What is Electrocardiograph?

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.