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P A R T 8
Drugs acting on the cardiovascular system
demands for oxygen. A faster-than-normal heart
rate—usually anything faster than 100 beats/min in an
adult—with a normal-appearing ECG pattern is called
sinus tachycardia. If sinus tachycardia becomes too fast,
it can lead to a decreased time for cardiac filling and a
decrease in cardiac output. Many activities or conditions
can cause a sinus tachycardia, such as exercise, fear or
stress. The underlying physical condition of the person
will determine whether this fast heart rate is problem
atic. Sinus bradycardia is a slower-than-normal heart
rate (usually less than 60 beats/min) with a normal-
appearing ECG pattern. Sinus bradycardia allows
increased time for ventricular filling and an increased
cardiac output. This is often seen with athletes who have
a slow heart rate. In other people, this rate might be too
slow to adequately perfuse all of the tissues.
Supraventricular arrhythmias
Arrhythmias that originate above the ventricles but not
in the SA node are called supraventricular arrhythmias.
These arrhythmias feature an abnormally-shaped P wave
because the site of origin is not the sinus node. However,
they show normal QRS complexes because the ventricles
are still conducting impulses normally. Supraventricular
arrhythmias include the following:
•
Premature atrial contractions (PACs)
, which reflect
an
ectopic focus
(a shift in the pacemaker of the heart
from the SA node to some other site) in the atria that
is generating an impulse out of the normal rhythm.
•
Paroxysmal atrial tachycardia (PAT)
, sporadically
occurring runs of rapid heart rate originating in the
atria.
•
Atrial flutter
, characterised by sawtooth-shaped
P waves reflecting a single ectopic focus that is
generating a regular, fast atrial depolarisation.
•
Atrial fibrillation
, with irregular P waves representing
many ectopic foci firing in an uncoordinated manner
through the atria.
With atrial flutter, often one of every two or one
of every three impulses is transmitted to the ventricles.
The person may have a 2:1 or 3:1 ratio of P waves to
QRS complexes. The ventricles beat faster than normal,
losing some efficiency. With atrial fibrillation, so many
impulses are bombarding the AV node that an unpre
dictable number of impulses are transmitted to the
ventricles. The ventricles are stimulated to beat in a fast,
irregular and often inefficient manner.
Atrioventricular block
Atrioventricular block, also called heart block, reflects
a slowing or lack of conduction at the AV node. This
can occur because of structural damage, hypoxia or
injury to the heart muscle. First-degree heart block, in
which all of the impulses from the SA node arrive in
the ventricles but after a longer-than-normal period,
is characterised by a lengthening of the P–R interval
beyond the normal 0.16 to 0.20 seconds. Each P wave
is followed by a QRS complex. In second-degree
heart block, some of the impulses are lost and do not
get through, resulting in a slow rate of ventricular
contraction. With this arrhythmia, a QRS complex
may follow one, two, three or four P waves. In third-
degree heart block, or complete heart block, no impulses
from the SA node get through to the ventricles, and the
much slower ventricular automaticity takes over. The
waveform shows a total dissociation of P waves from
QRS complexes and T waves. Because the P waves can
come at any time, the P–R interval is not constant. The
QRS complexes appear at a very slow rate and may not
be sufficient to meet the body’s needs.
Ventricular arrhythmias
Impulses that originate below the AV node originate
from ectopic foci that do not use the normal conduc
tion pathways. The QRS complexes appear wide and
prolonged, and the T waves are inverted, reflecting the
slower conduction across cardiac tissue that is not part
of the rapid conduction system. Premature ventricu
lar contractions (PVCs) can arise from a single ectopic
focus in the ventricles, with all of them having the same
shape, or from many ectopic foci, which produces PVCs
with different shapes. Runs or bursts of PVCs from
many different foci are more ominous because they can
reflect extensive damage or hypoxia in the myocardium.
Runs of several PVCs at a rapid rate are called ven
tricular tachycardia. Ventricular fibrillation is seen as a
bizarre, irregular, distorted wave. It is potentially fatal
because it reflects a lack of any coordinated stimulation
of the ventricles. The ventricles’ inability to contract in
a coordinated fashion results in no blood being pumped
to the body or the brain. Thus there is a total loss of
cardiac output.
■■
The normal ECG waveform is made up of five main
waves: the P wave, which is formed as impulses
originating in the SA node or pacemaker pass
through the atrial tissues; the QRS complex, which
represents depolarisation of the bundle of His (Q)
and the ventricles (RS); and the T wave, which
represents repolarisation of the ventricles.
■■
A person with a normal ECG pattern and a heart rate
within the normal range for that person’s age group is
said to be in normal sinus rhythm.
■■
When the generation of impulses is altered, the result
is known as an arrhythmia (or dysrhythmia) that
can upset the normal balance in the cardiovascular
system. A decrease in cardiac output, which affects
all of the cells of the body, follows.
KEY POINTS
