Filadelfio Puglisi

Since all of the clinical evaluations in this book were obtained using a commercial device, it is necessary to ascertain its consistency with the one specially built to take the measurements described in Chapter 6: Agiscop D, made by Sedatelec, France (see www.sedatelec.fr) does not use the single active electrode technique, but a double electrode.

The Agiscop is a small, portable, battery-operated apparatus, with a hand reference electrode held by the patient, while the operator holds a probe made of two concentric electrodes. The center one is a metal rod with a smoothed point 1 mm in diameter, surrounded by a coaxial electrode made from a metal tube with an external diameter of 3 mm (Fig. A2.1). Both electrodes are covered with an insulated coating except at their tip. The force to

Fig. A2.1 Agiscop's central and coaxial electrodes.

be exerted when pressing the probe in Agiscop D varies from about 50 g (both electrodes securely in contact with the skin) to about 80 g (end of mechanical range for the springs loading the electrodes).

From what can be inferred from the documentation supplied and from direct examination, the apparatus first measures the skin resistance between one of the two electrodes and a reference metal hand piece held by the patient. In a second step, it measures the resistance between the other electrode and the reference. Then it subtracts the two values and indicates if they differ by more than a specified amount. The difference is set by the operator within a continuous range conventionally indicated by the numbers 0 to 6 and this amount diminishes when going from 0 to 6.

With a second knob the operator chooses whether to be informed of one of two possibilities:

1. The coaxial electrode has a lower resistance than the central electrode (knob set as —).

2. The coaxial electrode has a higher resistance than the central electrode (knob set as +).

In either case a beep is heard and an LED flashes if the difference is greater than the set amount. Nothing happens if the difference is smaller.

The amounts corresponding to the different knob positions were measured using resistors of known value and modeling the double contact with the circuit shown in Figure A2.2. When the sensitivity knob is set at '1', the amount of difference is about 20 MO, diminishing in an exponential fashion as shown in Figure A2.3. The values of resistance shown in this figure are only approximated, probably around 10% more or less, since

Coaxial electrode

Center electrode

Coaxial electrode

Center electrode

^Threshold set to '4'

^Threshold set to '4'

Hand-held reference electrode

Fig. A2.2 Experimental setting for threshold measuring. The knob on the left regulates the threshold, the knob on the right decides the — or + modality.

Knob setting

AR

1

20 MQ

2

5 MQ

3

2 MQ

4

1 MQ

5

500 KQ

6

Knob setting

Fig. A2.3 Approximated values of resistance difference for knob settings 1 to 6 when light and sound switch on.

the knob is small and there is a dead range at the beginning for switching the device 'on'.

Almost all the measurements in this book have been taken with Agiscop's sensitivity knob set at '4' since this seemed a good compromise between an almost absolute absence of signal points (sensitivity knob set to '1'), and practically all touches signaled by the device (knob to '6').

It is consistently observed that when Agiscop's probe explores ear skin, — points are always much more numerous than + points, usually in the ratio of about 2 to 1. According to the explanation given above, this would mean that the resistance under the coaxial electrode is more often below the resistance under the central electrode than the reverse. This is difficult to explain, because if at a given spot the center electrode has a higher resistance, by moving it by about 0.5 to 1 mm the situation should reverse, since now the center has moved onto the lower resistance region while the coaxial electrode is now on an area that includes the former high resistance spot. But this assumption is not often confirmed by practice.

Probably at least part of the behavior of Agiscop can be explained by observing Figure A2.4. It is possible that the indentation of the skin under the coaxial electrode is larger and more skin area is in contact with the coaxial rather than the central electrode. Therefore the resistance under the center one is constantly somewhat higher. This would explain why — findings with the Agiscop are so much more frequent than + findings. This problem is addressed to the maker, whereas for clinical purposes it is sufficient to know that spots signaled by Agiscop in position '4' indicate a difference in resistance of about 1 MO between two areas of skin, concentric with one another and falling within a circular surface about 3 mm in diameter.

-Coaxial electrode

Center electrode

-Skin

Fig. A2.4 Skin indentation at Agiscop's tip.

It is interesting to note that the value of position '4' (decided empirically by Dr Romoli) is not very far from the threshold (2 MO) adopted in Chapter 6 after examining the histogram in Figure 6.7. Part of the study presented in Chapter 6 was dedicated to evaluating whether such discontinuities revealed by Agiscop corresponded to the locations where the resistance between a single electrode on the skin surface and a large electrode hand-held by the patient was significantly lower.

As observed in Chapter 6, 118 of the 228 points tested were indicated by the Agiscop as 1 MO discontinuity points (with the Agiscop's sensitivity knob in position '4'), either of the — or of the + modality, the — points being about double the + points.

It appears therefore that if a spot on the ear skin has a local discontinuity of at least 1 Mfi, then it also has an absolute low skin resistance, as measured with a device such as the circuit in Figure A2.2. This is a remarkable finding, for which again there is no ready physiological explanation.

Summing up the results above, a minimum set of assumptions can be reasonably made on the Agiscop detected points:

l Agiscop measures discontinuities in electrical skin resistance (ESR). l The discontinuity value that is signaled by the device depends on the position of the sensitivity knob, diminishing from 0 to 6. l Most of the points detected by Agiscop, in the — or + setting, are also points with an absolute low ESR.

It is more frequent to find — points than + points, probably in a ratio of 2 to 1.

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