Biomedical Engineering Reference
In-Depth Information
decrease in fiber resistance if other conditions remain the same. Immediately distal to the
AV bundle, the Purkinje fibers split into the right and left branch of the Purkinje fibers.
These branches follow the ventricular septum (the wall that separates the right and left
ventricles) all the way to the apex of the heart. At the heart apex, these fibers branch into
smaller fibers that penetrate the entire ventricular cardiac muscle mass. Therefore, this
acts to pass the action potential signal directly to every muscle cell within the ventricular
wall. The first cells that contract are those near the heart apex, and the wave of contraction
passes upward and outward to the remaining cardiac muscle cells. As the reader may
guess, the reason for the rapid conduction through the Purkinje fibers/ventricular mass is
due to the presence of many gap junctions connecting the cells. This high conductivity
allows all of the ventricular muscle cells to contract within 0.05 seconds of the cells at the
apex. The same fibrous mass that blocks the atrial action potential signal from entering the
ventricles blocks the ventricle action potential signal from entering the atria.
One of the most common ways to monitor the electrical system of the heart is through
an electrocardiogram (ECG). To measure an ECG, electrical leads are placed at particular
locations on the skin to measure the heart leak current that enters surrounding tissues.
The normal ECG is composed of three waves termed the P wave, the QRS complex (some-
times this can be considered as three independent waves), and the T wave ( Figure 4.6 ).
Each of these waves represents a particular portion of the cardiac action potential. The
P wave is associated with atrial depolarization and occurs at the beginning of atrial con-
traction. The P wave typically lasts for approximately 0.05 seconds under normal condi-
tions. The QRS complex is associated with ventricular depolarization and occurs at the
beginning of ventricular contraction. The QRS complex typically starts 0.15 seconds after
the P wave starts and lasts for approximately 0.09 seconds. The T wave is associated with
ventricular repolarization, and it occurs once the ventricular muscle cells recover from
depolarization. This wave lasts for approximately 0.05 seconds, occurring approximately
0.25 seconds after the QRS complex ends. The atrial repolarization wave does not show up
on the normal ECG because it occurs approximately 0.15 seconds after atrial
FIGURE 4.6 Schematic of a nor-
mal electrocardiogram trace, depict-
ing the various waveforms used to
determine if the heart is functioning
properly. The P wave is associated
with atria contraction, the QRS com-
plex is associated with ventricular
contraction, and the T wave is associ-
ated with ventricular repolarization.
1.2
1
R
0.8
0.6
0.4
P
T
0.2
0
0.2
Q
S
0.4
0.6
0
0.2
0.4
0.6
0.8
1
Time (sec)
1.2
1.4
1.6
1.8
2
 
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