Skin Conductance at 24 Source (Yuan) Acupoints in 8637 Patients: Influence of Age, Gender and Time of Day

Article Outline

• Abstract
• 1.. Introduction
• 2.. Materials and Methods
  • 2.1.. Skin conductance measuring system
  • 2.2.. Data preparation
  • 2.3.. Statistical analyses
• 3.. Results
  • 3.1.. Demographics
  • 3.2.. Average conductance at 24 acupoints
  • 3.3.. Gender and age group comparison for individual acupuncture meridians
  • 3.4.. Skin conductance change with age
• 4.. Discussion
• Declaration of Financial Support
• References
• Copyright

Abstract

The clinical practice of recording skin conductance (SC) at acupuncture points (acupoints), as a diagnostic and/or therapeutic monitoring aid may have scientific merit. However, influences of age, gender and time of day on these recordings are unknown and it is unclear whether SC at acupoints differs from SC levels in general (as reported in psychophysiology research). This paper summarizes SC data obtained with the AcuGraph 3 Digital Meridian Imaging System between June 2005 and March 31, 2010. An initial dataset of 117,725 SC examinations was scrubbed to include only the first SC examination on individual patients and exclude potentially faulty data. The final dataset consists of SC recordings at the 24 Source (Yuan) acupoints in 8637 patients, collected by 311 practitioners. Twelve left/right average conductance measures and an overall average of the 24 acupoints were assessed. Statistical analyses included two sample t tests, three way analyses of variance and linear regression. Results indicate that mean SC at acupoints, similar to SC in general, is higher in males, higher in afternoons and declines with age. Not previously reported, the rate of SC decline with age differs at different acupoints between males and females. These findings have substantial implications for acupuncture research and practice.

Key Words:  electrodermal activity at acupuncture points , skin impedance , skin resistance

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1. Introduction 

Limited evidence suggests that electrodermal activity (EDA) at acupoints and meridians is distinct from EDA in nearby tissue(s) [1, 2]. A few studies also suggest that skin conductance (or its reciprocal, resistance) at acupoints correlates with clinical diagnoses [3, 4, 5, 6, 7, 8] and with therapeutic outcomes [4, 9]. Whereas EDA at palmar sites has been validated and is extensively utilized by psychophysiologists in stress and emotion research [10, 11], acupuncturists measure SC at acupoints for a different, as yet un-validated purpose, based on the 1950s work of Voll [12, 13] and Nakatani [14]. Acupuncturists use electrical skin conductance (SC) at acupoints to identify which of 12 classically paired acupuncture meridians are out of balance. These SC measurements then inform the clinician's treatment plans and assist in monitoring therapeutic outcomes. Although SC at acupoints may prove to be a valid diagnostic aid and/or quantifiable outcome measure, more preliminary research is needed to characterize SC at acupoints including determining the accuracy and reliability of commercially available measuring systems, obtaining interexaminer and intraexaminer agreement among users of each system, defining normal physiological variability of SC at acupoints, characterizing SC at various acupoints compared to nonacupoints in the general population, and correlating SC with both acupuncture and western medical diagnoses and outcomes.

In order to exploit the potential utility of SC at acupoints as an outcome measure in clinical trials we, at the Helfgott Research Institute of the National College of Natural Medicine, have initiated a program of research to define electrical characteristics of acupoints. To date we have evaluated the accuracy and reliability of commercially available and custom-built skin impedance instruments [15, 16]; documented 24-hour physiological variability of skin resistance at acupoints in healthy volunteers [17]; and evaluated interexaminer and intraexaminer agreement among examiners using the AcuGraph 3 Digital Meridian Imaging System (manuscript under review with Acupuncture in Medicine).

We are currently collaborating with Miridia Technology, Inc. (Meridian, ID, USA) to analyze an existing database which consists of SC measurements recorded by the AcuGraph 3 Digital Meridian Imaging System between June 2005 and March 31, 2010. The dataset includes recordings collected by 804 AcuGraph users from more than 50 countries worldwide who completed 117,725 SC examinations on 43,088 patients. The objective of this report is to summarize findings from a final scrubbed dataset of 8637 first SC examinations on as many individual patients collected by 311 practitioners. We will compare SC recordings at the 24 Source (Yuan) acupoints (Figure 1) and analyze influences of age, gender and time of day on these recordings.

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• Figure 1. 

  (A, B) Yin source points. (C, D) Yang source points.

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2. Materials and Methods 

2.1. Skin conductance measuring system 

The AcuGraph 3 Digital Meridian Imaging System, a computer software and hardware system developed by Miridia Technology, Inc. consists of software, an electronic control unit, and a probe set. The system electrically measures current flowing between specific acupoints on the hands and feet and a handheld reference probe. The software program allows data collection at either, the 24 Source (Yuan) Points, the 24 Jing-well acupoints, or 24 Ryodoraku points. Typical usage consists of gathering patient identifying data, applying a water-moistened cotton electrode probe to the acupoints to collect data, and interpreting that data with charts, graphs, and recommendations produced by the software.

The original Ryodoraku system of measurement and treatment developed by Dr. Yoshio Nakatani expressed the conductance of each skin point on a scale of 0–200 microamperes (μA). For patient comfort and reliability, modern systems use much less current, but most have retained the original 0–200 scale of Ryodoraku. The AcuGraph system uses a measurement current of 0–40 μA to measure electrical resistance (R) and then converts resistance values to conductance (1/R), and reports normalized conductance units on a scale of 0–200, which corresponds to a range of resistance values from 75 kohms to 2 Mohms.

2.2. Data preparation 

As of March 31, 2010 the Miridia Technology, Inc. dataset contained SC measurements on 43,088 patients collected by 804 practitioners in the course of performing 117,725 SC examinations at 24 acupoints. Included in the dataset are anonymous practitioner and patient identification numbers, patient age and gender, dates and times of each SC examination, values for each of 24 meridian acupoints and six summary values for each patient visit. The initial dataset of 117,725 SC examinations underwent stepwise scrubbing procedures to evaluate only the first SC examination on any patient and to exclude potentially erroneous data (Figure 2). The first nine AcuGraph exams conducted by each practitioner were excluded so as to avoid any “practice exams” that were conducted while the practitioner was gaining competence using the AcuGraph system. To exclude examinations with errors in patient data entries, only patients aged 0–100 years were included. For individual patient examinations to be included they also had to have at least two SC examinations, not conducted on the same day to exclude possible non patient ID numbers. In addition, to assure that measurements were recorded from skin sites only, any 24 acupoint examinations that contained readings of zero, or had 12 or more readings > 200, or had 20 or more readings > 180 were excluded. Also excluded were patient examinations that contained recordings in which four or more acupoints had identical readings. The AcuGraph 3 Digital Meridian Imaging System computes a custom score, Personal Integrated Energetics (PIE), based on a 0–100 scale. PIE values < 5 were excluded because they were unlikely to have been derived from true acupoint recordings.

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• Figure 2. 

  Data preparation to exclude potentially faulty skin conductance examinations and include patient initial visits only. DOB = date of birth; PIE = personal integrated energetics.

2.3. Statistical analyses 

Two sample t tests were used to compare conductance measures for gender in each age group for each meridian acupoint (left and right averaged). Three-way analysis of variance was used to test differences between gender, age groups and time of day for the average conductance of all 24 acupoints. A linear regression was used to evaluate the relationship between age and conductance at acupoints for each meridian, by gender. All statistical analyses were performed with SAS version 9.1 (SAS Institute, Inc., Cary, NC, USA).

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3. Results 

3.1. Demographics 

The final data scrub yielded first SC examinations on 8637 patients (6069 females, mean age 49 years, and 2568 males, mean age 48 years) for analysis. The Source (Yuan) acupoints represented 85.28% of the total exams. Age and gender distributions for this patient population are depicted in Figure 3.

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• Figure 3. 

  Approximately two thirds of patients who underwent skin conductance examinations were female, except in the 0–15 year age group.

3.2. Average conductance at 24 acupoints 

A single average of the 24 acupoint readings was used for the first set of analyses. These analyses looked at differences of this measure across three factors: gender, seven age groups, and a time of day variable. The time of day variable was dichotomous, classifying the treatment as occurring before or after 1 PM (AM vs. PM). The means, standard deviations and cell sizes for this measure can be seen in Table 1. A three-way analysis of variance test was completed first, testing the differences between the three factors mentioned above. All three main effects showed significant differences for this summary measure (p < 0.0001 for each), and there was one significant interaction between gender and age groups (p = 0.002).

Table 1. Summary data for the average normalized skin conductance measurements for the 24 Source (Yuan) acupoints by gender, time of day and age groups

Based on the AcuGraph normalized point scale of 0–200, the average conductance for females was 86.5 and for males, 98.5. The average SC before 1 PM was 86.2, and after 1 PM was 93.2. Figure 4 shows an hourly time of day breakdown for the average SC measure, indicating where the break was made to create the dichotomous variable used in the analyses. The SC was higher after 1 PM for all gender and age group combinations, except for males over 70 years of age where the average difference was only 1.7 units lower for exams after 1 PM.

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• Figure 4. 

  Average skin conductance at 24 acupoints increased in the afternoon, peaking at 4 PM.

Because of the gender and age group interaction, age groups were investigated separately for each gender. Figure 5 shows a comparison of SC in males and females across age groups. SC in females was significantly lower than in males for all age groups (all p=0.002), except for the 0–15-year age group, where there was no significant difference. Using SC at the 0–15-year age group as the reference, females showed an overall decline in this mea sure from the 0–15-year age group to the 71+ year age group of 24.7% compared to 15.7% for males. Most of this decline occurred for females in the first four age groups (18.1% decline), while males only declined 3.5% up to age 50 years. The decline in males and females for this measure from the 41–50-year age group and higher was almost equal when using this age group as a reference (11.3% for females and 12.6% for males). Based on the results of this analysis, we conducted further testing to explore the possible contributions of individual meridian acupoints to these differences.

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• Figure 5. 

  Average skin conductance at 24 acupoints decreases with age in both males and females, however the rate of decline is greater for females in the first 50 years of life.

3.3. Gender and age group comparison for individual acupuncture meridians 

Figure 6 provides an overview of the 12 left/right averaged meridian acupoints with both genders and all age groups combined, paired to show yin/yang differences. To understand the gender differences in SC at these acupoints, a secondary analysis was performed with two sample t tests for each meridian and age group separately, using an average of the left and right SC for each acupoint. The results of the gender comparisons for each acupoint and age group are shown in Figure 7 with 95% confidence intervals. The 0–15-year age group is not shown since there were no significant differences between males and females for any of the 12 acupoints. The number of acupoints with differences between males and females increased with age up to the 41–50-year age group. The 16–30-year age group had four differences (ST (Stomach), KI (Kidney), LR (Liver), GB (Gall Bladder)); the 31–40-year age group had seven differences (ST, KI, LR, GB, LU (Lung), HT (heart), PC (Pericardium); and the 41–50-year age group had eleven differences (all acupoints except LI (Large Intestine)). The remaining age groups all had significant gender differences at nine acupoints, varying by age group. The 51–60-year age group showed gender differences at all acupoints except LI, SP (Spleen) and TE (Triple Energizer). The 61–70 year age group showed gender differences at all acupoints except SI (Small Intestine), SP and TE. And the 71+ year age group showed gender differences at all acupoints except LI, SI and TE. Males had higher SC than females at all acupoints except LI, for the 61–70-year age group. Significant male/female differences at four acupoints (ST, KI, LR and GB) were consistently present in all age groups except 0–15 years, and the magnitude of the difference for these four acupoints was generally larger than any of the other differences.

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• Figure 6. 

  Yin yang acupoint skin conductance comparisons. Yang points have consistently lower conductance than their matched yin counterparts, more so in the foot than the hand.

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• Figure 7. 

  Skin conductance (normalized units) differences between genders at 12 left/right averaged acupoints by age group, shown with 95% confidence intervals. Acupoints are sorted by magnitude of gender difference. Four meridians (ST, KI, LR, GB) are significantly higher for males in all six age groups shown. The 0–15 year is not shown because there were no gender differences. The number of meridian differences between genders increases up to the 41–50 year age group, with all but LI showing a significant difference, then declines slightly in the older age groups. For all significant differences, males have higher conductance, except for LI in the 61–70 year age group. All male/female paired meridians with overlapping confidence intervals are not significantly different.

3.4. Skin conductance change with age 

Another analysis was done to quantify the relationship between age, as a continuous variable, and SC. This analysis used a linear regression with normalized SC units as the dependent variable and age (in years) as the independent variable. The regression coefficient is an indication of the extent to which SC declines with incremental years of life. Figure 8 shows the regression coefficient (normalized SC units) for each acupoint (left and right SC averaged), by gender. All coefficients were significant with p < 0.0001. Females have a faster rate of SC decline than males at 11 of the 12 acupoints (all except LI). The chart has been ordered from left to right by the magnitude of the differences of SC decline coefficients between males and females. This ordering naturally separates the acupoints with large differences into the foot acupoints on the left in Figure 8, and much smaller differences in hand acupoints on the right.

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• Figure 8. 

  Acupoints are sorted by the magnitude of the difference in average rate of skin conductance decline with age between males and females. Maximum differences in rate of decline between males and females were observed in the foot acupoints.

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4. Discussion 

Our specific goals in this data analysis were to characterize SC measurements at the 24 Source (Yuan) acupoints and to assess the influence of age, gender and time of day on these measurements. To our knowledge this is the first attempt to report SC recordings at acupoints in a patient sample of > 8000 individuals. Our age, gender and time of day findings for SC measurements at acupoints are similar to SC in general, but more comprehensive in that we have documented SC characteristics at 24 skin sites not previously described. The majority of psychophysiology research evaluates SC measurements on the palmar aspects of the hand or fingers because SC at these sites is linked to emotional sweating and autonomic nervous system responsiveness. In contrast, acupuncturists, believe that SC at acupoints reflects the underlying condition of meridians represented at those skin sites and do not measure SC at palmar acupoints because the increased sweat activity confounds the measurements. Our long range mission is to determine if SC at acupoints has unique characteristics that are distinct from SC on the body in general, and if these differences can be used in the diagnosis and treatment of medical conditions. Our immediate goal in this study was to identify any SC differences at 24 clinically important acupoints between males and females in different age groups and at different times of day.

Comparable to reports in the psychophysiology literature [18] we found the average SC at the 24 Source (Yuan) acupoints is higher in males than in females, except in the youngest age group (0–15 years) where there are no significant gender differences. Also in agreement with psychophysiological research, we observed that SC at acupoints decreases with age in both genders [19, 20]. This finding generally coincides with the first clear age-related changes in the skin of adult humans that appear between the third and fourth decades of life, changes that may be accounted for by a relatively sudden decrease in skin thickness and elasticity occurring in the fourth decade [21]. In addition, we found significantly higher afternoon SC readings in both genders, consistent with increased sympathetic autonomic nervous system activity in the afternoon [22]. Other investigators' findings in younger age groups conflict with these morning/afternoon differences, however. Venables et al, performed EDA examinations on 640 Mauritian Island volunteers, aged 5–25 years and found significantly higher SC in females in the morning, in the cool season. However, their female volunteers had higher SC than males in the afternoon in the hot season [10].

Unlike previous reports, we scrutinized 24 skin sites in detail, and found unexplained between-acupoint and between-gender differences in our study sample. The number of male/female acupoint differences increased until age 41–50 years at which time SC in 11 of the 12 paired acupoints was statistically higher in males than females. Starting with the 51–60 year age group, the number of acupoint SC differences decreased slightly to 9 of the 12 meridian acupoints, and stayed at that number in all subsequent age groups. Notably with regard to acupuncture theory, four acupoints, (LR, GB, ST, KI) show the greatest male/female differences and these differences are significantly larger than other acupoint differences between genders for all age groups. We cannot account for these differences on the basis of anatomy alone since, for example, significant gender differences were found at the LR acupoint but not at the anatomically similar, nearby SP acupoint on the foot (Figure 1). Although no other studies have looked specifically at SC at the Source (Yuan) points, Rickles and Day [23] found that EDA measurements at nearby skin sites around the ankle are similar to EDA on the plantar surface of the foot. While we did not measure acupoints on the plantar surface of the foot, there was significant variation between SC on ankle acupoints (KI3, GB40) and other foot locations, and we did not see this level of SC variation at acupoints on the hand. In contrast to our data, Richter et al [24] recorded skin resistance (reciprocal of conductance) on the hand (again with no reference to specific acupoints) and found four times higher resistance (less conductance) on the dorsal than the palmar aspect, with a clear line of demarcation between the areas of lower and higher resistance. Dorsal/palmar differences were inconsistent in our study sample. For example, the SI 4 and HT 7 acupoints, which are located on the dorsal and palmar wrist, respectively (Figure 1) had similar SC, while LI 4 had higher conductance (lower skin resistance) than either PC 7 or HT 7, both located on the palmar wrist.

In addition to absolute differences between males and females, SC at different acupoints declined with age at different rates in males and females with more rapid SC decline for the overall average of the 24 acupoints in females than in males before age 50 years. Furthermore, SC in the foot meridian acupoints of females declines more rapidly than in hand meridian acupoints with age. This difference is not seen as clearly in males. Our findings of significant gender differences in SC decline at acupoints on the wrists and ankles warrant further analyses and interpretation in the context of both acupuncture theory and underlying skin anatomy and physiology. We are in the process of conducting these analyses and an in-depth discussion will be provided in a subsequent publication.

A number of limitations need to be acknowledged in this study. Multiple examiners recorded SC measurements on several thousand patients around the world. We cannot be assured of the skill of the 311 examiners or the quality of the 8637 SC measurements. Additionally the AcuGraph software guides practitioners to take acupoint measures in a specific order, with periodic rewetting of the probe tip. We attempted to examine possible influences due to these procedures. While no obvious distortions were evident we cannot say with certainty that the order and rewetting did not impact the data. We were gratified to find similarities in our data and that of the psychophysiology literature which suggests accurate SC readings in our data. Another limitation is that this study cannot ultimately answer the question of whether SC at acupoints is different from SC in general. This hypothesis could not be tested as we have no comparison data on SC collected at non acupoint sites. We are also unable to speak to whether SC at acupoints correlates with individual patient profiles, because the current Miridia Technology, Inc. database does not collect diagnostic, treatment or outcome information. Finally, our dataset does not represent “healthy individuals” but rather comprises a “normative” database of patients who sought the services of an acupuncturist.

Despite these limitations, we believe this study contributes substantially to existing knowledge on SC at acupoints and is important for several reasons. As the first comprehensive characterization of SC at the 24 Source (Yuan) acupoints in a large patient sample we have taken a fundamental step toward validating SC at acupoints for use in acupuncture research. In addition, we demonstrated that SC at acupoints, obtained by more than 300 clinicians can be efficiently collected, stored and analyzed, making a multisite clinical trial to assess the utility of SC at acupoints as an outcome measure entirely feasible. Moreover, we now have demographic data on which to base further development of SC at acupoints as an objective physiological measure. Most importantly, from this data we will be able to generate a number of testable hypotheses to help elucidate the mechanism(s) of acupuncture. In a rigorous manner we can start to answer questions such as these: Do SC measurements at acupoints differ in health and disease? Does normalization of SC at acupoints correlate with clinical improvement? Do specific acupuncture diagnoses correspond with specific SC patterns at different acupoints? Do specific SC patterns at acupoints correlate with western medical diagnoses? Are SC levels at acupoints predictive of health risk?

Our next steps are to modify the Miridia Technology, Inc. data collecting software, so as to be able to document patient diagnoses, treatments and outcomes. We will then correlate clinical outcomes with SC at acupoints in an observational study of patients being treated at the National College of Natural Medicine acupuncture clinics.

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Declaration of Financial Support 

The third author, Adrian Larsen is the President of Miridia Technology, Inc.

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