For many decades, practitioners have used electrical devices to locate specific points for acupuncture, both in association with, or to replace, manual methods such as the pain pressure test (PPT). The main assumption underlying such electrical devices is that an acupuncture point is characterized by its different electrical resistance (or its reciprocal, conductance) compared to that of the skin surrounding it. This statement derives mainly from the work of some researchers in the 1950s, for example Voll1 and Nakatani and Yamashita.2 They found that a lower skin resistance value often coincided with spots that traditional Chinese medicine indicated as effective for acupuncture treatment. Although Niboyet3 meticulously re-examined the method, it was only in 1975 that experiments by Reichmanis4 and colleagues truly awakened interest in the topic. This author employed a rod electrode for roughly locating higher electrical conduction points on the skin, and then a roller-type electrode for tracing conductance curves along lines passing through those points.
More than 30 years of episodic research has produced no consensus on the effectiveness of measuring electrical resistance for identifying acupuncture points. The work up to the late 1970s has been reviewed by Mannheimer and Lampe5 and, more recently, a comprehensive review was presented by Ahn and Martinsen.6
Over the last decades many devices for locating acupuncture points based on resistance measurement have become commercially available. Their very simple electronics and low cost have made
Electrical skin resistance (ESR) 164
ESR sources of variability 166
Design of an instrument for ESR 166
Clinical validation 168
Intra-rater repeatability 171 Inter-rater reliability 172
them widely accessible. However, acupuncturists rarely know more about them than that when an LED lights up or a beep is heard the machine has detected a variation in resistance. This is because very few commercial makers describe their devices or the principles behind them in depth.
It is the aim of this chapter to examine the physical principles on which such devices are based, and explain the structure of a simple machine for measuring ear skin electrical resistance, together with its clinical validation.
Is is suggested that the reader keep in mind that the acupuncturist can also derive many useful data from the work of other researchers, since electrical resistance of the skin is important in several fields of application. Broadly speaking these applications are of two different types. In the first, an endogenous generator establishes a flow of current through the body. This flow of current originates an electric potential that can be measured on the skin. In the second, an electrical generator outside the body applies a potential to two points of the skin and establishes current inside the body. Elec-tromyography, electrocardiography and electro-encephalography belong to the first category, and to the second impedance tomography, body composition scanners, psychogalvanic reflex, acupuncture point location and percutaneous transport in iontophoresis.
Also relevant are all electrostimulation techniques in which a current is applied for therapeutic reasons, such as transcutaneous nerve stimulation, therapeutical nerve stimulation and functional electrostimulation. Moreover, dermatological applications such as cosmetology, wound healing and percutaneous gas monitoring all have some interest in assessing the physical properties of the epidermis.
Research in the above fields has focused on some aspects of skin resistance and its controlling parameters, and is therefore worth studying for getting different angles of view on the subject.
Electrically speaking, the resistance R denotes the ratio of the voltage V applied to a conductor to the current I flowing in it: R = V/I.
When taking measurements, it must be kept in mind that resistance may be a non-linear parameter which depends on the value of applied voltage
(i.e. doubling the voltage, the current does not double) and on the type of tissue. If the voltage varies with time, as in the case of the commonly used alternating current, the parameter R is substituted by a complex parameter termed 'impedance' (which is a function of frequency). The situation is often confusing when trying to compare literature data, some obtained with constant, some with alternating currents of different frequencies. But even restricting the definition to the case of a constant voltage, when measuring skin resistance the current flows through a complex path, and which resistance is measured is not so clear.
Normal commercial devices for locating low skin resistance spots have a large common 'reference' or 'indifferent' electrode held by the subject in one hand, generally a cylinder of 2-3 cm diameter or a similar contrivance (Fig. 6.1).
A 'probe' consisting of a narrow electrode is put into contact with the skin at the location to be investigated, usually with the help of a small spring to keep the contact pressure within a known range. The main prerequisite is for the area of skin contact at the reference electrode to be much larger than that at the measuring probe, so that the resistance at the reference electrode compared with that
Fig. 6.1 Path followed by the electrical current.
Fig. 6.1 Path followed by the electrical current.
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