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Electric Currents Through the Human Body 79

FIGURE 5.8 Normal heart potentials as shown by an electrocardiogram.

correspondstonearly10%ofthewholecardiaccycle,themyocardium,
no longer driven by the SA node, is very vulnerable to foreign stimuli.
Any external current of sufficient magnitude applied to the heart in
the vulnerable interval will trigger the ventricular fibrillation.
The determination of the component of the actual current flow-
ing through the heart upon contact with a live part, with respect
to the total current flowing through the body, is extremely diffi-
cult. This component is the true culprit of the ventricular fibrillation
and depends on both the individual and the actual current path-
way through the human body. For this reason, we conventionally
use the total body current to identify the threshold of ventricular fib-
rillation, defined as the minimum value of body current that causes
ventricular fibrillation. This is a conservative definition as the cur-
rent through the heart is generally less than 10% of the body current
(Fig. 5.9).
For shock durations below 0.1 s, fibrillation may be triggered by
current in excess of 500 mA. For longer exposure, lower current inten-
sities will elicit fibrillation.

5.4.3 Thermal Shock
The circulation of electric current I through tissues, for a time t,
generates heat due to the Joule effect, and, thus, possible burns. 8
Let us consider a sample of tissue of length l, cross-sectional area
S, and resistivity . In addition, let us conservatively assume that
there is no thermal exchange between the body and the surrounding
environment (i.e., adiabatic process). In this case, all the heat devel-
oped by the current is absorbed in the tissues, whose temperature
rises.
This adiabatic process is described by the thermal balance of
Eq. (5.1):
l 2
I t = Slc (5.1)
S