C H A P T E R 4 5
Antiarrhythmic agents
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an ectopic focus, such as
premature atrial contractions
(PACs)
or
premature ventricular contractions (PVCs)
,
atrial flutter, atrial fibrillation (Box 45.1) or ventricular
fibrillation; or by alterations in conduction through the
muscle, such as
heart blocks
and bundle-branch blocks.
Figure 45.2 displays an electrocardiogram (ECG) strip
showing normal sinus rhythm; Figures 45.3 to 45.5
depict various arrhythmias.
■■
Arrhythmias (also called dysrhythmias) are
disruptions in the normal rate or rhythm of the
heart.
■■
The cardiac conduction system determines the
heart’s rate and rhythm. The property by which the
KEY POINTS
cardiac cells generate an action potential internally
to stimulate the cardiac muscle without other
stimulation is known as automaticity.
■■
Electrolyte disturbances, decreases in the oxygen
delivered to the cells, structural damage in the
conduction pathway, drug effects, acidosis or
the accumulation of waste products can trigger
arrhythmias.
■■
Changes in the heart rate, uncoordinated heart
muscle contractions or blocks that alter the
movement of impulses through the system can disrupt
heart rhythm.
■■
Arrhythmias change the mechanics of blood
circulation (haemodynamics), which can interrupt
delivery of blood to the brain, other tissues and
the heart.
FIGURE 45.2
Normal sinus rhythm. Rhythm: regular. Rate: 60 to 100 beats/minute. P–R interval: 0.12 to 0.20 seconds. QRS: 0.06 to
0.10 seconds.
FIGURE 45.3
Atrial arrhythmias: premature atrial contractions (PACs). Rhythm: irregular due to the origination of a beat outside the normal
conduction system (ectopic). Rate: normal sinus rate, except for PACs. P–R interval: P wave is abnormal, and interval may be slightly shortened
in ectopic beat. QRS: normal. Atrial fibrillation. Rhythm: irregularly irregular. Rate: variable; usually rapid on initiation of rhythm; decreases
when controlled by medication. P–R interval: no P waves are seen, replaced by an irregular wavy baseline. The atria are fibrillating because
impulses are arising at a rate greater than 350 per minute. The ventricles respond when the atrioventricular (AV) node is stimulated to
threshold and can receive the impulse. QRS: normal.
