Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 30

Origin of the Heartbeat & the Electrical Activity of the Heart 495

vector
or
axis
). The vector at any given moment in the two di-
mensions of the frontal plane can be calculated from any two
standard limb leads (Figure 30–8) if it is assumed that the three
electrode locations form the points of an equilateral triangle
(Einthoven’s triangle) and that the heart lies in the center of the
triangle. These assumptions are not completely warranted, but
calculated vectors are useful approximations. An approximate
mean QRS vector
(“electrical axis of the heart”) is often plotted
by using the average QRS deflection in each lead, as shown in
Figure 30–8. This is a
mean
vector as opposed to an
instanta-
neous
vector, and the average QRS deflections should be mea-
sured by integrating the QRS complexes. However, they can be
approximated by measuring the net differences between the
positive and negative peaks of the QRS. The normal direction

of the mean QRS vector is generally said to be –30 to +110 de-

grees on the coordinate system shown in Figure 30–8.

Left

or

right axis deviation

is said to be present if the calculated axis

falls to the left of –30 degrees or to the right of +110 degrees, re-

spectively. Right axis deviation suggests right ventricular hy-

pertrophy, and left axis deviation may be due to left ventricular

hypertrophy, but there are better and more reliable electrocar-

diographic criteria for ventricular hypertrophy.

VECTORCARDIOGRAPHY

If the tops of the arrows representing all the instantaneous car-

diac vectors in the frontal plane during the cardiac cycle are

connected, from first to last, the line connecting them forms a

FIGURE 30–7
Normal ECG.
(Reproduced with permission from Goldman MJ:
Principles of Clinical Electrocardiography,
12th ed. Originally published by Appleton &
Lange. Copyright © 1986 by McGraw-Hill.)

I

aVR aVL

II III

aVF

V 1 V 2 V 3 V 5 V 6

V (^1) V 2
V 3
V 4
V 5
V 6
V 4