The Cardiovascular System: The Heart
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
: the P wave, the QRS complex, and the
T wave (Figure 18.17). ±e ﬁrst, the small
, 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.
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.
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.
±e electrical currents generated in and transmitted through
the heart spread throughout the body and can be detected with
a device called an
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
, 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
decreases heart rate.
increase heart rate
and force of contraction.
Dorsal motor nucleus
Autonomic innervation of the heart.
An electrocardiogram (ECG) tracing.
identify the three normally recognizable deﬂections (waves) and the