The Normal Electrocardiogram pdf - The Heart

the record has risen to a maximum positive value.

meter. Note that when depolarization has reached the halfway mark in Figure

in potential between the two electrodes, as recorded by a high-speed recording

record positively. To the right of the muscle fiber is shown a record of changes

tivity, and the right electrode is in an area of positivity; this causes the meter to

Therefore, the left electrode on the outside of the fiber is in an area of nega-

the fiber has already depolarized, while the remaining half is still polarized.

and red negative charges outside, is traveling from left to right. The first half of

depolarization, demonstrated by red positive charges inside

In Figure 11–2

slightly positive inside and negative outside.

ization, the normal negative potential inside the fiber reverses and becomes

and repolarization, the color red designating depolarization. During depolar-

Figure 11–2 shows a single cardiac muscle fiber in four stages of depolarization

Depolarization Waves Versus Repolarization Waves

is needed.

discussed in Chapter 5. The distinction between depolarization waves and repo-

larization waves. The principles of depolarization and repolarization are

Thus, the electrocardiogram is composed of both depolarization and repo-

ization wave.

0.25 to 0.35 second after depolarization, and the T wave is known as a

the state of depolarization. This process normally occurs in ventricular muscle

The T wave is caused by potentials generated as the ventricles recover from

depolarization waves.

depolarization wave spreads through the ventricles. Therefore, both the P wave

tials generated when the ventricles depolarize before contraction, that is, as the

larize before atrial contraction begins. The QRS complex is caused by poten-

The P wave is caused by electrical potentials generated when the atria depo-

separate waves: the Q wave, the R wave, and the S wave.

QRS complex, and a T wave. The QRS complex is often, but not always, three

The normal electrocardiogram (see Figure 11–1) is composed of a P wave, a

Electrocardiogram

Characteristics of the Normal

A normal electrocardiogram for two beats of the heart is shown in Figure 11–1.

electrocardiogram.

skin on opposite sides of the heart, electrical

surface of the body. If electrodes are placed on the

the adjacent tissues surrounding the heart. A small

When the cardiac impulse passes through the heart,

The Normal Electrocardiogram

123

electrical current also spreads from the heart into

portion of the current spreads all the way to the

potentials generated by the current can be recorded;
the recording is known as an

and the components of the QRS complex are

repolar-

larization waves is so important in electrocardiography that further clarification

11–2A,

ventricle, which shows the QRS waves appearing at

muscle fiber. The upsweep of this action potential is

0.25 and 0.35 second. The top part of Figure 11–3 shows

muscle, discussed in Chapter 10, normally lasts between

The monophasic action potential of ventricular

Muscle to the QRS and T Waves in the Standard Electrocardio-

Relation of the Monophasic Action Potential of Ventricular

fiber membrane.

potential returns once more to zero. This completed

between them. Thus, in the recording to the right, the

positivity, so that no potential difference is recorded

repolarized, and both electrodes are now in areas of

In Figure 11–2

to the right, becomes negative.

Consequently, the recording, as shown

Figure 11–2

area of negativity. This is opposite to the polarity in

in an area of positivity, and the right electrode is in an

outside of the fiber. At this point, the left electrode is

same muscle fiber, with positivity returning to the

Figure 11–2

membrane.

trodes are now in areas of equal negativity. The com-

the entire muscle fiber, and the recording to the right

In Figure 11–2

124

Unit III

The Heart

B, depolarization has extended over

has returned to the zero baseline because both elec-

pleted wave is a depolarization wave because it results
from spread of depolarization along the muscle fiber

C shows halfway repolarization of the

D, the muscle fiber has completely

negative wave is a repolarization wave because it
results from spread of repolarization along the muscle

gram.

a monophasic action potential recorded from a micro-
electrode inserted to the inside of a single ventricular

caused by depolarization, and the return of the poten-
tial to the baseline is caused by repolarization.

Note in the lower half of the figure a simultaneous

recording of the electrocardiogram from this same

4 0.6

0.8 1.0

1.2

1.6

0.2 0.

Atria Ventricles

RR interval

P-R interval

= 0.16 sec

Q-T interval

S-T

segment

–1

1.4

Millivolts

Time (sec)

Normal electrocardiogram.

Figure 11–1

– – – – – – – – –

+ + + + + + +

+ + + + + + + + +

+ + + + + + + + + + + + + + + +

+ + + + + + + + +

+ + + + + + + + + + + + + + + +

+ + + + + + +

–

Depolarization

wave

Repolarization

wave

+ + + + + + + + +

– – – – – – – – –

+ + + + + + +

– – – – – – – – –

+ + + + + + +

– – – – – – – – – – – – – – – –

– – – – – – –

– – – – – – – – –

0.30 second

–

muscle fiber.

) from a cardiac

repolarization wave

Recording the

Figure 11–2

depolarization wave (A and B) and

the

(C and D

the Q wave is absent) to the end of the T wave. This

from the beginning of the Q wave (or R wave, if

Q-T Interval.

The normal P-Q interval is about 0.16 second. (Often

the ventricles. This period is called the P-Q interval.

The time between the beginning of

P-Q or P-R Interval.

and 0.3 millivolt; and that of the T wave is between 0.2

the S wave; the voltage of the P wave is between 0.1

electrodes on the two arms or on one arm and one leg,

brane. When electrocardiograms are recorded from

as 3 to 4 millivolts. Even this voltage is small in com-

heart, the voltage of the QRS complex may be as great

electrodes are to the heart. When one electrode is

The recorded

Normal Voltages in the Electrocardiogram.

thin lines, each of which represents 0.04 second.

these dark lines represent 0.20 second. The 0.20 second

segments by dark vertical lines; the intervals between

is 1 second, and each inch is usually broken into five

calibration lines. Each inch in the horizontal direction

The vertical lines on the electrocardiogram are time

gram represent 1 millivolt, with positivity in the upward

As shown in Figure 11–1, the horizontal calibration

with the photographic types of electrocardiographs.

the electrocardiogram is recorded, which is the case

paper, as is the case when a pen recorder is used, or

appropriate calibration lines on the recording paper.

Voltage and Time Calibration of the

QRS complex, partly because of its prolonged length.

the T wave is considerably less than the voltage of the

trocardiogram is a prolonged wave, but the voltage of

second. For this reason, the T wave in the normal elec-

repolarization extends over a long period, about 0.15

as long as 0.35 second. Thus, the process of ventricular

(the QRS complex), but in many other fibers, it takes

muscle begins to repolarize in some fibers about 0.20

the normal electrocardiogram. Ordinarily, ventricular

The ventricular repolarization wave is the T wave of

this reason, an atrial T wave seldom is observed in the

obscured by the much larger QRS complex. For

atrial T wave,

larization wave, known as the

the electrocardiogram. Therefore, the atrial repo-

termination of the P wave. This is also approxima-

The atria repolarize about 0.15 to 0.20 second after

has occurred, that is, until after the end of the T wave.

The

beginning of contraction of the ventricles.

11–1; the P wave occurs at the

cal processes of contraction. Refer again to Figure

Before contraction of muscle can occur, depolarization

Contraction to the Waves of the

Relationship of Atrial and Ventricular

or completely depolarized.

the ventricular muscle is either completely polarized

no potential is recorded in the electrocardiogram when

the T wave appearing at the end. Note especially that

The Normal Electrocardiogram

Chapter 11

125

the beginning of the monophasic action potential and

Only when the muscle is

partly polarized and partly depolarized does current
flow from one part of the ventricles to another part,
and therefore current also flows to the surface of the
body to produce the electrocardiogram.

Electrocardiogram

must spread through the muscle to initiate the chemi-

beginning of contrac-

tion of the atria, and the QRS complex of waves occurs
at the
ventricles remain contracted until after repolarization

tely when the QRS complex is being recorded in

is usually

electrocardiogram.

second after the beginning of the depolarization wave

Electrocardiogram

All recordings of electrocardiograms are made with

Either these calibration lines are already ruled on the

they are recorded on the paper at the same time that

lines are arranged so that 10 of the small line divisions
upward or downward in the standard electrocardio-

direction and negativity in the downward direction.

intervals are then broken into five smaller intervals by

voltages of the waves in the normal electrocardiogram
depend on the manner in which the electrodes are
applied to the surface of the body and how close the

placed directly over the ventricles and a second elec-
trode is placed elsewhere on the body remote from the

parison with the monophasic action potential of 110
millivolts recorded directly at the heart muscle mem-

the voltage of the QRS complex usually is 1.0 to 1.5
millivolt from the top of the R wave to the bottom of

and 0.3 millivolt.

the P wave and the beginning of the QRS complex is
the interval between the beginning of electrical exci-
tation of the atria and the beginning of excitation of

this interval is called the P-R interval because the Q
wave is likely to be absent.)

Contraction of the ventricle lasts almost

Depolarization

Repolarization

simultaneously.

Electrocardiogram recorded

Below,

but rapidly toward the end.

and then repolarization occurring slowly during the plateau stage

fiber during normal cardiac function, showing rapid depolarization

Monophasic action potential from a ventricular muscle

Figure 11–3

Above,

When one portion of the ventricles depolarizes and

electricity even more easily. Therefore, the heart

with air, conduct electricity to a surprising extent, and

the chest. Even the lungs, although mostly filled

Figure 11–5 shows the ventricular muscle lying within

Chest Around the Heart

Flow of Electrical Currents in the

apparatus.

a second, and the actual voltage measurements can

in the figure persist for only a few thousandths of

through the heart, the potential differences shown

explain the causes of the respective meter readings.

studied carefully, and the reader should be able to

are also demonstrated in Figure 11–4.

Two other electrode placements and meter readings

records positively.

in the figure,

still-polarized areas, as shown

is represented by the positive signs. Therefore, a meter

remaining surface of the heart, which is still polarized,

resented by the negative signs in Figure 11–4. The

making this part of the surface electronegative, as rep-

to the outsides of the depolarized muscle fibers,

syncytium becomes depolarized, negative charges leak

membrane potentials, as soon as an area of cardiac

had been positive and the interiors negative. For

Before stimulation, all the exteriors of the muscle cells

Figure 11–4 shows a syncytial mass of cardiac muscle

Recording Electrical Potentials from a

Cardiac Cycle

the Heart During the

Flow of Current Around

line on the paper where the stylus touches.

paper to an electrode at its back. This leaves a black

through its tip. Another type turns black when electri-

paper turns black when it is exposed to heat; the stylus

does not require ink in the recording stylus. One such

the electrocardiogram.The movement of the pen is con-

speed. As the paper moves forward, the pen records

at one end to an inkwell, and its recording end is con-

of paper. Sometimes the pen is a thin tube connected

puter-based systems and electronic display, while

changes in potentials.

sides of the heart in less than 0.01 second. Therefore,

Electrocardiograms

Methods for Recording

60/0.83 times per minute, or 72 beats per minute.

person is about 0.83 second. This is a heart rate of

heart rate is 60 beats per minute. The normal interval

mined from the time calibration lines is 1 second, the

heartbeats. If the interval between two beats as deter-

The rate of heartbeat can be determined easily from

126

Unit III

The Heart

interval is called the Q-T interval and ordinarily is
about 0.35 second.

Rate of Heartbeat as Determined from the Electrocardiogram.

an electrocardiogram because the heart rate is the
reciprocal of the time interval between two successive

between two successive QRS complexes in the adult

Sometimes the electrical currents generated by the
cardiac muscle during each beat of the heart change
electrical potentials and polarities on the respective

it is essential that any apparatus for recording electro-
cardiograms be capable of responding rapidly to these

Pen Recorder

Many modern clinical electrocardiographs use com-

others use a direct pen recorder that writes the elec-
trocardiogram with a pen directly on a moving sheet

nected to a powerful electromagnet system that is
capable of moving the pen back and forth at high

trolled by appropriate electronic amplifiers connected
to electrocardiographic electrodes on the patient.

Other pen recording systems use special paper that

itself is made very hot by electrical current flowing

cal current flows from the tip of the stylus through the

Partially Depolarized Mass of
Syncytial Cardiac Muscle

that has been stimulated at its centralmost point.

reasons presented in Chapter 5 in the discussion of

connected with its negative terminal on the area of
depolarization and its positive terminal on one of the

to the right

These should be

Because the depolarization spreads in all directions

be accomplished only with a high-speed recording

fluids in other tissues surrounding the heart conduct

is actually suspended in a conductive medium.

therefore becomes electronegative with respect to the

+++++

++++++

– – – – – – – – –

– – – – – – – –

– – – – –

–

+++++

++++++++

+++++

+++++++++++

++++++++++

++++++

+++++

++++++

+++++

++++++

– – – – – – –

– – – – – – – – –

+++++

+++

+ +

– – – – – – – – –

– – – – – – –

– – – –

– –

muscle mass that has been depolarized in its center.

Instantaneous potentials develop on the surface of a cardiac

Figure 11–4

The term “bipolar” means that the

bipolar limb leads.

standard

patient’s limbs and the electrocardiograph for record-

Figure 11–6 shows electrical connections between the

Electrocardiographic Leads

the electrode nearer the apex will be positive, and

electrode nearer the base will be negative, whereas

the surface of the body as shown in Figure 11–5, the

end. And if a meter is connected to electrodes on

the entire cycle of depolarization, except at the very

Thus, in normal heart ventricles, current flows from

the base, because the last part of the heart to become

0.01 second, flowing from the ventricular apex toward

has completed its course through the ventricles, the

muscle mass. Then, immediately before depolarization

direction, while depolarization spreads from the endo-

process, current also continues to flow in this same

with positivity toward the apex.

with negativity toward the base of the heart

elliptical lines), one finds that the average current flow

strated by the curving arrows in the figure. If one

ing the ventricles along elliptical paths, as demon-

tropositivity on the outer walls of the ventricles, with

negative signs in Figure 11–5. This provides elec-

der of the ventricles, as shown by the red areas and the

routes, as noted in the figure.

remainder, electrical current flows from the depolar-

The Normal Electrocardiogram

Chapter 11

127

ized area to the polarized area in large circuitous

It should be recalled from the discussion of the

Purkinje system in Chapter 10 that the cardiac impulse
first arrives in the ventricles in the septum and shortly
thereafter spreads to the inside surfaces of the remain-

tronegativity on the insides of the ventricles and elec-

electrical current flowing through the fluids surround-

algebraically averages all the lines of current flow (the

occurs

and

During most of the remainder of the depolarization

cardial surface outward through the ventricular

average direction of current flow reverses for about

depolarized is the outer walls of the ventricles near the
base of the heart.

negative to positive primarily in the direction from
the base of the heart toward the apex during almost

the recording meter will show positive recording in the
electrocardiogram.

Three Bipolar Limb Leads

ing electrocardiograms from the so-called

electrocardiogram is recorded from two electrodes

----

++++

Flow of current in the chest around partially depolarized

Figure 11–5

ventricles.

0 .2 mV

1 .2 mV

1 .0 mV

0 .5 mV

0 .7 mV

0 .3 mV

Lead III

Lead II

Lead I

dard electrocardiographic leads. Einthoven’s triangle is superim-

Conventional arrangement of electrodes for recording the stan-

Figure 11–6

posed on the chest.

cardiac impulse conduction

abnormalities of cardiac

are recorded, because

conducting system, it does matter greatly which leads

cycle. But when one wants to diagnose damage in

diagnose different cardiac arrhythmias, because diag-

leads are similar to one another, it does not matter

Einthoven’s law.

potential in lead II, thus illustrating the validity of

observance of polarities, that at any given instant the

be shown, with careful measurements and proper

On analysis of the three electrocardiograms, it can

P waves and positive T waves, and the major portion

electrocardiograms in leads I, II, and III. It is obvious

Figure 11–7 shows recordings of the

the three “standard” bipolar electrocardiograms are

Einthoven’s law, holds true at any given instant while

0.7 equals 1.2. Mathematically, this principle, called

that is, 0.5 plus

Now, note that

ences between the respective pairs of limbs.

0.7 millivolt, and

0.3 millivolt on the left arm. Similarly, lead

0.5 millivolt, because this is the

meters in the figure, it can be seen that lead I records

the body, the left arm is

noted in Figure 11–6, the right arm is

For instance, let us assume that momentarily, as

given instant, the third one can be determined

Einthoven’s law states that if the

s Law.

the point at which the left leg connects with the fluids.

with the fluids around the heart, and the lower apex is

form apices of a triangle surrounding the heart. The

heart. This illustrates that the two arms and the left leg

Einthoven’s triangle,

In Figure 11–6, the triangle, called

Einthoven’s Triangle.

arm is negative with respect to the left leg.

This means that

positive terminal to the left leg.

of the electrocardiograph is connected to the left arm

To record limb lead III, the

electrocardiograph records positively.

right arm is negative with respect to the left leg, the

Therefore, when the

positive terminal to the left leg.

the electrocardiograph is connected to the right arm

To record limb lead II, the

trocardiograph records below the line.

electrocardiogram. When the opposite is true, the elec-

positively, that is, above the zero voltage line in the

the left arm connects, the electrocardiograph records

Therefore,

of the electrocardiograph is connected to the right arm

In recording limb lead I, the

with a moving paper.

trical meter in the diagram, although the actual

the body and the electrocardiograph. The electrocar-

the limbs. Thus, a “lead” is not a single wire connect-

located on different sides of the heart, in this case, on

128

Unit III

The Heart

ing from the body but a combination of two wires and
their electrodes to make a complete circuit between

diograph in each instance is represented by an elec-

electrocardiograph is a high-speed recording meter

Lead I.

negative terminal

and the positive terminal to the left arm.
when the point where the right arm connects to the
chest is electronegative with respect to the point where

Lead II.

negative terminal of

and

the

Lead III.

negative terminal

and the
the electrocardiograph records positively when the left

is drawn around the area of the

two apices at the upper part of the triangle represent
the points at which the two arms connect electrically

Einthoven’

electrical potentials of any two of the three bipolar
limb electrocardiographic leads are known at any

mathematically by simply summing the first two (but
note that the positive and negative signs of the
different leads must be observed when making this
summation).

-0.2 millivolt

(negative) with respect to the average potential in

+ 0.3 millivolt (positive), and

the left leg is

+1.0 millivolt (positive). Observing the

a positive potential of

difference between the

-0.2 millivolt on the right arm

and the

III records a positive potential of

lead II records a positive potential of

+1.2 millivolts

because these are the instantaneous potential differ-

the sum of the voltages in leads I and

III equals the voltage in lead II;

being recorded.

Normal Electrocardiograms Recorded from the Three Standard
Bipolar Limb Leads.

that the electrocardiograms in these three leads are
similar to one another because they all record positive

of the QRS complex is also positive in each
electrocardiogram.

sum of the potentials in leads I and III equals the

Because the recordings from all the bipolar limb

greatly which lead is recorded when one wants to

nosis of arrhythmias depends mainly on the time
relations between the different waves of the cardiac

the ventricular or atrial muscle or in the Purkinje

muscle contraction or

III

electrocardiographic leads.

standard

Normal electrocardiograms recorded from the three

Figure 11–7

In this type of recording, two of

leads is nearer the heart apex, which is the direction

, and V

versely, the QRS complexes in leads V

most of the ventricular depolarization process. Con-

to the base of the heart than to the apex, and the base

Figure 11–8, the chest electrode in these leads is nearer

normal heart are mainly negative because, as shown in

, the QRS recordings of the

In leads V

vidual chest leads.

the anterior ventricular wall, can cause marked

minute abnormalities in the ventricles, particularly in

diately beneath the electrode. Therefore, relatively

to the chest wall, each chest lead records mainly the

chest leads. Because the heart surfaces are close

Figure 11–9 illustrates the electrocardiograms of the

, and V

are known as leads V

points shown in the diagram. The different recordings

recorded, one at a time, from the anterior chest wall,

in the figure. Usually six standard chest leads are

left arm, and left leg all at the same time, as also shown

indifferent electrode,

electrode, called the

terminal of the electrocardiograph, and the negative

Figure 11–8. This electrode is connected to the positive

markedly in some leads yet may not affect other leads.

The Normal Electrocardiogram

Chapter 11

129

change the patterns of the electrocardiograms

Electrocardiographic interpretation of these two
types of conditions—cardiac myopathies and cardiac
arrhythmias—is discussed separately in Chapters 12
and 13.

Chest Leads (Precordial Leads)

Often electrocardiograms are recorded with one elec-
trode placed on the anterior surface of the chest
directly over the heart at one of the points shown in

is connected

through equal electrical resistances to the right arm,

the chest electrode being placed sequentially at the six

, V

healthy heart as recorded from these six standard

electrical potential of the cardiac musculature imme-

changes in the electrocardiograms recorded from indi-

and V

of the heart is the direction of electronegativity during

, V

are

mainly positive because the chest electrode in these

of electropositivity during most of depolarization.

Augmented Unipolar Limb Leads

Another system of leads in wide use is the augmented
unipolar limb lead.
the limbs are connected through electrical resistances
to the negative terminal of the electrocardiograph,

1 2

3 456

5000

ohms

5000

ohms

5000
ohms

. LA, left arm; RA, right arm.

Connections of the body with the electrocardiograph for record-

Figure 11–8

ing chest leads

Normal electrocardiograms recorded from the six standard chest

Figure 11–9

leads.

aVR

aVL

aVF

Normal electrocardiograms recorded from the three

Figure 11–10

augmented

unipolar limb leads.

the electrocardiograph to determine this.)

recording from the aVR lead is inverted. (Why does

to the standard limb lead recordings, except that the

leads are shown in Figure 11–10. They are all similar

arm, the aVL lead; and when on the left leg, the aVF

the lead is known as the aVR lead; when on the left

nal. When the positive terminal is on the right arm,

130

Unit III

The Heart

and the third limb is connected to the positive termi-

lead.

Normal recordings of the augmented unipolar limb

this inversion occur? Study the polarity connections to

References

See references for Chapter 13.