Electrical Property of Acupoints

Technical Issues

Acupoints are reportedly 1 to 2 mm in diameter ^2,3 and located intramuscularly. ⁴ In the study, Ngai and Jones used electrocardiogram (ECG) electrodes (Red Dot, 3M, St. Paul, MN) for the skin impedance measurement. These electrodes are circular or rectangular in shape and made of a metal disc with a10 mm diameter and a gelled sponge or pad with a 15 mm diameter (Fig. 1, upper right). The size of the gelled sponge or pad is noticeably larger than that of the acupoint (15 mm² vs. 1 to 2 mm²), and the pad apparently covers untargeted tissue, that is, skin adjacent to and encircling the acupoint (see Fig. 1). Thus, the measurement setting is disposed to an unwanted signal-to-noise ratio, when noise is defined as the skin impedance of nonacupoint skin that is not part of the planned measurement.

OPEN IN VIEWER

FIG. 1.

Mismatch of electrocardiogram (ECG) electrodes and acupoints. The ECG electrodes are larger than, and distant from, the acupoints. The current density is focused around the electrode periphery, and the current runs above the acupoints.

Furthermore, the skin impedance measurement was done by applying a current to the skin and quantifying the skin's ability to conduct electricity. However, because the ECG electrodes are placed on the top of skin, the current flows superficially between the electrodes, and the greatest current density is found around the electrode periphery ⁵ (see Fig. 1). Consequently, surface electrodes placed over purported acupoints are likely to lead to much of the current bypassing the area of interest. In other words, the skin impedance reading is primarily from the nonacupoint skin, while the reading from the acupoint is negligible or even zero.

TENS and Cutaneous Blood Flow

It has been well documented that TENS can increase parasympathetic activation, ⁶ heart rate variability, ⁷ peripheral blood flow, ^8,9 and skin temperature, ^10,11 which indicates the vasodilation of superficial blood vessels in the dermis. The vessels, which include capillaries, arterioles, and venules, have thin, elastic walls. The blood moving inside them is electrically conductive. ¹² Under TENS, the vessels would dilate and further lower skin's electrical resistance. ^{8 –11} Thus, the observation by Ngai and Jones of a drop in acupoint skin impedance after 45-minutes of TENS is likely due to the dilated superficial vessels rather than the altered electrical property of the acupoints, which can be 10 mm below the top of skin. ¹³

Nonacupoint Skin Untested

The fact that the authors did not measure nonacupoint skin impedance at all appears to indicate an overgeneralization of the effect of TENS on acupoints in the study. Without a measure of nonacupoint skin impedance, it is open to doubt whether the alteration of skin impedance is detectable only over the supposed acupoints or may be detectable globally over the participants' skin.

Summary

Based on the preceding discussion, it is likely that the lower arm skin impedance after 45 minutes of TENS that was noted by Ngai and Jones is due to dilated cutaneous blood vessels, which act as pipelike electrical conductors, rather than changes in the electrical property of acupoints. The evidence of TENS specifically changing the electrical property of acupoints is not demonstrable.

Disclosure Statement

No competing financial interests exist.