Ganong's Review of Medical Physiology, 23rd Edition

494
SECTION VI
Cardiovascular Physiology

indifferent electrode, are commonly used in clinical electro-
cardiography. There are six unipolar chest leads (precordial
leads) designated V
1
–V
6
(Figure 30–6) and three unipolar
limb leads: VR (right arm), VL (left arm), and VF (left foot).
Augmented limb leads,
designated by the letter
a
(aVR, aVL,
aVF), are generally used. The augmented limb leads are re-
cordings between one limb and the other two limbs. This in-
creases the size of the potentials by 50% without any change in
configuration from the nonaugmented record.
Unipolar leads can also be placed at the tips of catheters and
inserted into the esophagus or heart.

NORMAL ECG

The ECG of a normal individual is shown in Figure 30–7. The
sequence in which the parts of the heart are depolarized (Figure

30–4) and the position of the heart relative to the electrodes are

the important considerations in interpreting the configurations

of the waves in each lead. The atria are located posteriorly in the

chest. The ventricles form the base and anterior surface of the

heart, and the right ventricle is anterolateral to the left. Thus,

aVR “looks at” the cavities of the ventricles. Atrial depolariza-

tion, ventricular depolarization, and ventricular repolarization

move away from the exploring electrode, and the P wave, QRS

complex, and T wave are therefore all negative (downward) de-

flections; aVL and aVF look at the ventricles, and the deflections

are therefore predominantly positive or biphasic. There is no Q

wave in V

1

and V

2

, and the initial portion of the QRS complex

is a small upward deflection because ventricular depolarization

first moves across the midportion of the septum from left to

right toward the exploring electrode. The wave of excitation

then moves down the septum and into the left ventricle away

from the electrode, producing a large S wave. Finally, it moves

back along the ventricular wall toward the electrode, producing

the return to the isoelectric line. Conversely, in the left ventric-

ular leads (V

4

–V

6

) there may be an initial small Q wave (left to

right septal depolarization), and there is a large R wave (septal

and left ventricular depolarization) followed in V

4

and V

5

by a

moderate S wave (late depolarization of the ventricular walls

moving back toward the AV junction).

There is considerable variation in the position of the nor-

mal heart, and the position affects the configuration of the

electrocardiographic complexes in the various leads.

BIPOLAR LIMB LEADS &

THE CARDIAC VECTOR

Because the standard limb leads are records of the potential dif-

ferences between two points, the deflection in each lead at any

instant indicates the magnitude and direction in the axis of the

lead of the electromotive force generated in the heart

(cardiac

FIGURE 30–5
Waves of the ECG.

TABLE 30–2
ECG intervals.

Normal Durations

Intervals Average Range

Events in the Heart

during Interval

PR interval

a

0.18

b

0.12–0.20 Atrial depolarization and

conduction through AV

node

QRS duration 0.08 to 0.10 Ventricular depolarization

and atrial repolarization

QT interval 0.40 to 0.43 Ventricular depolariza-

tion plus ventricular re-

polarization

ST interval

(QT minus QRS)

0.32... Ventricular repolariza-

tion (during T wave)

a
Measured from the beginning of the P wave to the beginning of the QRS complex.
b
Shortens as heart rate increases from average of 0.18 s at a rate of 70 beats/min to
0.14 s at a rate of 130 beats/min.

1.0

0.5

−0.5

0

0 0.2 0.4 0.6

P

R

S

Q

T

U

PR segment

PR interval

QT interval

ST segment

QRS duration

mV

Isoelectric

line

Time (s)

FIGURE 30–6

Unipolar electrocardiographic leads.

aVR

aVF

aVL

V 1 V 2

V 3

V 4 V 5 V^6