Chapter 18
The Cardiovascular System: The Heart
677
18
are bipolar leads that measure the voltage difference either
between the arms or between an arm and a leg, and nine are
unipolar leads. Together the 12 leads provide a comprehensive
picture of the heart’s electrical activity.
A typical ECG has three almost immediately distinguish-
able waves or
deflections
: the P wave, the QRS complex, and the
T wave (Figure 18.17). ±e first, the small
P wave
, lasts about
0.08 s and results from movement of the depolarization wave
from the SA node through the atria. Approximately 0.1 s a²er
the P wave begins, the atria contract.
±e large
QRS complex
results from ventricular depolariza-
tion and precedes ventricular contraction. It has a complicated
shape because the paths of the depolarization waves through the
ventricular walls change continuously, producing correspond-
ing changes in current direction. Additionally, the time required
for each ventricle to depolarize depends on its size relative to the
other ventricle. Average duration of the QRS complex is 0.08 s.
±e
T wave,
caused by ventricular repolarization, typically
lasts about 0.16 s. Repolarization is slower than depolarization,
so the T wave is more spread out and has a lower amplitude
(height) than the QRS complex. Because atrial repolarization
takes place during the period of ventricular excitation, the wave
in ganglia in the heart wall and their fibers project most heavily
to the SA and AV nodes.
Electrocardiography
±e electrical currents generated in and transmitted through
the heart spread throughout the body and can be detected with
a device called an
electrocardiograph
. An
electrocardiogram
(ECG)
is a graphic record of heart activity. An ECG is a com-
posite of all the action potentials generated by nodal and con-
tractile cells at a given time
(Figure 18.17)
not
, as sometimes
assumed, a tracing of a single action potential.
To record an ECG, recording electrodes (typically 12 leads)
are placed at various sites on the body surface. ±ree electrodes
Thoracic spinal cord
The
vagus nerve
(parasympathetic)
decreases heart rate.
Cardioinhibitory
center
Cardioacceleratory
center
Sympathetic cardiac
nerves
increase heart rate
and force of contraction.
Medulla oblongata
Sympathetic
trunk
ganglion
Dorsal motor nucleus
of vagus
Sympathetic trunk
AV
node
SA
node
Parasympathetic fibers
Sympathetic fibers
Interneurons
Figure 18.16
Autonomic innervation of the heart.
Sinoatrial
node
Atrioventricular
node
Atrial
depolarization
QRS complex
Ventricular
depolarization
Ventricular
repolarization
P-R
Interval
S-T
Segment
P
R
Q
S
T
Q-T
Interval
Time (s)
0
0.2
0.4
0.6
0.8
Figure 18.17
An electrocardiogram (ECG) tracing.
The labels
identify the three normally recognizable deflections (waves) and the
important intervals.
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