Vol. 17, #2. January 2006

Physiologic Variability Of Electrical Skin Resistance Measurements At The Ting Acupuncture Points
Agatha P. Colbert, MD
Meg Hayes, MD
Mikel Aickin, PhD
Richard Hammerschlag, PhD

ABSTRACT
Background No previous reports have assessed the 24-hour physiologic variability of electrical skin resistance measurements (ESRMs) at acupuncture points.
Objectives To evaluate within- and between-subject variability of ESRMs at the 24 Ting points; to identify any inherent rhythmic patterns in these measurements; and to test the hypothesis that peak and trough activity of the 12 Meridians manifest in ESRMs at the relevant traditional Chinese clock times.
Design, Setting, and Participants Twenty healthy subjects, aged 20-35 years, participated in an analytical study at the General Clinical Research Center inpatient unit of the Oregon Health and Science University (OHSU) between October 2004 and January 2005. Participants were admitted in groups of 1-3 for a 24-hour period.
Main Outcome Measure Within- and between-subject variability of ESRMs at the 24 Ting points.
Results Variability in ESRMs between participants was significant at the 1% level (range: 11 kW to 59,963 kW). High ESRMs correlated with low activity counts (surrogate for sleep) and low ESRMs with high activity counts. Hypothesis testing for evidence of the Chinese clock entailed computing P values for peak and trough periods for each person separately; 11% of these P values (6% more than expected by chance) were statistically significant.
Conclusions This first summary of evidence on the variability of ESRMs at the Ting acupuncture points suggests that ESRMs behave similarly to basal skin resistance, a parameter of the galvanic skin response. Values increase dramatically during sleep and decrease on awakening. The most repeatable ESRM readings were recorded when individuals were fully alert. ESRM evidence for the classic 2-hour energy ebb-and-flow described by the Chinese clock was minimal in this study.
KEY WORDS
Electrical Skin Resistance, Skin Conductance, Electrodermal Activity, Galvanic Skin Response, Acupuncture Points, Physiologic Variability,
Chinese Clock

INTRODUCTION
Electrodermal screening at acupuncture points is a technique used by practitioners worldwide to diagnose energetic imbalances and monitor the effects of acupuncture and/or homeopathic remedies.1-9 Despite the widespread clinical usage of electrodermal screening, little scientific data have been collected that validate either the instruments used or the physiological responses obtained. Electrodermal screening may have substantial clinical relevance and important implications for complementary and alternative medicine. However, until the reliability of the instruments and procedures are established and the underlying physiological variability defined, these measurements lack scientific credibility.10

The use of electrical skin resistance or conductance (the reciprocal of resistance) is based on previous work.6,11,12 Researchers observed that electrical skin resistance measurments (ESRMs) varied over the body and that acupuncture points showed greater correlation with lower ESRMs than did other areas. Further work13-16 demonstrated that acupuncture points represent punctate loci of significantly lower electrical resistance than the immediate surrounding skin. Other researchers observed that changes in ESRMs at auricular acupuncture points reflect changes in related organ systems in humans.17-19 ESRMs at related acupuncture points also occur in animals with experimentally induced pathology.20,21

Since ESRM recordings are notoriously prone to error, the trustworthiness of previously reported clinical studies has been questioned.22-24 Instrument-related artifacts arise primarily from the pressure and duration of probe application, which may lead to skin damage and/or polarization, resulting in spurious readings. Skin conditions, including thickness of the stratum corneum, levels of hydration, amount of sweat, and the presence of cracks or abrasions markedly influence ESRM readings. When Motoyama25 applied a 3-V DC electrical potential between the right and left LR 3 acupuncture points, the current was approximately 46 µA. When the stratum corneum was incrementally stripped to the level of the dermis, the current flow increased to approximately 1460 µA.25 ESRMs, similar to the galvanic skin response (GSR), reflect autonomic nervous system activity, which is known to fluctuate continuously with different emotional, physiologic, and environmental influences.3,25,26

ACCURACY AND RELIABILITY OF
ELECTRODERMAL MEASURING INSTRUMENTS
Few assessments of the precision and reliability of electrodermal test instruments for measuring ESRMs at acupuncture points have been reported. When the accuracy of measurements recorded from 16 different EAV (Electro-acupuncture, according to Voll) devices was evaluated, substantial discrepancies in performance were identified.27 A test-retest reliability assessment of the AMI (Apparatus to diagnose the function of the Meridians and the corresponding Internal organs), in which 30 subjects were measured at the same time on 2 consecutive days, found highly repeatable measurements: correlation coefficients between 0.923 and 0.947 for the 4 parameters tested.28 However, this study has not been replicated.

In separate settings, 2 groups of investigators found clinical reliabilities of 0.72 and 0.76 with 30 and 31 participants, respectively.29,30 The Prognos device, a constant voltage ohm meter developed for recording skin resistance at the Ting points, was used.

Physiologic Variability of ESRMs at Acupuncture Points
Throughout 24 Hours
Only 1 other group of researchers has evaluated ESR at the Ting acupuncture points over a 24-hour period (Gaetan Chevalier, oral communication, 2003). When recording skin conductance measurements with the AMI, this group found a prominent diurnal pattern that they correlated with their subjects being either "morning," "evening," or "intermediate" personality types.

THE CHINESE MIDDAY/MIDNIGHT CLOCK
According to Chinese medical theory, "nourishing Qi" circulates through each of the 12 paired organs and their associated Meridians during a regular 24-hour cycle, with a 2-hour period of maximum energetic output and a 2-hour period of minimum activity for each organ31 (Table 1). The purported "Chinese clock," although having clinical implications for acupuncturists, has no obvious counterpart in conventional medical diagnoses.

Table 1. Peak and trough meridian hours

 

Maximum output

Minimum output

Organ

(Peak)

(Trough)

Lung

03:00-05:00

15:00-17:00

Large Intestine

05:00-07:00

17:00-19:00

Stomach

07:00-09:00

19:00-21:00

Spleen

09:00-11:00

21:00-23:00

Heart

11:00-13:00

23:00-01:00

Small Intestine

13:00-15:00

01:00-03:00

Bladder

15:00-17:00

03:00-05:00

Kidney

17:00-19:00

05:00-07:00

Pericardium

19:00-21:00

07:00-09:00

Triple Heater

21:00-23:00

09:00-11:00

Gall Bladder

23:00-01:00

11:00-13:00

Liver

01:00-03:00

13:00-15:00

To ascertain scientific evidence of a timed energy ebb-and-flow within specific Meridians, we studied 5 participants over an 8-hour period and recorded ESRMs at the Ting acupuncture points every 20 minutes. The preliminary data from this exploratory study in 2003 provided the rationale for recording ESRMs at 20-minute intervals in the current protocol.

Our objectives for this study were to evaluate within- and between-subject ESRM variability at 24 Ting acupuncture points; to identify any inherent rhythmic patterns in these measurements; and to test the hypothesis that peak and trough activity of the 12 Meridians manifest in ESR recordings at the Ting acupuncture points.

METHODS
Participants
Twenty healthy volunteers were recruited from the general public, the Oregon College of Oriental Medicine (OCOM), the National College of Naturopathic Medicine (NCNM), and the OHSU School of Medicine to participate in the study. Inclusion criteria included general good health (no acute or chronic medical problems); age 20-35 years; both sexes (female admissions were timed so that participants were in the early follicular phase of their menstrual cycle); the ability to understand and sign an informed consent form; and the ability to rest quietly for 24 hours. Exclusion criteria were acute or chronic illness; current use of any over-the-counter (OTC) or prescription medication, including birth control pills; and pregnancy.

Female participants were administered a urine pregnancy test prior to enrollment. The study was approved by the OHSU Office of Research Integrity. All participants signed an informed consent form before admission.

Setting and Times of Measurements
The complete experiment was conducted at the General Clinical Research Center (GCRC) of OHSU during 4 different weekends between October 2004 and January 2005. Participants were admitted in groups of 1-3 for a 24-hour period. The first 11 participants were studied from 7 AM on day 1 to 6:55 AM of day 2. The other 9 participants were studied between 3:00 pm on day 1 and 2:55 pm on day 2.

Equipment and Procedures
Each participant wore a Holter monitor and an Actiwatch. Salivary cortisol specimens were collected every 2 hours. (Results and correlations of heart rate variability [HRV] and salivary cortisol levels will be reported in a future publication.) Five technicians, each working in 8-hour shifts, took ESRMs at acupuncture points over the 24-hour cycles.

ESRMs at the 24 Ting acupuncture points were obtained at 20-minute intervals using the Prognos (which was connected to a computer through an infrared port). Data were acquired using Medprevent's Prognos software (version 5.62.01; http://www.germanmedtec.com/Download/Prognos-dl1/Info.html). The data were later imported into SPSS version 13 statistical package for numerical analysis (SPSS Inc, Chicago, Ill).

All measurements were taken with participants lying supine in a dimly lit hospital room where the temperature was 21-22°C (70-72°F). Participants refrained from conversation while the ESRMs were taken. They were allowed to engage in quiet activities between measurements. Three meals were provided and participants were allowed to snack between recordings.

The skin over each of the 24 Ting acupuncture points was cleansed with ethyl alcohol and allowed to dry. The points were masked with a circular adhesive tape with a 5-mm diameter central perforation to ensure repeatability of probe tip placement at the 24 acupuncture points during 72 consecutive measurements.

Participants were required to rest in bed for 5 minutes prior to each ESRM recording session. Recording a set of 24 ESRMs on each participant took 1-2 minutes.

ESRMs were taken in the order of left hand, right hand, left foot, right foot in the following point sequence: LU 11, LI 1, PC 9, TE 1, HT 9, SI 1, SP 1, LR 1, ST 45, GB 44, KI 1 (at the medial corner of the nail bed on the small toe), and BL 67.

Statistical Analysis
Experimental Design. A repeated measures design was used to determine the variability of ESRMs. Microsoft Excel (Microsoft Inc, Redmond, Wash) charts were used to portray inherent rhythms in these measurements.

Analysis for Within- and Between-Subject Variability. The analses for this pilot study are descriptive in nature. Plots of ESRMs over time for each subject were examined to explore temporal patterns. Sample means and standard deviations for ESRMs at acupuncture points were obtained for each subject. Parametric models relating ESRMs to time were explored. A general repeated measures analysis was conducted to obtain summary information regarding variability over specific time periods (sleep and wake), and variability between subjects. Asymmetries between left and right ESRMs were evaluated in each individual.

Hypothesis Testing for Evidence of the Chinese Clock. Hypothesis testing was based on the assumption that low ESRMs (high conductance) correspond to peak activity and high ESRMs (low conductance) correspond to trough activity for each acupuncture point during the 2-hour intervals specified by the classic Chinese clock (Table 1).

Sample Size Justification. This study was performed in preparation for a larger study to assess the feasibility of developing ESRMs at acupuncture points into valid biomarkers. Limited useful preliminary data were available to guide sample size selection for our protocol.

Preliminary data assessing the variability of ESRMs at 1 point on 5 subjects over 2 sequential 1-hour periods showed the average difference in ESR between the 2 time periods to be 1136 kV and the SD of the differences to be 1908 kV. With 24 subjects, a 2-sided paired t test at level a=.05 was expected to have approximately 80% power to detect a difference of 1136 kV, if the true SD was 1908 kV. For this measurement, we expected to have reasonable power to detect differences over time. Hence, with multiple measurements over a 24-hour period, we anticipated discerning patterns that would provide hypotheses for future work.

RESULTS
Twenty healthy participants (11 men and 9 women) were studied. All women were in the prefollicular phase of their menstrual cycle. After reviewing data on the first 11 participants who were tested between 7 am on day 1 and 6:55 am on day 2, it appeared there was extremely high intra- and inter-subject variability in ESRMs. The highest values occurred between 1 Am-9 Am, including the time of admission/discharge. Due to the marked differences in ESRMs observed, we were able to reduce our sample size to 20. Additionally, so as not to miss any potentially informative transitional data around 7 am, the start time was changed to 3:00 pm on day 1, with completion at 2:55 pm on day 2 for the last 9 participants.

Variability in ESRMs between subjects was significant at the 1% level ranging from 11 kV to 59,963 kV. This marked variability among participants was exemplified in subjects 10, 11 and 19, 20 (Figures 1 and 2). ESRMs in 2 same-age men, recorded during the same test session, show marked variability in kV values. Furthermore, these male participants had inherent rhythmic patterns that were substantially different from each other. Figure 2 shows 2 same-age women (subjects 19, 20) who were recorded during the same test session. These female participants had similar inherent patterns but marked differences in ESRM kV values.

ESRMs and activity counts plotted over 24 hours showed an inherent pattern in which high ESRMs were associated with low activity counts (surrogate for sleep), and low ESRMs with high activity counts. Increases in ESRMs with sleep, which were abrupt in some participants and gradual in others, correlated highly with daytime naps as well as nighttime sleep (Figure 3). A 2-hour cycle of precipitous falls in ESRMs, coinciding with participants being awakened every 2 hours for the salivary cortisol collections, was evident in the graphs of all 20 participants. Variances in ESRMs within individuals also differed during sleep and arousal (variance range, 2.6 MV to 149.8 MV, and 0.40 MV to 75.9 MV, respectively). In all but 1 of the 20 participants, higher variance in ESRMs occurred more often during sleep than during arousal.

Of the 240 pairs of acupuncture points tested, only 10 (4%) showed right/left asymmetries, and 5 of the right/left asymmetric pairs were present in a single individual (subject 15) (Figure 4).
 
Hypothesis Testing for Evidence of the Chinese Clock
All data were smoothed to remove spike artifacts and gross short-term variations. The smoothing process allowed for nearly all the variability in the data to remain. This was done blindly by a smoothing routine that was tuned to case samples from the data to ensure that it neither oversmoothed (which would remove meaningful peaks/ troughs) nor undersmoothed (leaving spikes and short aberrant sections). P values for peak and trough periods were computed for each person separately. The point-specific trajectories were standardized to have mean 0, variance 1. The average trajectory over 24 hours was computed and then subtracted from each point-specific trajectory. This was an attempt to put all the points on the same scale and remove the strong sleep/wake effect. (Example: high values at night would appear as troughs for the point that happened to be predicted to trough at night.)

The P values for each person were moved through a multiple testing screen, which suggested that 11% of them were statistically significant. The P values were 1-sided in the predicted direction. (Thus, for a peak, the ESRM value had to be significantly low, and vice versa for a trough.) Although 5% of P values would be expected to fall below .05 inadvertently 11% did, indicating that perhaps 6% of the cases showed statistical evidence of following the Chinese clock hypothesis.

DISCUSSION
This study is perhaps the first to document 24-hour within- and between-subject variability in ESRMs at the Ting acupuncture points. A highly significant negative correlation between ESRMs and activity level (sleep) was observed in all but 1 of 20 participants (subject 10). There were no gender differences in ESRM readings among these young healthy participants. Right/left differences occurred primarily in 1 participant (subject 15).

Since there are no other published data on the 24-hour variability of ESRMs for comparison, we plan to evaluate our findings in the broader context of GSR research. However, comparisons of ESRMs at acupuncture points with GSR measurements should be considered only approximate for the reasons described below.

GSR Parameters
Any of 6 electrodermal activity parameters are typically measured in GSR studies: skin conductance level (SCL), skin conductance response (SCR), skin resistance level (SRL), skin resistance response (SRR), skin potential level (SPL), and skin potential response (SPR). Each of these is reported in a series of quantifiable metrics including number, amplitude, morphology, latency, duration, and recovery rate.32 The ESRMs recorded in the present study are essentially a single component of the GSR, equivalent to the tonic background level of SRL (also known as basal skin resistance), which correlates most closely with spontaneous SPRs.
Measuring Instruments, Electrodes, and Skin Preparation Absolute kV values of ESR differ enormously, depending on the instrument and method used to take recordings. Most commercially available electrodermal measuring devices, including the Prognos and various GSR devices, are constant voltage direct current (DC) instruments. However, alternating current (AC) is also used by some psychophysiologists,33 electrophysiologists,32,34 and researchers measuring impedances at acupuncture points and control sites.35

The measuring techniques and electrodes used in GSR studies also differ considerably from the Prognos device and methods. The Prognos records ESR at 24 distinct acupuncture points located at the corners of the finger and toenail beds. GSR is most often recorded on the thenar and hypothenar eminences of the hand or on the dorsal and palmar surfaces of the index and middle or ring fingers. When GSR was measured on non-palmar parts of the body, including areas on the head, trunk, and legs, spontaneous SPRs were found at all sites except the scalp.36 Greater electrodermal activity and less resistance has been measured on the distal phalanx than on the proximal phalanx.37 Venables and Mitchell observed that ESR measurements differed between the 4 fingertips, but offered no explanation for that finding.26 Dorsal skin measurements compared with palmar measurements have less spontaneous SPR excitability and fewer sweat glands.38

ESRMs are proportional to the size of the electrode and to the area of skin in contact with the
electrode paste.39 The Prognos probe tip is 4.5 mm in diameter, whereas GSR electrodes are usually about 1-2 cm in diameter. When measuring ESRMs (acupuncture points are purportedly 1-5 mm in diameter), recordings are usually taken on dry skin to limit the possibility of gel spreading to a larger area around the acupuncture points.40 In contrast, conductive gel is typically used in GSR studies.

Diurnal Variability of GSR
Diurnal variability in GSR has been documented by several researchers.26,41 Anticipating such a rhythm in the ESRMs, we started our 24-hour recordings at 7 am. However, when data on the first 11 participants revealed a rhythmic pattern that appeared to be interrupted at the 7 am admission/discharge time and did not appear to be diurnally related, we changed our start time to early afternoon (3:00 pm). This allowed us to define a clear pattern related to sleep/arousal rather than day/ night or light/dark rhythms.

Sleep/Arousal and Left/Right Differences in GSR
Numerous investigators have demonstrated that the basal skin resistance is inversely related to level of arousal.42-46 States of relaxation are accompanied by high skin resistance, which reach maxima during sleep. Gradual or abrupt rises on falling asleep as noted in our study are also reported in the GSR literature.43 When our subjects awoke in the morning, the ESRMs at acupuncture points decreased rapidly over a period of a few seconds. Also reflected in our ESRM data is the fact that if sleep was interrupted completely, i.e., when participants were awakened for the 2-hourly salivary cortisol collection, ESRMs dropped precipitously. But, when participants were minimally disturbed (during the every-20-minute ESRM recordings), the drop was less dramatic.

A high proportion of spontaneous electrodermal variabilty (80%) has been observed during sleep, with random right/left differences also more prevalent during sleep.47-51 These right/left differences in GSR have been interpreted as random and "inconsistent variables."51 When data on participants in our study were analyzed individually, significant right/left differences at acupuncture points occurred in only 5 participants, with the majority of the asymmetries in a single participant.

Our 20 participants were considered "healthy" because they had no acute or chronic illnesses and were not taking medications. However, subtle energetic imbalances were not ruled out. It is possible that the asymmetries observed among paired acupuncture points such as the TE, PC, HT, LU, and LI in subject 15 (Figure 4) may represent incipient illness or changes at the pre-organic level, which is the phenomenon that electrodermal screening claims to diagnose.

Data from our study revealed some occasional aberrant ESRMs exemplified in subject 11 (Figure 2). Such fluctuations were also reported by Zhang.52 These fluctuations might be perceived as within-person variability or an artifact of the measuring technique. It is clear that for as yet unexplained reasons, ESRMs periodically become volatile and this appears to happen more frequently during sleep when ESR values are highest.

During daytime hours, no distinct patterns supporting the Chinese clock hypothesis were demonstrated in Meridian peak or trough activity, but some significant peak and trough patterns did occur during nighttime readings.

CONCLUSIONS
This first summary of evidence on the variability of ESR at the Ting acupuncture points suggests that ESRs behave similarly to basal skin resistance, a parameter of the GSR. Both demonstrate significantly higher, more variable kV values during sleep and lower, more stable values during times of arousal. These findings have obvious implications for clinicians who use electroderm screening in their practices. ESRM evidence for the classic 2-hourly intervals of energy ebb and flow described by the midday/midnight Chinese clock was minimal in this study.

This observational study has generated a number of new hypotheses to be tested in the process of developing and validating ESRMs at acupuncture points as medical biomarkers.

Funding/Support
This project was supported in part by Public Health Service grant 5 M01 RR000334 and the National Center for Complementary and Alternative Medicine/National Institutes of Health grant AT00076 through the Oregon Center for Complementary and Alternative Medicine. 

ACKNOWLEDGEMENTS
We wish to thank Dawn Peters, PhD, for her assistance with setting up the statistical analysis plan, and Sarah Giardanelli for her help with data handling. We also wish to thank Richard Hark for loaning us the Prognos, and for his technical and clinical support.

Figure 1. Significant between-subject variability is demonstrated in 2 same-age male participants tested on the same day. Marked differences are observed in absolute kohm values as well as in the inherent rhythmic patterns.

 

Figure 2. Significant difference in between-subject variability is demonstrated in 2 same-age female participants tested on the same day. Although the inherent rhythmic patterns are similar, the absolute kohm values are markedly different.

 

Figures 3A and 3B. High ESR correlates significantly with low activity counts (surrogate for sleep) during daytime naps and night time sleep.

Figures 3A.

Figures 3B.

 

Figure 4. Right/left differences were significant in 5 pairs of acupuncture points in Subject #15. The right-sided points were consistently higher for the TE, HT, and PC points while the left-sided LU and LI points were intermittently higher than the right.

 

 

 

 

 


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AUTHOR INFORMATION
Dr Agatha P. Colbert is an MD Investigator at the Helfgott Research Institute, National College of Naturopathic Medicine, in Portland, Oregon.

Agatha P. Colbert, MD*
Helfgott Research Institute
National College of Naturopathic Medicine
049 SW Porter St
Portland, OR 97201
Phone: 503-552-1745 • Fax: 503-227-3750 • E-mail: acolbert@ncnm.edu

Dr Meg Hayes is Assistant Professor in the Department of Family Medicine, Oregon Health and Sciences University, in Portland, Oregon.
Meg Hayes, MD
E-mail: hayesm@ohsu.edu

Mikel Aickin, PhD, is Professor, Program of Integrative Medicine, at the University of Arizona, in Tucson, Arizona.
Mikel Aickin, PhD
E-mail: maickin@comcast.net

Richard Hammerschlag, PhD, is Research Director at the Oregon College of Oriental Medicine, in Portland, Oregon.
Richard Hammerschlag, PhD
E-mail: rhammerschlag@ocom.edu

*Correspondence and reprint requests

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