Effect of foot bathing on distal-proximal skin temperature gradient in elders

Abstract

Increased distal (foot)-proximal (abdominal) skin temperature gradient (DPG) has been associated with better sleep initiation. Warm footbath can affect distal skin temperature to change DPG. However, the optimum water temperature and duration necessary to raise DPG has not been established. This study explored the effects of 1-h foot bathing at two water temperatures of 40 and 41 °C, respectively, on DPG in Taiwanese elders ($n=6$, ages 60–73 years). Each subject's feet and legs were immersed in a temperature-controlled water tub to 20 cm above the ankles for 60 min in each of two water temperatures. Oral, abdominal, and foot temperatures were taken during (at 10-min intervals), and after (at 1-min intervals) foot bathing. DPG was calculated by subtracting abdominal temperature from foot temperature. Results showed the value of DPG was significantly increased in the 10th min bathing at both water temperatures and maintained above 0 °C. DPG gradually declined after bathing at both water temperatures. The value of DPG with 41 °C water was slightly higher than 40 °C. All subjects tolerated both bathing temperatures well for 1 h. Both 40 and 41 °C foot bathing for 1 h can increase the DPG and may be an effective way to affect whole body skin blood flow and trigger heat dissipation.

Introduction

Sleep disturbances are common in older people. The overall prevalence of insomnia ranges from 14.0% to 42.2% in the general older population (Chiu et al., 1999; Foley et al., 1995; Kim et al., 2000; Newman et al., 1997; Ohayon, 1996, Ohayon, 1997; Ohayon and Zulley, 2001; Rocha et al., 2002). Sleep disturbances in older people are partially due to a decline of heat dissipation from the body core to the periphery. Interventions that enhance heat dissipation prior to sleep may improve sleep in older people.

Thermoregulation exhibits powerful interactions with sleep. Studies have shown that decreased core (rectal) temperature and increased distal (foot) temperature are associated with shortened sleep-onset latency (Krauchi and Wirz-Justice, 1994; van den Heuvel et al., 1998) and increased NREM sleep (Burgess et al., 2001). Decreased rectal temperature is induced by the vasodilatation of peripheral vessels in the skin, which produces an increase of distal temperature and facilitates heat loss from the core of the body to the periphery (Krauchi and Wirz-Justice, 2001). The gradient of temperature from proximal body sites (infraclavicular, thigh, stomach, forehead) to peripheral sites (feet and hands) is an indirect measure of heat dissipation or heat loss from the core to the periphery. This gradient of temperature is called distal-proximal skin temperature gradient (DPG). It has been used as a measure of skin blood flood and as an indirect index of distal heat loss (Krauchi et al., 1999). From observational studies, DPG (reaches 0 °C and above) is associated with sleepiness that occurs before sleep onset (van den Heuvel et al., 1998) and has been reported as the best predictor of the body's readiness for sleep (Krauchi et al., 2000, Krauchi et al., 1999; Krauchi and Wirz-Justice, 2001). Sleep latency (time to fall asleep) was significantly shorter when DPG value reached 0 °C before lights out.

Skin blood flow plays an important role in body heat conservation and dissipation. Warming of skin causes vessel dilation and induces heat dissipation from the core to the periphery. It has been established that local temperature of 42 °C for 35–55 min causes maximal dilation of local skin blood vessels in adults ages 18–75 years old (Charkoudian, 2003; Kellogg et al., 1998; Minson, 2003; Taylor et al., 1984). Minson et al. (2001) demonstrated a typical biphasic skin blood flow during 50–80 min of 42 °C local heating of the right arm in healthy young adults. Skin blood flow was measured by cutaneous red blood cell flux via laser-Doppler. In their local heating protocol, skin temperature rapidly increased to 39–39.5 °C and resulted in a fast increase in skin blood flow to an initial peak during the first 3–5 min, followed by a transient drop to a nadir, and then a secondary progressive rise to a plateau at 25–30 min of warming. After 50 min of warming, skin blood flow began to decline in some subjects. The local sensory nerves were involved in the initial rapid peak, whereas the nitric oxide mediated the second slow increase phase of skin blood flow (Charkoudian et al., 2002; Kellogg et al., 1998; Michikami et al., 2001; Minson et al., 2001; Roberts et al., 2002; Wilkins et al., 2003). The general pattern of cutaneous vasodilatation response to local warming at 42 °C was similar in both the older and young adults, but response was slower in older people than in young adults (Holowatz et al., 2003; Martin et al., 1995).

The above studies establish mechanisms of local warming on local skin blood flow as measured by local red blood cell flux. To our knowledge, no study has reported the effect of local warming on whole body skin blood flow as measured by DPG. We do not know if local warming can affect whole body vessel dilatation. Moreover, the sensory nerves involved in the first vasodilatation response are primarily c-fibre afferents, which are also nerves that conduct pain sensation. Heat sensation along with pain is vital to protect skin from acute damage. A diminished ability in nerve response to local warming makes older people susceptible to local tissue damage. Therefore, though 42 °C local heating for 35–55 min can exhibit a maximum skin blood flow, such a high temperature may cause pain sensation and injury. Optimum water temperature and duration of local warming such as footbath to raise whole body skin blood flow has not been established. This study explored the effect of warm foot bathing at 40 and 41 °C water on skin blood flow measured by DPG in Taiwanese elders. Findings will guide the design of foot bathing intervention on the sleep quality in elders with insomnia.