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56 Chapter 2: Cardiovascular system

Ventricular fibrillation death’ where a subject is fortunate to survive such an
event. It is now customary to use these in patients
Definition
known to have a high risk of sudden cardiac death.
Chaoticelectromechanicalactivityoftheventriclescaus-
ing a loss of cardiac output.
Conduction disturbances
Incidence
The most common cause of sudden death and the most Atrioventricular block
common primary arrhythmia in cardiac arrest.
Atrioventricular or heart block describes an alteration
in the normal pattern of transmission of action poten-
Aetiology
tials between the atria and the ventricles. The normal
Mayoccur de novo, as a sequelae to a myocardial infarc-
function of the AV node is to delay the transmission of
tion, post-electrocution or as a result of other arrhyth-
action potentials and hence protect the ventricles from
mias or drug overdose including digoxin and adrenaline.
transmission of atrial rates fast enough to impair cardiac
It may be preceded by another arrhythmia such as tor-
filling. Block at the AV node may result in (see Fig. 2.10)
sades de pointes or develop in the context of complete
aprolongation of the time required for transmission
heart block. Hypokalaemia and hypomagnesaemia may
(first degree block).
also result in ventricular fibrillation.
intermittent failure of transmission (second degree
block).
Pathophysiology
complete failure of transmission (third-degree heart
The underlying electrical activity consists of multiple ec-
block).
topic foci and small re-entry circuits with resulting un-
coordinated contractions such that cardiac ventricular
filling and cardiac output fall to zero. First degree atrioventricular block
Definition
Clinical features
Atrioventricular block describes an alteration in the
The clinical picture is of cardiac arrest with loss of ar-
transmission of action potentials between the atria and
terial pulsation, loss of consciousness and cessation of
the ventricles. First degree block is a prolongation of the
breathing.
time required for AV transmission.
Investigations
Aetiology
ECG shows the chaotic rhythm with ventricular com-
First degree block occurs occasionally in normal indi-
plexes of varying amplitude, rate and form distinguish-
viduals but may follow rheumatic fever, ischaemic heart
ing it from pulseless electrical activity and asystole (the
disease or digoxin toxicity. In the young it is often caused
other causes of cardiac arrest).
by high vagal tone. First degree block may progress onto
other forms of heart block.
Management
Early defibrillation is the most important treatment,

as the longer it is delayed the less likely reversion to Clinical features
sinus rhythm is possible. Cardiopulmonary resuscita- First degree atrioventricular block is asymptomatic.
tion should be initiated to maintain organ perfusion
until defibrillation can be given. Investigations
Prevention of recurrent of ventricular fibrillation is ECG shows a prolongation of the PR interval >0.2

with antiarrhythmics usually amiodarone. seconds (one large square on a standard ECG) with
Increasingly automatic implantable cardiac defibrilla- QRS complexes following every P wave (see Fig. 2.11).

tors (AICDs) are implanted to prevent sudden death. Digoxin level in patients on digoxin to exclude
The most common indication is for ‘failed sudden overdose.