Michelle Prowse

Skin heat dissipation: the influence of diabetes, skin thickness, and subcutaneous fat thickness.

BACKGROUND It is well established that diabetes impairs vascular endothelial function. However, the impact of impaired endothelial function on thermal conductivity of the skin, especially in relation to a constant versus a sudden heat stress, has not been established. Further, there is some evidence that aging reduces skin dermal thickness and subcutaneous fat thickness. Since these are important determinates of heat dissipation by the skin, these parameters also need to be examined in people with diabetes. METHODS Ninety subjects (30 younger individuals, 30 patients with diabetes, and 30 patients age-matched to the diabetes subjects) participated in two series of experiments to determine (1) the thickness of the subcutaneous fat layer and skin thickness and the skin response to a sudden heat stress and (2) the response to a continuous heat stress on the lower back. Skin thickness and subcutaneous fat thickness were assessed by ultrasound, and skin blood flow was examined by infrared laser Doppler flow meter. RESULTS People with diabetes had significantly less resting blood flow, blood flow in response to a single or continuous heat load, less subcutaneous fat, and thinner skin than either age-matched controls or younger people (P < 0.05). Subjects with diabetes also had the lowest concentration of red blood cells in their skin, implying a reduction in the number of capillaries in the skin. CONCLUSIONS Thinning of the skin and probably a reduction in capillaries in the dermal layer contribute to a reduction in the blood flow response to heat. People with diabetes, in particular, have reduced skin heat dissipation because of less resting blood flow and thinner skin than that seen in age-matched controls.

Dry heat, moist heat and body fat: are heating modalities really effective in people who are overweight?

Surface heating modalities are commonly used in physical therapy and physical medicine for increasing circulation, especially in deep tissues, to promote healing. However, recent evidence seems to indicate that in people who are overweight, heat transfer is impaired by the subcutaneous fat layer. The present investigation was conducted on 10 subjects aged 22–54 years, whose body mass index averaged 25.8±4.6. Subcutaneous fat above the quadriceps muscle varied from 0.51 to 0.86 cm of thickness. Three heating modalities were examined: the application of dry heat with a commercial chemical heat pack, hydrocollator heat packs (providing a type of moist heat), and a whirlpool, where conductive heat loss through water contact would be very high. The temperature of the skin and the temperature in the muscle (25 mm below the skin surface) were assessed by thermocouples. The results of the experiments showed that for heating modalities that are maintained in skin contact for long periods of time, such as dry heat packs (in place for 6 hours), subcutaneous fat did not impair the change in deep muscle temperature. In contrast, when rapid heat modalities were used, such as the hydrocollator and the whirlpool (15 minutes of sustained skin contact), the transfer of heat from the skin to deep muscle was significantly impaired in people with thicker subcutaneous fat layers. We observed that the greater the impairment in heat transfer to muscle from skin covered by body fat, the warmer the skin temperature increase during the modality.

The effects of skin moisture and subcutaneous fat thickness on the ability of the skin to dissipate heat in young and old subjects, with and without diabetes, at three environmental room temperatures

The Pennes model predicts the ability of the skin to dissipate heat as a function of conductive heat transfer and blood flow. Conductive heat exchange may be affected by skin moisture and subcutaneous fat thickness, factors not considered by Pennes. In the present investigation, we sought to expand the Pennes model by examining subcutaneous fat and skin moisture as factors of heat dissipation and their effects on heat exchange and blood flow. Subjects who were older (O) (mean age 64.2+/-5.9 years, n=15), had diabetes (D) (mean age 62+/-5.9 years, mean duration 13.2+/-9.1 years, n=15), and were younger (Y) (mean age 25.7+/-2.9 years, n=15) participated. Thermisters were placed in an iron heat probe and on the skin to measure the change in skin temperature to create a thermal change index to demonstrate the ability of the skin to dissipate heat. The lower back had the thickest subcutaneous fat layer for all subjects, which contributed to higher skin temperatures than the foot and hand in response to local and global heat. There was a significant inverse correlation between skin moisture and skin temperature after 5s of heat application (r=-0.73, p<0.001) with O and D having significantly less skin moisture than Y (p<0.05). O and D had significantly increased skin temperatures in response to local heat, as compared to Y, in all global temperatures (p<0.05). Thus, the Pennes model may need to be adjusted to take into consideration aging, diabetes, skin moisture, and subcutaneous fat thickness.

Does skin moisture influence the blood flow response to local heat? A re-evaluation of the Pennes model

Pennes first described a model of heat transfer through the limb based only on calories delivered from a heat source, calories produced by metabolism and skin blood flow. The purpose of this study was to determine the effect of a moist versus a dry heat source on the skin in eliciting a blood flow response to add data to this model. Ten subjects were examined, both male and female, with a mean age of 32.5 ± 11.6 years, mean height of 172.8 ± 12.3 cm, and mean weight of 77.6 ± 19.5 kg. Skin temperature was measured by a thermocouple placed on the skin and skin blood flow measured by a laser Doppler flow meter. The results of the experiments using a dry heat pack (commercially available chemical 42°C cell dry heat source), moist hydrocollator pack (72.8°C) separated from the skin by eight layers of towels, and whirlpool at 40°C, showed that moist heat caused a significantly higher skin blood flow (about 500% greater) than dry heat (p < 0.01). Most of the greater increase in skin blood flow with moist heat was due to the greater rate of rise of skin temperature with moist versus dry heat while some of the increase in blood flow was due to the moisture itself. This could either be related to the greater heat flux across the skin with moist air or due to changing the ionic environment around skin thermo receptors by keeping the skin moist during heating. Skin thermo receptors are believed to be temperature sensitive calcium gated channels in endothelial cells which couple calcium influx to a release of nitric oxide. If true, reducing moisture in the skin might have the effect of altering ionic flux through these receptors. A correct model of skin heat flux should therefore take heat moisture content into consideration.

Interrelationships between body fat and skin blood flow and the current required for electrical stimulation of human muscle

There is variability between individuals in the current needed to elicit a contraction in human muscle with surface electrodes. To understand what might be causing some of this variability, 25 subjects whose average age was 24.4+/-2.3 years, whose height was 165.5+/-9.5 cm, and whose average weight was 70.3+/-21 kg were examined. Electrical stimulation was applied above the motor point of the quadriceps, biceps, and lateral gastrocnemius muscles. To assess body fat, 2D ultrasound was used with a 1cm stand off. Electrical stimulation was applied with sine wave stimulation at 100 micros pulse width and at a frequency of 30 Hz. To alter skin blood flow, aside from the natural difference in skin blood flow at rest, hot packs and cold packs were used for 5 min. The average fat thickness below the quadriceps and gastrocnemius muscles was 0.75+/-0.13 cm and under the biceps was 0.48+/-0.16 cm. Without the use of hot or cold packs, the currents for the quadriceps and gastrocnemius muscles were significantly higher than that of the biceps (p<0.01). While there was some relationship between stimulation current and blood flow without the application of hot or cold packs, when hot packs were applied, skin blood flow increased as did the current required to stimulate muscle to threshold. When cold packs were applied, there was a decrease in the current required to stimulate these muscles. In conclusion, there is a causal relationship between skin blood flow, the thickness of the fat layer below the skin, and the current required to stimulate the muscle.

The Influence of Environmental Temperature on the Response of the Skin to Local Pressure: The Impact of Aging and Diabetes

BACKGROUND To protect against ischemia, pressure-induced vasodilation (PIV) causes an increase in skin blood flow. Endothelial dysfunction, which is commonly found in older patients and those with diabetes, and global temperatures can affect the resting blood flow in skin, which may reduce the blood flow during and after the application of local pressure. The present study investigated the PIV of the skin with exposure to three global temperatures in younger and older populations and those with diabetes. MATERIALS AND METHODS Older subjects (n = 15, mean age 64.2 +/- 14.0 years), subjects with diabetes (n = 15, mean age 62 +/- 5.9 years, mean duration 13.2 +/- 9.1 years), or younger subjects (n = 15, mean age 25.7 +/- 2.9 years) participated. An infrared laser Dopler flow meter was used to measure skin blood flow on the bottom of the foot, lower back, and hand during and after applications of pressure at 7.5, 15, 30, 45, and 60 kPa at 16 degrees C, 24 degrees C, and 32 degrees C global temperatures. RESULTS The resting blood flow for all subjects was significantly lower in the 16 degrees C environment (P < 0.05). Blood flow in the group with diabetes was significantly lower at rest, during the application of all pressure, and after the release of pressure in all global temperatures (P < 0.05). The younger group showed a significant increase in blood flow after every pressure application, except 7.5 kPa, in all global conditions (P < 0.001). Older subjects and patients with diabetes did not have a significant reactive hyperemia, especially in the 16 degrees C environment. CONCLUSION The protective mechanism of PIV is severely reduced in older populations and those with diabetes, especially in colder environments where skin blood flow is already diminished.

Multiple stressors and the response of vascular endothelial cells: the effect of aging and diabetes.

BACKGROUND The present study examined the effects of local heat, global heat, and the interaction between these two endothelial stressors on the blood flow of the skin of the foot in people who are older and who have diabetes. METHODS Subjects who were older (mean age 64.2 +/- 5.9 years) and were younger (mean age 25.7 +/- 2.9 years) and subjects who had diabetes (mean age 62 +/- 5.9 years, mean duration 13.2 +/- 9.1 years) participated. Subjects were exposed to three global temperatures (16 degrees C, 24 degrees C, and 32 degrees C), and the blood flow response was recorded on the foot with a laser Doppler flow meter for 30 s following applications of local heat (30 degrees C, 33.5 degrees C, and 37 degrees C) using a Peltier junction to clamp the skin for 2 min. RESULTS All three groups significantly increased blood flow from the 16-24 degrees C environments for the 37 degrees C application of local heat (P(Younger) = 0.02, P(Older) = 0.02, P(Diabetes) = 0.01). Those with diabetes and those who were older only increased blood flow 5% and 6% from the 24-32 degrees C environment, which was not statistically significant (P(Older) = 0.12, P(Diabetes) = 0.14). CONCLUSIONS There appears to be considerable blood flow reserve in younger subjects to tolerate heat stress. In contrast, older subjects and those with diabetes reach a critical level after which additional heat does not cause in increase in blood flow.

The use of an isometric handgrip test to show autonomic damage in people with diabetes.

BACKGROUND Vascular endothelial and autonomic damage are hallmarks of type 1 and type 2 diabetes. However, while much has been published on impairment of the autonomic nervous system, much less has been published on the interrelationship between autonomic damage and exercise. STUDY DESIGN The present investigation examined the change in heart rate, blood pressure, skin and limb blood flow, and sweat during non-fatiguing (10% and 25% maximum strength [maximal voluntary contraction (MVC)]) and a fatiguing isometric contraction (40% MVC) in people with type 2 diabetes compared to younger and older controls to see if a simple handgrip test could show the extent of autonomic damage in people with diabetes. Fifteen younger subjects (30.6 +/- 8.6 years), 15 older subjects (65.8 +/- 8.8 years), and 15 subjects with diabetes (63.4 +/- 14.4 years) whose average percentage body fat was 40.1 +/- 12.9%, 36.1 +/- 9.3%, and 39.6 +/- 15.5%, respectively, participated in these studies. Whole forearm blood flow, skin blood flow, and sweat on the forearm, chest, and forehead were measured at rest and during and after a contraction at 10% MVC, 25% MVC, and 40% MVC. RESULTS Blood flows and sweat rates were greatest in younger subjects, significantly less in older subjects, and even significantly less in subjects with diabetes (P < 0.05). The heart rate response was unaltered during contractions at 10% and 25% MVC and less in diabetes than in the other two groups with 40% MVC. Strength was about half in the diabetes group than with the other two groups, but endurance was similar. CONCLUSIONS Diabetes is associated with a reduction in handgrip strength and significantly impaired autonomic function during and after isometric exercise.

Effects of a 2-, 3- and 4-electrode stimulator design on current dispersion on the surface and into the limb during electrical stimulation in controls and patients with wounds

Electrical stimulation is a widely used modality in the field of physical therapy and exercise physiology. The most common method for the application of electrical stimulation is a two-electrode system where one electrode is the source and the other is a reference. However, recent studies report that a more effective delivery system can be achieved if more than two electrodes are used. In the present investigation, the circuitry to deliver electrical stimulation through a 2-, 3- or 4-electrode delivery system was designed. The system was evaluated by its ability to deliver current on the surface of the skin as well as deep into the quadriceps muscle in six control subjects and in and around wounds in six other subjects. The results of the experiments showed that much better depth of penetration was achieved in a 4-electrode system (one electrode was on the opposite side of the limb and three electrodes were on top of the limb) than in either a 2- or a 3-electrode delivery system. In non-wounded skin, given the same current from the stimulator, the current in the quadriceps muscle was found to be double with a 4-electrode versus a 2-electrode system. In wounds, this same finding was seen. Here, blood flow, an indicator of the effectiveness of electrical stimulation in wounds, was three times higher if a multi-channel stimulator was used versus a 2-channel stimulator. Thus a multi-channel electrical stimulation system is more effective than a 2-electrode system.

The effect of body fat, aging, and diabetes on vertical and shear pressure in and under a waist belt and its effect on skin blood flow.

BACKGROUND Much attention has been given to the effect of pressure on skin blood flow in the feet of older people and people with diabetes. However, little attention has been paid to other areas of the body, especially under the belt at the waist where pressure might be high during body movements associated with exercise. This may be very important when devices such as heat packs are worn during the day under the belt because their safety relies on appropriate skin blood flow to dissipate the heat; in diabetes populations burns have been seen. METHODS Forty male and female subjects, with and without diabetes, were examined in two series of experiments to assess the vertical and shear pressure under a belt worn during different common exercises. Vertical and shear pressure under the belt, belt tension, and shear pressure were measured with a Tactilus (Sensor Products, Madison, NJ) pressure mapping system. Eleven different body movements were examined. Then, from the recorded pressures, a second series of experiments examined skin blood flow at these same pressures. RESULTS The results of the experiments showed that there was little shear and vertical pressure in thin subjects during 10 different exercises. However, for overweight subjects, pressure under the belt was as high as 150 kPa. At these high levels of pressure, skin circulation was occluded. CONCLUSIONS In subjects with diabetes who are generally overweight and have impaired circulation, hot packs should be used with caution because of the low blood flows at rest and occlusion of the circulation under the belt with body movement.