending on the site of this pacemaker the QRS com-
plexes may be either narrow or wide: Prognosis
Narrow complex disease is due to disease of the AV Untreated chronic AV block with Stokes–Adams

node or proximal bundle of His. The ectopic pace- episodes has a 1-year mortality of 35–50%.
maker within the specialised conducting fibres dis-
tal to the lesion gives a reliable rate of 50–60 bpm
and is associated with congenital heart disease, infe- Left bundle branch block
rior infarction, rheumatic fever and cardiac drugs, e.g.
Definition
β-blockers.
Block of conduction in the left branch of the bundle of
Broad complex disease is due to more distal disease of

His, which normally facilitates transmission of impulses
the Purkinje system. The pacing thus arises within the
to the left ventricle
myocardium giving an unreliable 15–40 bpm rate. In
theelderlycausesincludefibrosisofthecentralbundle
Aetiology/pathophysiology
branches (Lenegre’s disease). It may also be associated
The most common cause is ischaemic heart disease. The
with ischaemic heart disease.
block may be complete or partial.
Clinical features
Clinical features
Severity of symptoms is dependent on the rate and re-

Most patients are asymptomatic but reversed splitting of
liability of the ectopic pacemaker, and whether or not
the second heart sound may be observed. Normally, the
the myocardium can compensate for the bradycardia.
second heart sound splits during inspiration because the
Patients with underlying ischaemic heart disease, par-
increased flow of blood into the right side of the heart
ticularly recent myocardial infarction are most at risk
delays the pulmonary component. In left bundle branch
of complications. Symptoms include those of cardiac
block the second heart sound is split on expiration, be-
failure, dizziness and Stokes–Adams attacks (syncopal
cause left ventricular conduction delay causes the aortic
episodes lasting 5–30 seconds due to failure of ven-
valvetocloseafterthepulmonaryvalve.Oninspirationit
tricular activity).
occurs together with the pulmonary component because
On examination, there are occasional cannon waves

the pulmonary component is also delayed.
in the JVP due to the atria contracting on a closed
tricuspid valve, with a variable intensity of the first
Investigations
heart sound.
ECG shows a characteristic RsR’ M shape in seen in
lead V5 or V6. There is a deep ‘S’ wave in leads V1–
Investigations V3. Left bundle branch block results in a QRS complex
The ECG is diagnostic revealing independent, unrelated of greater than 0.12 seconds. There is left axis deviation
atrial and ventricular activity. and the morphology of the QRS complex is abnormal