What is Electrocardiograph?

The human heart beats about 70 times per minute. Each beat begins
with a contraction of the atria which is followed a moment later by the
contraction of the muscle. The contractions of the heart muscle, like
the contractions of other muscles, are triggered by electric signals. But
in contrast to other muscles, where the electric signals travel along
nerve fibers, the electric signals in heart muscle travel along the muscle
fibers. These electric signals involve changes of the electric potential
in and around the muscle fibers. The changes of the electric potential
associated with the heartbeats are strongest in the immediate vicinity of
the muscle fibers; but, with a sensitive detector, the changes in the
potential can be measured at some distance from the heart, on the
surface of the skin. The detector used to measure such changes of
potential is called electrocardiograph; it is widely used by physicians
to monitor the operation of the heart and to discover defects in the
heart muscle.

The mechanism for the propagation of electric signals along muscle
fibers is almost exactly the same as for the propagation along nerve
fibers. Each muscle fiber is a single, long cell, sheathed in a
membrane. When the muscle fiber is at rest (inactive), the membrane
carries a layer of positive charge on its outside surface and a layer of
negative charge on its inside surface. A membrane with such layers of
opposite charges is said to be polarized. The positive charges are
mainly K+ ions, and the negative charges are mainly Cl- ions. Between
these layers of charge there is a uniform electric field between
oppositely charged parallel plates consequently there is a potential
difference between the outside and the inside of the cell. The potential
induced of the cell is negative. This resting potential is about 90 millivolts.

When a muscle fiber is stimulated by an electric signal from an
adjacent muscle fiber, positive charges (mostly Na+ ions) from the
interstitial fluid surrounding the fiber flows through the membrane into
the fiber. The accumulation of these positive charges reverse the
potential difference, from - 90 millivolts to about + 30 millivolts. The
membrane is then said to be depolarized (actually, polarized in the
reverse direction). The depolarization propagates along the length of
the muscle fiber and constitutes the electric signal. This depolarization
triggers the contraction process of the fiber. Within a short time,
chemical processes within the fiber pump the positive charges out of
the fiber and restore the charge distribution to the initial resting state.
The depolarized fiber is then ready for the next depolarization and the
next contraction.

The depolarization of muscle fibers can be detected at some distance
from the fibers by the changes in the electric potential. Consider a
muscle fiber that is initially polarized, and progressively becomes
depolarized from left to right. Initially the positive charge distribution
is centered on the negative charge distribution. The potential at a point
M beyond one end of the fiber is nearly zero, since the average
positions of the positive and the negative charge distributions are
slightly different - the average position of the positive charge
distribution is slightly to the right of the average position of the
negative charge distribution. The separation between the average
positions of the charge distributions implies that the muscle fiber has a
dipole moment. The electric fields then do not cancel, and the
potential at the exterior point is not zero.

In the heart, the depolarization and contraction of the muscle fibers of
the atria and the ventricles are initiated respectively by the sinus node
and the atrioventricular node (A-V node). These nodes are small
clumps of muscle fibers. The sinus node is the primary pacemaker of
the heart. Its muscle cells depolarize and contract spontaneously at
regular intervals. The electric signals produced during this activity are
transmitted to the muscles of the atria and initiate their contraction.
When the electric signals reach the A-V node, it relays these signals to
the ventricles and thereby initiates their contraction.

To observe the potential changes generated during the depolarization
of the muscle fibers in the heart, electrodes (contacts) are placed on the
skin and connected by wire leads to a sensitive voltmeter. The potential
changes on the skin are of the order of a few millivolts, and to detect
such small potentials, the voltmeters used in the electrocardiograph are
equipped with amplifier circuits. The standard procedure for
electrocardiography is to attach electrodes to the left wrist, the right
wrist and the left ankle (other electrodes are attached).

The sinus node is the primary pacemaker of the heart to the chest wall,
but we will ignore these. The electrodes are covered with a salt paste,
for good electric contact. The choice of location of the electrodes on
the arms and legs is a matter of convenience. The arms and the legs act
as conducting segments, and attaching electrodes to the ends of the
arms or legs. Measurements with the wrist and ankle electrodes are
roughly equivalent to measurements at the vertices of a triangle
centered on the heart called the Einthoven triangle.

With these three electrodes at the vertices of the triangle, six
measurements are routinely made and plotted by the
electrocardiograph: three “unipolar” measurements and three “bipolar”
measurements. The unipolar measurements simply give the values of
the potential at the three vertices of the triangle. These potentials are
designated VR (for right wrist), VL (for left wrist), and VF (for left
ankle or foot). The plus signs included next to these symbols indicate
that the +e terminal of the voltmeter is connected to this electrode
during the measurement. The -e terminal of the voltmeter is connected
to a reference terminal, or “indifferent terminal,” whose potential is
zero.

The bipolar measurements give the potential differences between
adjacent vertices. These potential differences are designated I, II, and
III. The plus and minus signs indicate where the + and - terminals of
the voltmeter are connected during the measurement. Note that I, II,
and III can be expressed as differences between VR, VL, and VF.
I = VL - V
II = VF - VR
III = VF - VL.
Figure 2 : is an example of an electrocardiogram (ECG) measures on a
healthy individual. This ECG displays the potentials I, II, III, VR, VL,
and VF as a function of time. Figure 2 is an enlarged sketch of the
potential II. The main features of this plot are the maxim and
minimum, called the P, Q, R, S, and T “waves”. These waves indicate
the potentials generated by depolarizations and repolarizations of
muscle fibers in different parts of the heart. The P wave indicates the
depolarization of the atria; the Q wave that of the septum, the R wave
that of the main mass of the ventricles, and the S wave that of a
smaller, lateral portion of the right ventricle. The T wave indicates the
depolarization of the ventricles.

These waves are of different magnitudes because the muscle mass is
different (largest for ventricle, smallest for septum), and because the
directions of the progressive depolarization of the muscle fibers are
different. The direction of depolarization in the ventricle is on the
average, aligned with the direction to which the potential II belongs
and therefore this depolarization gives a large contribution to this
potential.

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