Although we limit,
always, teaching the ohm law, we noted for a very
long time that resistance R was variable. We timidly
introduced the concept of impedance (Z).
The impedance of
the human body decreases when the frequency
increases. This paramount concept, is alas
considered, by simplification, like only one entity.
We however know that we are in presence of strongly
heterogeneous mediums. We know, in the same way,
there are nonlocal factors which modify our
perception of current. Not to take account of these
subsets entail a bad comprehension and an erroneous
analysis of the phenomena observed.
Moreover, since
several decade, clash two groups as for method of
application of currents, taking into account this
variation of impedance. Some recommend to work with
generators called *constant voltage* and the others,
with apparatuses delivering *constant current*.
On paper the
designs are defended, namely :
The generator with
constant tension seeks to maintain constant delivered
tension, it is the current which is variable
according to Z. That wants to say that if Z
decreases, the current increases and if, on the
contrary, Z increases, the current crossing the
stimulated zone decreases. The major risk is a brutal
reduction of Z (ex; cutaneous problem) : Very
unpleasant feelings and skin burns. A minor risk is
the absence of effectiveness by increase of Z (ex;
electrodes falling apart).
The generator with
constant current tries to maintain constant the
delivered intensity, it is the tension which becomes
variable according to Z. If Z decreases, the terminal
voltage of the stimulated zone decreases and if Z
increases, the tension increases. The major risks are
an increase or a fast reduction of Z (ex; electrodes
falling apart, cutaneous problem) : Very
unpleasant feelings and skin burns.
The only
conclusion, there are two imperfect methods of
application. However, the patients prefer, to choose,
the constant tension. The experts, in their majority,
use apparatuses with constant current. But, we are, of course,
still listening to our patients !
