Galvanic Current
Direct current (galvanic current) represents a constant electron
flow from the negative electrode to the positive one, with no oscillations
or alternations. Direct current (straight-line) has only one use for
iontophoresis but therapeutically it is not used today.
During galvanic application, the area of tissue under the cathode
will show alkaline reaction (strong), while that under the anode shows
acidic reaction (weak). There is danger of chemical burn under the area of
cathode due to capillary hyperemia, which leads to marked red coloration
of the skin. The entire region between the electrodes is involved
according to the conductivity of the tissue but the most marked
circulatory changes take place in the skin and superficial tissues; just
under the electrodes.
Physicochemical and physiological effects:
Although the direct current is the simplest current so far as physics
is concerned, the explanation of its mode of action upon the body is
somewhat complicated. The human body may be considered from the
viewpoint of electrotherapy as a bag of skin, holding a solution of
common salt; “sodium chloride”. When the molecules of sodium
chloride (NaCl) dissolve in water, they dissociate into sodium ions (Na+),
bearing a positive charge and chlorine ions (Cl-), bearing a negative
charge. The flow of a direct current through the salt solution causes these
ions to move in a definite direction. The sodium ions (+) migrate toward
the negative pole (cathode) and the chloride ions (-) toward the positive
pole (anode); this process is known as “ion transfer” (iontophoresis).
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When the positively-charged sodium ions arrive at the negative
pole and the negatively-charged chlorine ions arrive at the positive pole,
they lose their charges and become free atoms. This in turn causes a
secondary chemical reaction: the formation of sodium hydroxide (NaOH)
and liberation of hydrogen (H) at the negative pole.
The principal physical effect of ion transfer by the direct current is
an acid reaction (hydrochloric acid; HCl) at the positive pole and an
alkaline reaction (sodium hydroxide; NaOH) at the negative one; the
intensity of each reaction varies with the strength and relative density of
the current at each pole. This acid and alkaline effect may be referred to
as “polar effect” because it occurs only under the electrodes.
In addition to the polar effect caused by ion transfer, nondissociated colloid molecules (glue-like substance, diffusible but not
soluble in water and cannot pass through the animal membrane) such as
droplets of fat, albumin, particles of starch, blood cells, bacteria and other
single cells have another role. All these colloids have an electrical charge
due to the adsorption of ions; also move under the influence of a direct
current toward the negative pole. This process is known as
“cataphoresis”.
Finally, a shifting of the water content of the tissues occurs through
membrane structures, with an electrical charge known as “electroosmosis” (passage of water across the membrane due to osmotic
pressure). All this makes it evident that the passage of the direct current,
across or around cell membranes of variable permeability and along
fluids of different conductivity and ionic composition, presents a
complicated bio-physical process. There is also of a minimal “thermal
effect”.
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The most important physico-chemical effect of a direct current is
the fact that when it passes through a chemical solution, it causes a
migration of its component ions in a definite direction. “Those with a
positive charge are attracted to the negative pole and those with a
negative charge are attracted to the positive pole”. As a large number of
drugs are soluble in water in an ionic state; one can introduce the ions of
such drugs into the skin or mucous membranes by the polarity effect of a
direct current. All what is needed is to saturate a pad electrode with the
solution, connect it to the proper pole and start the current flowing. This
is known as “ionic medication”, “iontophoresis” or “medicinal ion
transfer”.
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