Thermal Properties of Direct and Indirect Moxibustion

1. Introduction 

In traditional medicine from the Western Pacific region, moxibustion is one of the main therapeutic modalities, delivering heat to specific areas such as meridian points. Heat stimulation [1, 2, 3] and some pharmacological effects from moxa (Artemisia spp.; mugwort) [4, 5] have been attributed to the therapeutic efficacy of moxibustion. The efficacy of the chemicals in moxa is enhanced by the combustion heat of the moxa, promoting the hypodermic transmission of beneficial chemicals. Therefore, it is apparent that heat plays a major role in the efficacy of moxibustion. However, moxibustion has some disadvantages, including the possibility of burns from excessive heat [6]. To resolve this issue, it is imperative to study the thermal characteristics of moxibustion.

Moxibustion therapy incorporates direct moxibustion and indirect moxibustion. In direct moxibustion, the moxa contacts the skin surface directly and for indirect moxibustion, buffer substances such as ginger, garlic or salt are placed between the skin and moxa. Air or paper can be used as buffer layers for modern indirect moxa devices [7]. The presence of additional buffer layers gives indirect moxibustion unique thermal characteristics. Chiba et al reported the thermal and antiradical properties of indirect moxibustion and noted that the gap between moxa and the skin was more important than moxa weight [8]. They also stated that the maximum temperature induced by indirect moxibustion was about 65°C on the skin surface, and 45°C in the subcutaneous layer [9]. A research group measured the radiation curve of direct and indirect moxibustion to develop moxibustion instruments [10]. A study of infrared radiation (IR) from indirect moxibustion suggested that there is a difference in IR between non-traditional and traditional thermal materials [11].

In spite of its many clinical applications, few studies have been reported on the efficacy of moxibustion, partly due to the lack of a reliable sham moxibustion. To further study themoxibustion effect, a robust sham moxibustion was recently introduced [12].

In this report, we measured the temperature of both direct and indirect moxibustion on garlic at different time course. The optimum condition for indirect moxibustion was determined and was in accordance with the traditional method on the consecutive number of cones and thickness of a garlic slices. To examine the environmental effect on the combustion of moxa, the airflow dependency of both types of moxibustion was also studied. Some guidelines for the practicing clinical environment were suggested.

2. Materials & Methods 

2.2. Experimental set up 

The measurement set up is depicted in Figure 1. The set up was installed on a stainless steel lab bench with a forced ventilation system. An automatic temperature acquisition module and 0.08 mm thermocouple were used. As shown in Figure 1, a thermocouple was placed between an aluminum hot plate and the moxa cone for direct measurement, and between a garlic slice and the hot plate for indirect moxibustion. Detailed procedure is described in Kim et al 2009 [14].

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Figure 1. A schematic diagram illustrating the experimental set up.

3. Results 

3.1. Direct moxibustion 

We measured the temperature at the lower center of the moxibustion during various airflows (0-0.8 m/s), as shown in Figure 2. No abrupt change in the temperature was observed. With an increase in airflow, peak temperature increased and the curve became gradually narrower. The shape of the curve became considerably more symmetrical.

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Figure 2. Temperature curves of direct moxibustion with four different airflows.

To quantify the broadness of the temperature curve, we introduced duration time and maximum time. The duration time was defined as the time when the moxa temperature was higher than the hot plate temperature (34°C) during its combustion. The maximum time was determined as the time taken to reach the maximum temperature.

Figure 3 shows the duration time and maximum temperature of each moxibustion. There is an anti-correlation between the duration time and maximum temperature. The maximum time is presented in Table 1 and clearly demonstrates the amount of peak shift. To find the change in rate on a temperature curve, we simply calculated the slopes of both sides at each temperature curve, as shown in Figure 4. To obtain the values in the figure, two points on the same side of a curve were selected, which corresponded to 20% and 80% of its peak value.

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Figure 3. Duration time and maximum temperature of direct moxibustion with all airflows. A clear anti-correlation between the two properties can be observed.

Table 1. Time taken to reach the maximum temperature of direct moxibustion with various air flows

	Airflow (m/s)	0.0	0.3	0.4	0.5	0.6	0.7	0.8
	Time (s)	224	223	151	118	108	117	104

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Figure 4. Comparison between increase rate and decrease rate of direct moxibustion temperature.

As shown in Figure 4, an anti-correlation is observed between the increase rate and decrease rate of each temperature curve. The change in rate varied rapidly at airflows of 0.3-0.4 m/s. Ac cording to these results, there seems to be a demarcation level between the airflows of 0.3 m/s and 0.5 m/s. The peak position, peak value and shape of the temperature curves clearly show the presence of the level.

3.2. Indirect moxibustion 

3.2.1. Determination of the optimum garlic thickness 

We measured the temperature of indirect moxibustion on garlic slices of various thicknesses. Some results are shown in Figure 5. Results of four consecutive moxibustion tests with each single garlic slice are depicted in the same figure for easy comparison.

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Figure 5. Results of four consecutive moxibustion on a single garlic slice of (A) 1.0 mm, (B) 1.3 mm, (C) 1.5 mm, (D) 1.7 mm, (E) 2.5 mm and (F) 3.0 mm.

With 1.0-1.7 mm slices, therapeutic temperature was achieved up to the third moxibustion and the curves showed broad therapeutic windows in this condition. The maximum temperature of the fourth moxibustion was higher than 50°C, which was outside the therapeutic window. By contrast, the temperature using the slice thickness of 2.5 mm and 3.0 mm never reached 50°C. However, these indirect moxibustions revealed relatively small therapeutic windows. For the 3.0 mm slice, the first two moxibustions did not even reach 40°C. These results implied that there is little therapeutic effect with slice thickness of 2.5 mm and 3.0 mm and these indirect moxibustions are almost of no use. Therefore, based on these results, we selected the 1.7 mm thickness as the optimum condition for indirect moxibustion and executed airflow dependency measurement with it.

In Figure 5, the temperature curve of a 0.5 mm slice was omitted as the maximum temperature reached 90°C, which was obviously much higher than the other thicknesses. This result clearly demonstrated that there is a lower limit for the thickness of a buffer layer for insuring efficacies of indirect moxibustion.

3.2.2. Airflow dependency 

Figure 6 represents typical temperature curves obtained by an airflow dependency experiment. As airflow increases, the curves shift to the left and their peak temperature decreases from 45°C to below 40°C. The decrease in temperature is exactly opposite to that of direct moxibustion where temperature goes from 130°C up to higher than 300°C. The curve became narrower with increased airflow, demonstrating that both the duration and therapeutic time were shortened. Airflow dependency of the maximum temperature and therapeutic time is summarized in Figure 7 and the duration time in Table 2.

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Figure 6. The temperature time course of airflow dependency measurement with the optimum garlic thickness of 1.7 mm.

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Figure 7. Maximum temperature and duration time of a 1.7 mm garlic slice, extracted from the results in Figure 6.

Table 2. Duration time of indirect moxibustion using a 1.7 mm garlic slice with various airflows

	Airflow (m/s)	0.0	0.3	0.4	0.5	0.6	0.7
	Time (s)	231	228	238	199	152	149

4. Discussion 

Our studies have focused mainly on the primary factors that affect the thermal characteristics of moxibustion and the optimum therapeutic conditions of indirect moxibustion. We believe that these studies are necessary to understand the mechanisms involved in moxibustion therapy and to develop effective moxibustion devices.

Our data showed the importance of airflow control, regardless of the nature of moxibustion. Differences in the maximum temperature of both types of moxibustion were attributed to the presence of a garlic slice. However, the decrease of the maximum temperature of indirect moxibustion with airflow requires explanation. The transport rate of thermal energy to the thermocouple is reduced for a reason.

We hypothesized that the difference in the specific heats of air inside the moxa and water in the garlic slice as well as the upflow might be responsible for the result. Ignited moxa heated the air surrounding moxa tissues and this hot air transferred thermal energy to the bottom of the moxa by convection, where a thermal detector was installed. However, for indirect moxibustion, hot air must first warm a garlic slice, which has a very high specific heat compared with air. As airflow increased, more hot air moved to the top of the moxa and less hot air delivered its thermal energy to the slice. Since the thermal detector was located below the slice, the temperature acquired should be lower with airflow. If this explanation is valid, both the importance of the amount of air contained inside moxa and the affect of convection as a heat transport mechanism need to be reconsidered.

According to the results of direct moxibustion, environment control, such as ambient airflow, seems to be crucial to ensure reliable and repeatable moxibustion. Results in Figure 3, Figure 4 show that airflow dependency of the duration time and maximum temperature in the slower flow regime is less severe than that in the faster flow regime. Any action increasing the airflow around moxibustion might cause unwanted heat stimulus to patients during the treatment. Practically, controlling temperature and heat of moxibustion as a function of time is not easy in clinical application and this may cause some inconsistency in moxibustion efficacy.

The results of indirect moxibustion on garlic slices with different thicknesses provided valuable clues on the mechanism of efficacy in moxibustion treatment. For moxibustion on garlic or ginger, there have been certain rules in Traditional Korean Medicine on the usage of the buffer layers for multiple moxibustions at a single position. The thickness of the buffer layer is approximately a half of 10th cun (1 cun = 33 mm) and the layer must be replaced after three cones (the number of moxibustion) have been used [16]. The recommended thickness of the layer is about 1.6-1.7 mm, which is in agreement with our optimal thickness judged by its thermal properties. During consecutive moxibustions with slices of 1.0-1.7 mm thickness, the fourth moxibustion showed a peculiarly high temperature of more than 50°C, which called for replacement with a new slice. These two facts strongly suggested that the reason for introducing these buffer layers is to control or modulate the heat from ignited moxa. The potential for utilizing the chemicals from moxa seems to be unlikely as the garlic slice prevents them from reaching the skin surface.

Our experiments have several limitations. We used forced upstream airflows for our measurements as the simplest possible case. In reality, the direction of airflow is arbitrary and therefore, further study is necessary to reflect the effect of arbitrary airflow occurring in the clinic. Measurement results would be affected by the aluminum hot plate. Aluminum and the skin have very different thermal properties such as specific heat and thermal conductivity and, in addition, the skin has its own fluid cooling system to cope with excessive external heat. Therefore, in clinical application, the moxibustion temperature needs to be higher to stimulate the skin.