Heterogeneity of Skin Oxygen Density Distribution: Relation to Location of Acupuncture Points

1. Introduction 

Acupuncture has been used for longer than 2500 years in traditional Eastern medicine. Nowadays, it is widely used as an alternative and complementary therapeutic treatment even in Western cultures [1]. Fourteen known main meridians, relating to internal organs, pass through and connect acupuncture points (APs) on the skin. Previous research indicated that acupuncture increases blood flow [2], and that APs and meridians show high electrical conductance [3, 4]. Recently, modern techniques, such as positron emission tomography imaging [5] and functional magnetic resonance imaging [5, 6], have been used to study the neuronal activity changes in the brain during acupuncture treatments. However, the mechanisms of action of acupuncture at the various meridians are not yet clearly understood.

In acupuncture practice, Qi, the vital energy, is considered to flow throughout the whole body via meridians which interconnect APs. Recently, it was reported that nitric oxide synthase expression was higher at skin APs and meridians compared with other areas [7]. Nitric oxide is known to be an important signaling molecule in vasodilatation directly connected to blood flow and volume [8], thus increasing delivery of oxygen (O2) to tissue is necessary for energy metabolism. These separate observations indicate that APs are possibly associated with body O2 supply.

In this paper, we demonstrate the real-time quantitative measurements of O2 levels on two different APs and on the corresponding non-APs using a highly sensitive electrochemical O2 microsensor. The sensor had the small planar sensing disk (diameter = 25 μm). Therefore, it was able to monitor the localized O2 levels at the confined APs with effective dimensions were known to be small.

2. Materials and Methods 

2.2. O2 measurements on skin 

For the measurements of O2 levels, a drop of PBS (pH = 7.4) solution was applied to the skin area of interest. Then the planar O2 sensor was immersed in the PBS solution and carefully positioned above the skin AP maintaining the separation ∼ 1 mm while the sensor currents were recorded as shown in Figure 1A. Once the measured currents, proportional to the O2 levels, was stabilized, the sensor was moved horizontally to the corresponding non-APs where the sensor currents were measured until stable currents were obtained. The whole procedure was repeated three times.

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Figure 1. (A) Schematic representation of the positioning of an oxygen (O2) microsensor for the measurement of O2 levels over hand skin. (B) The positions of analyzed points on the hand skin: LI 4 (▴) and the corresponding non-acupuncture point (▵), PC 8 (●) and the corresponding non-acupuncture point (○). PBS = phosphate-buffered saline.

Sensor currents responding to O2 levels were measured at two different APs [large intestine 4 (LI 4, Hegu), pericardium 8 (PC 8, Laogong)] and the corresponding non-APs, or non-APs, (3-5 cm away from the APs) using the prepared O2 microsensors. The positions analyzed are shown in Figure 1B. The measurements were performed at room temperature on five healthy volunteers (average age = 23 years) in calm and restful conditions. None of the subjects had previously inserted acupuncture needles at the skin locations investigated. The measured sensor currents were converted to the corresponding O2 levels using prior calibration data.

3. Results 

The sensor was positioned near the skin surface of the LI 4 point to monitor O2 levels. After measurement for a certain time to obtain stable signals, the sensor was moved horizontally to the non-APs and positioned near the skin surface while maintaining the sensor vertical, away from the skin in a similar manner to that used to measure the APs.

Figure 2 shows typical partial O2 pressure-time curves obtained. The partial O2 pressures (pO2) were calculated from the measured sensor currents using the sensor calibration data prior to the measurements. For all five volunteers, without exception, high O2 densities were observed at the LI 4 and PC 8 APs compared with the non-APs. The measured pO2 were 146.9 ± 23.7 mmHg (vs. 141.8 ± 22.7 mmHg at the corresponding non-AP) and 169.7 ± 28.5 mmHg (vs. 164.4 ± 27.4 mmHg at the corresponding non-APs) at LI 4 and at PC 8, respectively. A wide range of background O2 levels (with large standard deviation) near the skin were observed, depending on the subject. The variation may be due to the individual physiological condition of the volunteers. Thus the measured pO2 were normalized for statistical analyses and comparison. In fact, the normalized pO2 was obtained as follows:

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Figure 2. Representative measured partial oxygen pressures as a function of time when the sensor was positioned over (A) LI 4 and the corresponding non-acupuncture point (AP) and (B) PC8 and the corresponding non-APs. Gray-colored regions represent the measurements made over non-APs.

pO2normalized = pO2average/pO2non-APs

where pO2normalized is the normalized partial oxygen pressure, pO2normalized is the average of the partial oxygen pressures measured, pO 2non-APs is the average of the partial oxygen pressures measured at non-APs.

Thus pO2normalized at non-APs was always 1. The calculated pO2normalized values at the APs (LI 4 and PC 8) compared with pO2normalized at non-APs are shown in Figure 3. Each pO2normalized value was obtained by averaging three repetitive measurements at the corresponding same location for each subject. The average values of the pO2normalized for five subjects were 1.036 ± 0.016 and 1.032 ± 0.012 at LI 4 and PC 8, respectively.

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Figure 3. Normalized oxygen levels measured at (A) LI 4 and (B) PC 8 points for five subjects. Dashed lines represent normalized oxygen levels measured at the corresponding non-acupuncture points.

The normal distribution of the obtained data was confirmed by a normal probability plot. Using a two-tailed t test (with a significance level of p < 0.05), the measured O2 levels at LI 4 or PC 8 points were significantly different (p = 0.009 and p = 0.002, respectively) compared with the corresponding non-APs.

4. Discussion 

According to the experimental results, APs were clearly distinguished from non-APs in terms of higher O2 densities around the APs. It should be noted that the measured O2 levels were dependent on the distance between the sensor end plane and skin. Indeed, higher O2 levels were observed when the sensor-skin separation was small. Therefore, the measurements of O2 levels were carried out while maintaining the sensor-skin vertical distances as similar as possible at both APs and non-APs. Even with care, there could be slight differences in the distance between skin and sensor when the sensor was moved and repositioned over different locations. However, the observed O2 level variations depending on the vertical distances (at the same locations) were smaller than those between APs and non-APs: ≤ 1.75 mmHg changes in pO2normalized between 0.5-1.5 mm separations versus 4.7-8.8 mmHg difference in pO2normalized between APs and non-APs. Therefore, the observed higher pO2normalized at the APs can be considered meaningful.

Although the pO2normalized differences between the APs and the non-APs were small (ca. 2-5% in pO2normalized; i.e. ca. 4.7-8.8 mmHg in absolute pO2), they were sufficiently large to be differentiated with our high sensitive O2 microsensor. Recently, Zhang et al reported that higher transcutaneous carbon dioxide emissions were observed on 12 points along the Pericardium meridian line compared with control points beside the meridian line [10]. Our observation of higher O2 levels at the APs than at the non-APs correlates with Zhang's report and may provide a crucial component of the biological/physiological functions of the APs.

The observed high contents of O2 at the APs may reflect the relatively reduced O2 uptake from the atmosphere through the APs in comparison to the non-APs. Stücker et al suggested that the O2 supply to skin is a balance between O2 transported by blood and uptake from the atmosphere [11] and also demonstrated that the transcutaneous O2 flux is increased by interruption of blood flow to skin on the volar forearm [12]. Therefore, the higher O2 levels measured at the APs may imply that the O2 supply by capillary O2 transport is greater at the APs than at the non-APs. Possibly large blood vessels or primo-nodes (Bonghan corpuscle)/vessels, which were proposed to be corresponded to APs by Dr B.H. Kim [13] and recently rediscovered by a group from Seoul National University [14], could be present directly underneath the skin APs. To confirm this conjecture, further research (e.g. anatomical studies) is required.

Our current investigation may provide scientific evidences for the physical existence and physiological functions of APs in which traditional Eastern medicine has believed for many years.