Clinical applications of bioelectricity - Biomedical Engineering Desk Reference - page 380

electrode contact the constrictional zone resistance will
dominate. The segmental resistance may be estimated
from the equation R sr ¼ L/ s A . With constant s the seg-
mental resistance depends on the ratio L/A , and ac-
cordingly varies according to body or limb size.

main stroke there are continuing currents of typically
100A and 200ms duration.
The mechanical hazards are due to the pressure rise
in the lightning channel. The energy per meter is
equivalent to about 22 g of TNT per meter. A direct hit
from the main stroke is usually lethal, but often the
current path is via a tree, the ground (current path
from foot to foot, current through tissue determined by
the step voltage ), or from a part of the house or
building. The current path is of vital importance, for
humans a current from foot to foot does not pass vital
organs, for a cow it may do.
It is believed that there are around 500 deaths caused
by lightning per year in the USA.

Table of threshold values
From the threshold current levels, the corresponding
voltages are found by estimating the minimum current
limiting resistance. These worst case minima are found
by assuming no protective action from the skin at all, only
from the volume resistance of the living parts of the body.
The levels are summarized in Table 4.1-2 .
The question of current canalizing effects in tissue is
one of the issues of our field. It is well known that the
myelin sheet around the nerves serves electrical insu-
lation and current canalization to the Ranvier nodes. But
the extent of current canalization in many parts of the
body is largely unknown. The blood has a high conduc-
tivity, but what are the electrical properties of the en-
dothelium? Does electrosurgery current follow the bile
duct? The current through skin is canalized through the
sweat ducts. The high acid concentration in the stomach
must have high electrical conductivity.

Electrocution
The current path in an electric chair is from a scalp
electrode to a calf electrode. The current is therefore
passing the brain and the brain stem, the lung and the
heart. It is believed that the person gets unconscious
immediately after current onset, but it is well known that
death is not immediate. The electric chair was used for
the first time in 1890, and the first jolt was with 1400 V
60 Hz applied for 17 seconds, which proved insufficient.
At present a voltage of about 2000 V applied for 30
seconds is common, followed by a lower voltage, for
example, a minute. The initial 60 Hz AC current is about
5 A, and the total circuit resistance is therefore around
400 U . The power is around l0 kW and the temperature
rise in the body, particularly in the regions of highest
current densities in the head, neck and leg region, must
be substantial. Because of the cranium the current dis-
tribution in the head may be very non-uniform. Tem-
perature rise is proportional to time and the square of
current density, so there are probably local high tem-
perature zones in the head.
The scalp electrode is a concave metal device with
a diameter about 7 cm and an area of about 30 cm 2 .
A sponge soaked with saline is used as contact medium.

Single pulses
Trigger and safety levels have been examined by Zoll and
Linenthal (1964) for external pacing of the heart. For
TENS single monophasic pulses of duration < 1 ms, FDA
has set up the following values for the electric charge
through the thorax ( Table 4.1-3 ).

4.1.17.3 Lightning and electrocution

Lightning
An average lightning stroke may have a rise time of 3 m s
and duration of 30 m s, energy dissipated 10 5 J/m, length
3 km, peak current 50 000 A, power 10 13 W. After the

4.1.17.4 Electric fence

The electric fence is used to control animals and live-
stock. There are two types of controllers: one type de-
livers a continuous controlled AC current of about 5 mA.
The other delivers a capacitive discharge, like the
working principle of a deflbrillator. The repetitive fre-
quency is around 1 Hz, and the capacitor is charged to
a DC voltage up to 10 kV. The large voltage secures that
the shock will pass the animals' hair-covered skin. The
shock is similar to an electrostatic discharge, even if the
electric fence shock energy is higher and the duration
longer. The capacitor is charged to an energy typically in
the range 0.25-10 J.

Table 4.1-3 Electric charge values (microcoulomb) for single
monophasic pulses through the chest

m C

TENS threshold 3

TENS, max 7

Safe level 20

Hazard threshold 75

Heart pacing 100