This study heated the left feet of subjects using two different thermal stimuli in order to investigate their effects on cutaneous temperature and blood flow responses in the contralateral plantar foot. The results showed that both the warm bath and infrared radiation increase the cutaneous temperature and blood flow in the contralateral foot, and the warm bath has a stronger residual thermal effect.
Studies have shown that physiological thermoregulation is mainly controlled by the preoptic/anterior hypothalamus (POAH), which regulates vasodilation and vasoconstriction of blood vessels by the internal and/or skin temperature [16]. When applying a thermal stimulus to one side of the foot, the skin thermoreceptors in the heated area will signal the POAH and activate temperature-sensitive neurons. In the efferent pathway, the projections from the rostral ventrolateral medulla and hypothalamus to the intermediolateral column of the spinal cord are predominantly ipsilateral, but also produce contralateral signals. Thus, the contralateral thermoregulatory effectors will also receive signals to induce skin vasodilation [17]. Cranson et al. reported that the temperature of arterial blood flow increased by 0.5 °С within 4 minutes when the forearm was placed in water at 40 °C[18]. The increased temperature of arterial blood flow would activate the central thermal receptors, consequently inducing vasodilation of bilateral skin vessels and increasing peripheral blood flow and cardiac output[19, 20]. Thus, a thermal stimulus applied to one side of the foot can induce contralateral responses in skin temperature and blood flow by the innervation of peripheral and central thermal receptors.
The plot in Fig. 2 shows that the residual effects of the two thermal stimuli on contralateral skin blood flow are different, and the skin temperature in the initial recovery stage is also different between the two tests (Fig. 3). The skin blood flow of M2 in the contralateral foot was significantly higher than the basal blood flow for 5 minutes after removal from the warm bath, while the temperature was only significantly higher for the first 1 minute after removing the infrared radiation heat. Moreover, when considering the increase in temperature from the baseline, the warm bath test showed a significantly greater increase in temperature than the infrared radiation test during the first minute of the Recovery stage. This indicates that heating the foot with a warm bath leaves a stronger residual contralateral effect than infrared radiation, which may be due to the different contralateral regulatory mechanisms between the two thermal stimuli.
Marshall et al. reported that if one hand was heated by a warm bath for 2 min, blood flow in the contralateral hand would increase, and there would be a further increase in blood flow and cardiac output after 5 minutes of heating[11]. The elevated cardiac output during thermal stimulation is primarily caused by the increase in internal temperature[21]. This suggests that the reflexive vasodilation of skin vessels in the contralateral limb during heating is not only mediated by skin thermoreceptors in the heated limb, but also by the central thermoreceptor activated by the increased temperature of arterial blood flow. Moreover, the perception and conduction of temperature by skin thermoreceptors occurs rapidly, but the peripheral response mediated by the central thermoreceptor is relatively slow. It can be speculated that when the source of heat is removed, the stimulation of skin thermoreceptors ceases and afferent signals to the POAH are greatly reduced. However, the internal temperature of arterial blood would not drop immediately, and would take longer for the volume and temperature of blood to recover to baseline values because the heat needs to be dissipated though vessel vasodilation. Thus, in this study, heating in a warm bath for 10 minutes may stimulate both the peripheral and central thermoreceptors by increasing the temperature of skin and arterial blood flow, resulting in a longer recovery time and a stronger residual effect on subjects’ contralateral feet.
When soft tissue is heated by infrared radiation, heat will be generated by resonance and friction between molecules because the vibration frequency of partial far infrared rays is close to that of intracellular molecules in soft tissue, which causes an increase in temperature and accelerates blood circulation[22, 23]. The thermal effect in the heated side of limbs will be regulated by the thermoregulation of central nervous systems, making the temperatures in the contralateral side and the irradiated side close[24]. Another theory is that, the infrared radiation can induce the release of cytokines and growth factors in the circulation, contributing to the vasodilation of ipsilateral and contralateral vessels[12, 25]. However, the far infrared rays can only be transmitted into skin tissue in a depth of 3–5 mm, and may have a relatively slight effect than warm bath, which can induce vessels vasodilation regulated by both peripheral and central thermoreceptors[11, 13, 21].
Another reason for the different residual effects may be the conducting medium. Water is a good conductor and can transfer heat to the whole foot. When heating with infrared radiation, the stagnant air may attenuate heat from radiator to plantar foot[26]. To mitigate the difference in the heat transferred to the foot, the temperature of the measurement areas under infrared light was ensured to be 40 °С by controlling the device.
The second metatarsal head (M2) typically experiences high plantar pressure when walking and, as such, is a common site for foot ulcers in diabetics with impaired microcirculation [27]. Thus, this study chose M2 to investigate the thermal effects of two heating methods. Moreover, since 10 minutes of thermal stimulation can transmit heat stress into skin tissue to a depth of 1 cm, and a thermal stimulus with a temperature below 40 °С is generally safe[13, 28], this study used these parameters to achieve a thermal effect and avoid skin burns.
This study has some limitations that should be noted. Although the temperature of both thermal stimuli was 40 °С, it was not verified whether the two sources provided an equal level of heat to the foot needs to be further verified. In addition, this study only considered heating the feet of healthy subjects. Future studies may consider other limbs like dominant side or enrolling subjects with impaired microcirculation.
In conclusion, this study found that both the warm bath and infrared radiation are effective at increasing the temperature and skin blood flow in the contralateral foot, but there are differences in the duration and intensity of the residual effects. Heating with infrared radiation is generally more convenient for home use, and is more suitable for detecting nerve and vascular function because of the instantaneous change in temperature and blood flow in the contralateral limb. The warm bath has a better thermal effect, and may be more suitable for use by people who suffers from skin lesions or wears an external fixator, and cannot accept the heat source being focused on one side of the limb.