PHYSICS

16.4. PROPAGATION OF ACTION POTENTIALS
When a cell is excited and generates an action potential, an
ionic current begin to flow. This process can, in turn, excite
similar neighboring cells. With nerve cells that have a long fiber,
the action potential is generated over a very small segment of the
fiber's length, however, it is propagated in both directions from
the original point of excitation. The rate at which an action potential
moves down a fiber or is propagated from cell to cell is
called the propagation rate. The velocity of the propagation rate
varies widely. The usual velocity range in nerves cells is 20 ­
140 rn/s; propagation through heart muscle is slower, with an
average rate of 0.2 - 0.4 m/s,
16.5. ELECTROCARDIOGRAPHY
The biopotentials generated by the muscles of the heart result
in the electrocardiogram (abbreviated ECG) and the technique
used to measure these potentials is called electrocardiography.
Each action potential in the heart originates near the top of the
right atrium at a point called the sinoartrial node - a group of
specialized cells that spontaneously generate action potentials at a
regular rate. The action potential propagates in all directions
along the surface of both
atria. A graphic recording
or display of the time-variant
voltages produced by
the myocardium during the
cardiac cycle is called an
electrocardiogram. Fig. 16.4
shows the basic waveform
of a normal electrocardiogram.
The P, QRS, and
T waves reflect the rhythmic
electrical depolarization
and repolarization of
the myocardium associated
with the contractions of
the atria and ventricles. The
124
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P-R S T T
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f--
Fig. 16.4. The important intervals
and segments in the electrocardiogram
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electrocardiogram is used clinically in diagnosting various diseases
and conditions associated with the heart.
16.6. THE EINTHOVEN'S TRIANGLE
An ECG can be described in terms of the cardiac vector. The
electrical activity of the heart corresponds to the movement of an
electrical di pole which consists of a positive and a negative charge
separated by a variable distance. The cardiac vector is the line
joining the two charges. To fully describe the cardiac vector, its
magnitude and direction must be known. Usually the cardiac
0
vector is described by its length in three directions at 60 to each
other. The resulting geometric pattern (fig. 16.5) is known as
Einthoven 's triangle. The sides of the triangle represent the lines
along which the three projections of the ECG vector are measured.
The magnitudes and directions of these projections depend
strongly on the state of the patient's heart. The direction of normal
cardiac vector runs between 0 - +90'; the other directions
correspond to various pathological situations.
.~ -90 0
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a
c
AI
AlII
AI
VI Fig. 16.5. The Einthoven's
triangle: the dependence
of the magnitudes and
All directions of the ECG
vector projections
VIII on the state of the
patient's heart
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