Biomedical Engineering Reference
In-Depth Information
cross sectional area, so most of the resistance is in
the proximal zone in the stratum corneum. The current
density is probably far out in the non-linear breakdown
region of the skin, but because of the short pulse
duration the impedance is presumably also determined
by the capacitive properties of stratum corneum.
A rough calculation with s
¼
0.1 S/m and calculated
resistance from measured current maximum: 20 k
U
,
gives
for practical reasons, safe and hazard levels are more
often quoted as current, energy or quantity of current in
the external circuit, and not current density in the tissue
concerned.
Macro/microshock
A
macroshock
situation is when current is applied to
tissue far from the organ of interest, usually the heart.
The current is then spread out more or less uniformly,
and rather large currents are needed in the external cir-
cuit (usually quoted
>
50 mA @ 50/60 Hz) in order to
attain dangerous levels (
Fig. 4.1-30
).
The heart and the brain stem are particularly sensitive
for small areas of high current density. Small area con-
tacts occur, for example, with pacemaker electrodes,
catheter electrodes and current carrying fluid-filled car-
diac catheters. Small area contact implies a monopolar
system with possible high local current densities at
low
current levels
in the external circuit. This is called a
microshock
situation. The internationally accepted
50/60 Hz safety current limit for an applied part to the
heart is therefore l0
m
A in normal mode, and 50
m
A under
single fault condition (e.g. if the patient comes into
contact with the mains voltage because of defective
insulation). The macro- and microshock safety current
levels differ by more than three orders of magnitude.
The heart is most vulnerable for an electric shock in the
repolarization interval, that is in the T-wave of the ECG
waveform. Therefore the probability of current passage
during the approximate 100 ms duration of the T-wave is
important. If the current lasts more than one heart cycle,
the T-wave is certainly touched. For short current dura-
tions
<
1 seconds, the risk of heart stop is determined by
the chance of coincidence with the T-wave.
the
arc
contact diameter with the
skin:
2
a ¼
200
m
m.
The charge transferred around threshold level is of the
order of 0.2 C. The threshold of perception as a function
of stored energy is about 10
m
J. The formation of an arc in
the air between the conductor and the skin is possible
when the voltage difference is larger than about 400 V.
The arc discharge can be heard as a click and felt as
a prick in the skin.
4.1.17.2 Electrical hazards
Electromagnetic field effects
Coupling without galvanic tissue contact and electro-
magnetic hazards are outside the scope of this chapter.
There is a vast amount of experimental data on this
subject, and the interested reader is recommended the
CRC handbook (Polk and Postow, 1986).
Continuous current
The risk of sudden death is related to stimulating the
cells of three vital organs of the body: the heart, the lungs
and the brain stem. Involuntary movements may in-
directly lead to sudden deaths (loss of balance, falling).
Heat and electrochemical effects may also be fatal by
inducing injuries that develop during hours and days after
the injury. In electrical injuries the question often arises
as to whether the current is evenly distributed in the
tissue, or follows certain high-conductance paths. Cur-
rent marks and tissue destruction often reveal an uneven
current distribution (cf. Ugland, 1967).
The current path is important, and organs without
current flow are only indirectly affected: to be directly
dangerous for the healthy heart, the current must pass
the heart region.
Cell, nerve and muscle excitation
Heat effects are certainly related to current density in
volume conductors, but this is not necessarily so for
nerve and muscle excitation. Excitation under a plate
electrode on the skin is more highly correlated to current
than current density (cf. Section 4.1.17.1). The stimulus
summation in the nerve system may reduce the current
density dependence if the same current is spread out
over a larger volume of the same organ. Therefore, and
Figure 4.1-30 Macroshock (left) and microshock (right) situations.
