C H A P T E R 4 5
Antiarrhythmic agents
697
A
s discussed in earlier chapters, disruptions in impulse
formation and in the conduction of impulses through
the myocardium are called arrhythmias. (They also
are called dysrhythmias by some healthcare providers.)
Arrhythmias occur in the heart because all of the cells of
the heart possess the property of automaticity (discussed
later in this chapter) and therefore can generate an excit-
atory impulse. Disruptions in the normal rhythm of the
heart can interfere with myocardial contractions and
affect the
cardiac output
, the amount of blood pumped
with each beat. Arrhythmias that seriously disrupt
cardiac output can be fatal. Drugs used to treat arrhyth-
mias, called antiarrhythmics, suppress automaticity or
alter the conductivity of the heart.
ARRHYTHMIAS
Arrhythmias involve changes to the automaticity or
conductivity of the heart cells. These changes can result
from several factors, including electrolyte imbalances
that alter the action potential, decreased oxygen delivery
to cells that changes their action potential, struc-
tural damage that changes the conduction pathway,
or acidosis or waste product accumulation that alters the
action potential. In some cases, changes to the heart’s
automaticity or conductivity may result from drugs that
alter the action potential or cardiac conduction.
Conductivity
With normal heart function, each cycle of cardiac con-
traction and relaxation is controlled by impulses arising
spontaneously in the sinoatrial (SA) node and transmit-
ted via a specialised conducting system to activate all
parts of the heart muscle almost simultaneously (see
Chapter 42) (Figure 45.1). These continuous, rhythmic
contractions are controlled by the heart itself. This
property allows the heart to beat as long as it has enough
nutrients and oxygen to survive, regardless of the status
of the rest of the body.
Automaticity
All cardiac cells possess some degree of automaticity
(see Chapter 42) in which the cells undergo a spontane-
ous depolarisation during diastole or rest because they
decrease the flow of potassium ions out of the cell and
probably leak sodium into the cell, causing an action
potential.
The action potential of the cardiac muscle cell
consists of five phases:
•
Phase 0
occurs when the cell reaches a point of
stimulation. The sodium gates open along the cell
membrane and sodium rushes into the cell; this
positive flow of electrons into the cell results in an
electrical potential. This is called depolarisation.
•
Phase 1
is a very short period during which the
sodium ion concentration equalises inside and outside
the cell.
•
Phase 2
, or the plateau stage, occurs as the cell
membrane becomes less permeable to sodium, calcium
slowly enters the cell and potassium begins to leave
the cell. The cell membrane is trying to return to its
resting state, a process called repolarisation.
•
Phase 3
is a time of rapid repolarisation as the
sodium gates are closed and potassium flows out
of the cell.
•
Phase 4
occurs when the cell comes to rest; the
sodium–potassium pump returns the membrane
to its resting membrane potential and spontaneous
depolarisation begins again.
Each area of the heart has a slightly different-
appearing action potential that reflects the complexity
of the cells in that area. Because of these differences in
the action potential, each area of the heart has a slightly
different rate of rhythmicity. The SA node generates an
impulse about 60 to 100 times per minute, the atrio-
ventricular (AV) node about 40 to 50 times per minute
and the complex ventricular muscle cells about 10 to
20 times per minute.
Haemodynamics
The study of the forces that move blood throughout the
cardiovascular system is called
haemodynamics
. The
ability of the heart to effectively pump blood depends on
the coordinated contraction of the atrial and ventricular
muscles, which are stimulated to contract via the con-
duction system. The conduction system is designed so
that atrial stimulation is followed by total atrial con-
traction and ventricular stimulation is followed by total
ventricular contraction.
SA node
AV node
Right bundle
branch
Left bundle
branch
Purkinje
fibres
Bundle of His
Right
ventricle
Left
atrium
Right
atrium
Left
ventricle
FIGURE 45.1
The conducting system of the heart. Impulses originating
in the sinoatrial (SA) node are transmitted through the atrial bundles
to the atrioventricular (AV) node and down the bundle of His and
the bundle branches by way of the Purkinje fibres through the
ventricles.
