Electrical Skin Resistance

at the measuring probe can be disregarded. The current flows from the large to the small electrode, passing through a number of biological tissues and two skin/electrode interfaces. The usual approximation is to consider the electrical skin resistance (ESR) as V/I where V is known because it is preset by the generator, while I is measured.

The measured value of ESR includes contributions from several elements, but the most determining arises from the stratum corneum of the skin. On the derma lies a complex of tissues which constitute the epidermis, whose thickness is on average about 50-150 mm but can be as high as 1500 mm on the plantar aspect of the foot.7 It is made up of several superimposed specialized cell strata, of which the external is the corneum, consisting of regular rows of dead flat cells filled with keratin, a fibrous protein.

This stratum corneum continuously loses dead cells at its surface, which are replaced by new cells originating from the lower stratum. Its thickness (both absolute and as a fraction of the epidermis) depends on location. For instance, Sandby-M0ller et al8 found a thickness from 11 mm on the shoulder to 18.3 mm at the dorsal aspect of the forearm, while Jacobi and Kaiser9 give 17 mm to 28 mm for the stratum corneum for porcine ear skin which they judge to give similar results in humans. The stratum corneum is characterized by very low water permeability, low ion mobility and high electrical resistance. Its interest here lies in the fact that, from experiments where the stratum corneum is removed and the decrease of resistance monitored, it is evident that skin resistance is produced mainly in the stratum corneum.

There are several well documented methods of skin ablation. The commonest, probably because it does not require any special equipment, consists of stripping the dead cells by successive applications of adhesive tape. Examples of this technique are described in Yamamoto and Yamamoto,10 Kalia and Guy,11 Kalia et al12 and Bashir et al.13 Another method consists of laser ablation,14 while the most recent method uses a flow of abrasive particles, as described in Gill et al.15

The removal of the stratum corneum is commonly used in esthetic applications for skin remodeling, and in methods of preparing the skin for drug conveyance, such as iontophoresis. They interest us only for the electrical measurements taken to correlate quantity and thickness of removed material and skin resistance.

Tape stripping seems to increase ion mobility by two orders of magnitude (that is a hundredfold increase).12 This correlates very well with the findings on microdermabrasion, which decreases resistance from thousands of kfi to tens of kfi.15 Gill and colleagues15 give a value of about 20-80 kfi for skin resistance after complete stratum removal.

To give an order of magnitude for skin resistance, in an earlier work Inada et al16 had given 12-120 kfi/cm2, while Kalia and Guy11 found a value of 187 kfi for the real part of the complex skin impedance in hydrated skin with intact stratum corneum at 1 Hz, with electrodes of 3.14 cm2. Oleson et al17 give, instead of resistance, the amount of current flowing through the skin under a voltage of 9 V. They found spots of low resistance allowing the passage of 300 mA, but unfortunately they do not specify the internal electrical resistance of the device used.

In fact, the very strong influence of the stratum corneum had been noted extremely early in articles seeking to locate acupuncture points by electrical methods. McCarrol and Rowley18 deliberately damaged the stratum corneum by applying a very high pressure (2 kg/mm2) at the tip of the searching electrode, observing a dramatic drop in skin impedance, which, in one case, amounted to 94% of the impedance measured previously using a tip loaded with 0.1kg/mm2. Their experiment was conducted at 1000 Hz, but no details of the tip's dimension are given. Their conclusion was that when trying to locate an acupuncture point by measuring skin conductance with a small electrode, the chances are that we are simply measuring the integrity of the stratum corneum. Therefore, we can suspect that any macroscopic variations are probably due to accidental stratum corneum abrasions.

As an alternative assumption, acupuncture points may be associated with areas of thinner or more hydrated stratum corneum.

This takes us to the definition of a 'gold standard' for an 'acupuncture point', or at least to some kind of specific histological findings for acupuncture loci.

There is almost nothing in the literature on this subject. A valuable contribution is the experiment by Terral and Rabischong.19 They made histologi-cal sections of the skin of rabbits on the spots where they had located acupoints and found that at locations with lower electrical resistance the connective tissue was more lax, and contained a number of nerve fibres and other structures. It is, however, to be noted that rabbits had been carefully shaved, therefore it is not possible to know how the stratum corneum had been altered.

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Acupuncture For Cynics

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