Differential Responses in the Temperature of the Palmar Forearm Following Acupoint Stimulation in the 3 Yin Meridians of the Hand

Xiang Zhu¹, Wei Gao¹, Yong Chi¹, Zhiyong Wang1 and Junjie Shao^1*

Abstract

To evaluate the temperature on the palmar forearm of the three yin meridians of the hand, to investigate the relationships between temperature and human physiological conditions, such as age and sex, and to examine changes in temperature following stimulation of different acupoints along the meridians. Methods: Two experiments were conducted. Experiment 1: The temperature of the palmar forearm from a total of 150 healthy volunteers was scanned and measured by infrared thermometers (IRT) to provide a comprehensive assessment of skin temperature associated with the meridians. Experiment 2: Sixty young healthy volunteers (age<40) received acupuncture stimulation on the palmar forearm in the area of the three yin meridians of the hand. Temperatures were recorded and analyzed. Results: Mean temperatures of the palmar forearm region of the three yin meridians of the hand were higher in the middle-aged group than in the young group (p<0.05). There were no other clear differences between temperatures based on physical conditions, such as age and sex. Importantly, we found that stimulating fire acupoints led to increases in meridian temperatures, while stimulating water acupoints resulted in decreases in meridian temperature. Conclusion: Our study suggested that meridian temperatures had sensitivity and unique characteristics, and might be a valuable indicator for monitoring the health condition of individuals. The differential temperature responses to acupoint stimulation indicated that the therapeutic mechanism of acupuncture is related to its ability to regulate body temperature.

Keywords: Temperature, Palmar Forearm, Water and Fire Acupoints, Three Yin Meridians of the Hand

Introduction

Acupuncture meridians, with acupoints as nodes, are considered “energy channels” in traditional Chinese medicine (TCM) theory^［1,2］. It is believed by most TCM researchers that meridians are physically real anatomical structures and that every acupoint is associated with unique therapeutic effects^［3-5］. In the past 50 years, scientists have made many efforts to prove the existence of meridians, such as studies examining electrical communication and anatomical structure ^(6-9). Recently, Li visualized the pericardium meridian in the palmar forearm by injecting 2 fluorescent dyes and excluded the possibility of corresponding vessels or lymph glands to account for the results^［10］. However, there is still no convincing evidence regarding the standards for meridian temperature and explaining the potential mechanisms underlying acupoint activity^{［4，5，11，12］}. According to TCM theory, acupoints were named and classified based on functional treatment effects and Chinese natural philosophy, such as the so called “water” and “fire” acupoints^{［5，13-15］}. It is easy to understand that water acupoints are able to eliminate heat and are suitable for patients with high fever or acute inflammation, while fire acupoints are good at producing heat and are mainly used for patients who have low energy or whose body temperature is low. Clinical practice has hinted at their treatment effects for many years, but there have been no experiments showing changes in temperature, which would further elucidate their mechanisms using modern techniques^{［16，17］}.

Recently, infrared thermography, which can measure infrared radiation waves emanating from the human body and convert them into temperature values, has been widely used in the field to investigate the properties of meridians^［18-20］. It is generally believed that body temperature, which is closely related to energy metabolism, provides a quick evaluation of a persons health and an ideal tool for monitoring meridian activities^{［16-18，21］}. For example, Wei measured and summarized the properties of 14 main meridian acupoint temperatures by IRT. Lou used IRT to investigate temperature changes in forearm acupoints from 20 volunteers stimulated by moxibustion^［8］. Through IRT, Tsuruoka found that the skin temperature of fingers and toes were different after stimulation of PC-6^［22］.

Thus, the purpose of our study was to verify whether there are patterns in temperatures on the palmar forearm over the 3 yin meridians of the hand and to identify the factors that influence meridian temperatures. More importantly, we examined how the skin temperature of meridians changed following stimulation of acupoints with different characteristics. Thus, we plan to validate some meridian theories that provide the foundation of TCM. Skin temperatures of meridians may also provide valuable information regarding human energy metabolism and acupoint combinations for treatment.

Materials and Methods

Subjects

Experiment 1 included a total of 150 adults (72 males, age: 40.05±2.68 years, height: 175.06±2.08 cm, weight: 70.23±8.29 kg; 78 females, age: 40.29±1.73 years, height: 160.38±5.45 cm, weight: 60.56±6.57 kg) who were 18 to 66 years f age, had no underlying diseases, and were not prescribed any medications.

Experiment 2 included a total of 60 adults (30 males, age: 31.05±1.68 years, height: 172.06±1.08 cm, weight: 71.23±6.35 kg; 30 females, age: 30.29±2.04 years, height: 159.48±3.95 cm, weight: 62.66±4.56 kg) who were 18 to 40 years of age, had no underlying diseases, and were not prescribed any medications.

The criteria for exclusion were as follows: mental and psychological disorders, arrhythmia, atherosclerosis, arterial thrombosis, systolic blood pressure > 140, diastolic blood pressure < 60, wounds or scars in the region of pulse measurement, BMI < 17 or > 30, pregnancy, and menstruation.

All participants provided written informed consent. This study was approved by the Institutional Review Board of Oriental Hospital of Ruijing Hospital, Shanghai, China (approval no. KY2020-360).

Experimental equipment

Body temperature was measured using a TSI-2000 IRT manufactured by the Bioyear Group company (Beijing, China). A thermograph camera (Bioyear Group, Ltd, Beijing, China), with a measurement range from 20 to 60 °C and a resolution of 640 × 480 pixels, was used to obtain thermographic images.

For Experiment 2, we used disposable stainless needles (diameter of 0.25 mm and length of 40 mm, provided by Suzhou Huatuo Medical Products Factory Co Ltd, Suzhou, China).

Experimental procedure

Experiment 1

The room temperature was controlled to be between 22 and 24 °C, and the participants were asked to place their hands so that they did not touch the body in an air-conditioned room (humidity, 40-50%). The IRT was used to obtain the skin temperature of the palmar forearm. Images were acquired with the subjects standing and the camera fixed approximately 70 cm away from the front of the body. A total of 15 acupoints on the 3 yin meridians of the hand (lung, heart and pericardium) are shown in Fig. 1, and their relationship to the human body is noted in Table 1. It is worth noting that we defined cold and warm weather based on the change of seasons. Winter and spring start on November 1st and end on April 30th in the northern hemisphere, while summer and autumn begin on May 1st and end on October 31st in the same hemisphere. The temperature of the 3 meridians was measured, and the data were acquired through software (Pico Log Recorder) on a Windows system.

Experiment 2

In Experiment 2, IRT measurements were obtained under the same conditions as in Experiment 1. We performed acupuncture stimulation along the 3 yin meridians of the hand, and bilateral body images were obtained at the following time periods: before and 5, 10, 20 and 30 min after the administration of acupuncture. Three fire acupoints (LU-10, PC-8 and HT-8) and 3 water acupoints (LU-5, PC-3 and HT-3) were chosen for stimulation (red dots). The region of the 3 yin meridians of the hand and the software used were the same as in Experiment 1.

Statistical analysis

Data are expressed as the mean±standard deviation (SD). The data were analyzed by SPSS version 15.0 (SPSS Inc., Chicago, IL, USA). Independent t-tests were used to examine the potential differences in both intrinsic and extrinsic variables between the 2 groups. Serial changes among the 3 groups were analyzed based on 2-way analysis of variance (ANOVA). Differences with P-values<0.05 were considered statistically significant, with significance levels marked as^*p<0.05, ^**p<0.01, and ^***p<0.001. As a measure of effect size, Cohen’s d value was calculated (low:d=0.20-0.49; moderate: d=0.50-0.79; high: d=0.80-1.29; and very high:d≥1.30)^［23］.

Result

Experiment 1

IRT measurements for 150 volunteers are shown in Tables 2 and 3. As illustrated, the mean temperature range of the palmar forearm on the 3 yin meridians of the hand was approximately 32.08 and 31.98°C on the left and right sides, respectively, and there was no difference between the sides (p=0.367). Furthermore, we did not find any differences in skin temperature based on sex (p=0.422) and weather (p=0.143). However, temperatures of the meridians were significantly. different in relation to age (p=0.03). The average temperature in the middle-aged group was 0.63°C higher than that in the young group. Second, as shown in Table 3, the mean temperature of the PC meridian was 32.34°C, which was higher than that of the HT (31.82°C) and LU (31.95°C) meridians, and there was a slightly significant difference between the HT and LU meridians (p=0.045). To assess effect sizes, Cohen’s d analyses were used. As displayed in Table 4, the values for the comparison of PC with the other 2 meridians were 0.504 and 0.639, indicating a significant difference in the temperature, while the value for the comparison between LU and HT was 0.163, which indicated that the difference was negligible.

Experiment 2

The changes in temperature of the 3 yin meridians of the hand stimulated by fire acupoints are shown in Figures 2-4. Fig. 2a shows that the PC meridian temperature raduall ncreased from 31.7°C to 32.3°C, and there was a significant correlated increase in the bilateral meridian temperatures after stimulation at the PC-8 point. Figs. 3a and 4a further confirmed that the meridian temperatures increased when we applied needles on the LU and HT meridiansat the fire acupoints (LU-10 and HT-8), and a synchronized response on the untreated side also appeared. In addition, we found that the stimulation of fire acupoints resulted in temperature increases in other meridians. For instance, with acupuncture on the PC meridian, the temperature f the HT and LU meridians increased by 0.71°C and 0.65°C, respectively Fig. 2b). As displayed in Fig. 3b and Fig. 4b, similar to the results in the lung and heart meridian intervention groups, synchronization of the temperature responses between the treated and untreated meridians was observed when acupuncture was performed on other fire acupoints (LU-10 and HT-8). Figs. 2c, 3c and 4c provide a clear picture of the temperature changes in the meridians stimulated by fire acupoints.

The temperature changes in the meridians induced by stimulation of the water acupoints are shown in Figures 5-7. Water acupoint stimulation exhibited opposite effects in regulating skin temperature in the region of the meridians. As shown in Fig. 5a, the PC meridian temperature gradually decreased from 32.25°C to 31.73°C with PC-3 stimulation, while skin temperature essentially did not change on the untreated side. Meanwhile, temperatures associated with the LU and HT meridians tended to decrease in the stimulated group, and a significant difference in the temperature existed between the groups after 20 min of stimulation (p<0.05), while the unstimulated group remained at a stable level (Figs. 6a and 7a). We also found that skin temperature decreased in all 3 meridians on the side receiving water acupoint stimulation. For example, PC-3 stimulation led to a decrease in temperature in the HT and LU channels, dropping from 32.3°C to 31.69°C and from 32.2°C to 31.71°C, respectively (Fig. 5b). Figs. 5c, 6c and 7c show that the color of the forearm turned from orange to green in the before to after stimulation conditions, illustrating that changes in temperature appeared only on the treated side.

Discussion

While the time of meridians being scientifically undetectable recently ended after experiments with tracer dyes were injected along the acupoints of the pericardium channel confirmed the existence of a continuity not accounted for by other anatomical structures^［10］, there are still potential mechanisms related to acupoints that need explanation. First, many studies have demonstrated that local and general warming effects are induced by both manual needling and electrically stimulated acupuncture^{［20,22，24］}. Other studies proved that acupuncture or moxibustion produced significant cooling effects based on skin temperature of the fingers^［18,25］. Thus, the question of whether surface temperature changes in connection with acupoints follow some natural rules is in urgent need of an answer. It has been hypothesized that the different skin temperature responses induced by acupoint stimulation were due to excitation of thermal nociceptors or the autonomic nervous system^{［18,26-28］}. However, all these findings failed to explain exactly which acupoints exhibited cooling effects and which acupoints had a warming function. Here, we reported that acupuncture stimulation at particular acupoints from the same meridian elicited opposite changes in temperature. According to the philosophy of TCM, the 12 meridians and acupoints are classified based on five elements (metal, wood, water, fire, and earth) ^［5,14］. For instance, acupoints with warm and cool properties are believed to have opposite therapeutic effects in terms of temperature regulation. Our study showed that skin temperatures increased with fire acupoint stimulation, while temperatures decreased with water acupoint stimulation. However, we found that the fire acupoints are sometimes used to reduce heat in clinical practice. Nevertheless, the clinical effects of fire points are mainly generated with the help of specific acupuncture manipulations, such as drainage. In our experiment, the solo acupuncture stimulation excluded the interference of acupuncture manipulation. Specifically, volunteers were more sensitive and had a quicker response to the fire acupoint stimulation than to the water acupoint stimulation. However, not all the individuals showed positive results. There were 2 individuals in the fire acupoint stimulation group who failed to show increased temperatures, whereas there were 5 subjects who exhibited negative findings with water acupoint stimulation (Table 5). This phenomenon might relate to personal responsiveness to the acupoints, and future studies with a longer observation period and a greater sample size are needed.

Second, we found that the skin temperature in the fire acupoint-stimulated group increased bilaterally, while stimulation of the water acupoints failed to exhibit this synchronized effect. In fact, synchronous findings have already appeared in TCM literature and have been used in TCM practice for a long tim^［29,30］. Paulson and Shay claimed that the synchronous activation was due to a sympathetic nervous system response^［31］. Li reported that stimulation of acupoints might trigger cutaneous sympathetic nerve fibres and activate local microcirculation, further leading to the synchronous effects^［18］. Zheng claimed that acupuncture stimulation might evoke changes in energy metabolism, including transcutaneous oxygen pressure/carbon dioxide pressure and microcirculation blood perfusion, that lead to visceral function adjustments such as synchronous temperature changes^［2］. Our findings not only provided strong evidence that acupuncture can have therapeutic effects through stimulating the healthy side, but also led to the hypothesis that there may be a close connection between meridians on both sides of the body.

Third, our experiment revealed some particular characteristics of the 3 yin meridians of the hand. First, we found that the temperature of the PC meridian was higher than that of the other 2 meridians before stimulation. Some studies have suggested that differences in temperature might be based on the anatomical location of the meridians^［8,32］. There are ulnar and radial vessels located near the lung and heart channels where the skin temperature easily changes, while there are few vessels passing along the PC meridian. Another finding was that the meridian temperatures had a close relationship with age. The middle-aged group had a higher temperature than the young group. Wang reported that middle-aged people had more skin blood perfusion in PC meridians than young people^［9］. Finally, it is worth noting that the LU meridian temperature fluctuated sharply and returned to baseline, while the PC and HT meridian temperatures continued to change after 30 min of stimulation. This indicated that the ideal therapeutic time for acupuncture treatment of the lung meridian might be 30 min, while the effective time for PC and HT meridians should be longer.

Table 1: Anatomical locations of the hand 3 yin meridians (LU, HT and PC)

Table 2：Relationship between the hand 3 yin meridians and influential factors

Table 3: Temperature of PC, LU, and HT Meridians

Table 4：Temperature of PC, LU, and HT Meridians Analyzed by Cohen’s Classification Scheme

Table 5: Number of Cases with Reverse Responses to Fire and Water Acupoint Stimulation

Conclusions

Our study monitored the average skin temperature of the 3 yin meridians and found that the PC meridian temperature was the highest in healthy people. The meridian temperatures showed no relationship with sex, body side, or season, but did show a relationship with age. The main findings of the present study revealed that acupuncture stimulation at fire and water acupoints resulted in differential temperature responses in the same meridian. However, given that only 3 meridians at a limited number of sites were measured and compared, our investigation was not sufficiently robust. More systematic research should be conducted to investigate more meridians and more measurement sites in the future.

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