2007, Piran, Slovenia
2007, Piran, Slovenia 2007, Piran, Slovenia
Environmental Ergonomics XII Igor B. Mekjavic, Stelios N. Kounalakis & Nigel A.S. Taylor (Eds.), © BIOMED, Ljubljana 2007 THE EFFECT OF THERMAL AND NON-THERMAL FACTORS ON SKIN BLOOD FLOW Eugene H. Wissler The University of Texas at Austin Contact person: ehwissler@mail.utexas.edu INTRODUCTION Skin blood flow (SkBF) plays an important role in cardiovascular function and in human thermoregulation. For that reason, and also because skin is the only organ for which blood flow can be easily measured, cutaneous blood flow has been studied by physiologists for more than half a century. While those studies have established that SkBF is affected by central temperature (Tc), mean skin temperature ( s T ), local skin temperature (Ts), exercise & ), and posture, they have not produced a clear phenomenological description of the ( O r , V 2 relationship between skin blood flow and the controlling factors. All human thermal models require a description of skin blood flow. Generally, models define cutaneous perfusion rate as an empirical function of central temperature and mean skin temperature. Arbitrary parameters in the fitting function are usually chosen to achieve a good fit between key measured and computed temperatures for a limited number of cases, but seldom, if ever, are perfusion rates determined in that way compared with measured values. The objective of this investigation was to use published physiological data to define a suitable algorithm for computing skin blood flow in a new two-dimensional human thermal model. METHODS The classic books by Rowell and several hundred papers were read, and re-read, in an effort to understand how the independent variables mentioned above affect cutaneous blood flow. For a long time, that was a confusing, frustrating endeavor, because the papers were often contradictive. It finally became clear that a significant problem was that investigators were ignoring the multidimensional nature of the system. For example, several authors reported that exercise diminishes SkBF, while others reported that exercise either has no effect on SkBF, or enhances it, but no one asked whether the effect of exercise on SkBF was different when s T = 38 o C than when s T = 32 o C. The algorithm described below is faithful to underlying physiology and provides reasonable results when used in a new two-dimensional human thermal model. RESULTS The principal conclusion is that the local cutaneous perfusion rate can be represented as the product of three factors; that is, SkBF = DILAT(T , T , V& C s O , r ) x CVCM( Ts ) x CVCL(Ts ) . 2 (1) , T , V& , ) is the centrally mediated drive for active vasodilation, T ) DILAT(TC s O2 r 460 CVCM( s accounts for the reflex effect of T s on cutaneous vascular conductance, and ) CVCL(Ts accounts for the effect of Ts on cutaneous vascular constriction. Active vasodilation increases linearly with Tc above a threshold temperature (Tc,th) that depends on T s , O r , & and posture. V 2
Modelling (T , T , V& , ) = 1. 0 + α (T T , ) if Tc,th < Tc (2) DILAT C s O r c − 2 c th = 1.0 if Tc < Tc,th α is a gain constant. Experimental studies since 1970 established that Tc,th decreases approximately 0.1 o C for each degree C increase in T s . The effect of O r , V& on Tc,th was 2 controversial until Taylor, et al. (1988) and Smolander, et al. (1991) showed rather clearly that Tc,th is not affected by moderate levels of O r , V& (say 2 O r , V& < 0.5) when T 2 s is less than 34 o C. At higher intensities of exercise, Tc,th increases with O r , V& , as shown in Figure 1. The 2 solid line in Fig. 1 is a graph of the relationship 2 ΔTc , th = 1.41 ΔV − 0.194 ΔV + 0.01 (3) V& , − V& , / 1.0 − V& , & = 0.5. in which ( ) ( ) THRESHOLD CENTRAL TEMPERATURE INCREASE: C 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 ΔV = O 2 r O2 crit O2 crit , and O crit , V 2 40 50 60 70 80 90 100 OXYGEN CONSUMPTION RATE: % VO2 max Figure 1. Δ Tc,th for active vasodilation as a function of O r , V 2 & when TS < 33.0 . Filled and open circles identify plethysmographic and laser-Doppler data, respectively, of Smolander, et al., 1991; open triangles identify data of Taylor, et al., 1988. Also shown is the graph of Eqn. 3. While Eqn. 2 accounts for the effect of heavy exercise on SkBF at normal skin temperatures, it fails to account for the effect of moderate exercise on Δ Tc,th at high skin temperatures. For example, Eqn. 1 yields Δ Tc,th = 0 o C for V& , r ≈ 0. 5, which is at variance with the threshold O2 increments of 0.29 o C observed by Johnson and Park (1981) and 0.28 o C observed by Kellogg et al. (1991). However, those studies were carried out with s T = 38 o C. A possible solution for that dilemma is to assume that V& decreases with increasing T s , which is not O2 , crit unreasonable, because cutaneous blood flow and volume both increase when the skin temperature is high. We simply assume that V& decreases linearly from V& = 0.5 for T s ≤ 33 o C to O2 , crit O2 , crit O2 , crit V& = 0 for 38 o C. Then, if s T = 38 o C and & = 0. 5 , as in the study of V O2 , r 461
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Environmental Ergonomics XII<br />
Igor B. Mekjavic, Stelios N. Kounalakis & Nigel A.S. Taylor (Eds.), © BIOMED, Ljubljana <strong>2007</strong><br />
THE EFFECT OF THERMAL AND NON-THERMAL FACTORS ON SKIN<br />
BLOOD FLOW<br />
Eugene H. Wissler<br />
The University of Texas at Austin<br />
Contact person: ehwissler@mail.utexas.edu<br />
INTRODUCTION<br />
Skin blood flow (SkBF) plays an important role in cardiovascular function and in human<br />
thermoregulation. For that reason, and also because skin is the only organ for which blood<br />
flow can be easily measured, cutaneous blood flow has been studied by physiologists for<br />
more than half a century. While those studies have established that SkBF is affected by<br />
central temperature (Tc), mean skin temperature ( s T ), local skin temperature (Ts), exercise<br />
& ), and posture, they have not produced a clear phenomenological description of the<br />
( O r , V 2<br />
relationship between skin blood flow and the controlling factors.<br />
All human thermal models require a description of skin blood flow. Generally, models<br />
define cutaneous perfusion rate as an empirical function of central temperature and mean<br />
skin temperature. Arbitrary parameters in the fitting function are usually chosen to achieve a<br />
good fit between key measured and computed temperatures for a limited number of cases,<br />
but seldom, if ever, are perfusion rates determined in that way compared with measured<br />
values. The objective of this investigation was to use published physiological data to define<br />
a suitable algorithm for computing skin blood flow in a new two-dimensional human thermal<br />
model.<br />
METHODS<br />
The classic books by Rowell and several hundred papers were read, and re-read, in an effort<br />
to understand how the independent variables mentioned above affect cutaneous blood flow.<br />
For a long time, that was a confusing, frustrating endeavor, because the papers were often<br />
contradictive. It finally became clear that a significant problem was that investigators were<br />
ignoring the multidimensional nature of the system. For example, several authors reported<br />
that exercise diminishes SkBF, while others reported that exercise either has no effect on<br />
SkBF, or enhances it, but no one asked whether the effect of exercise on SkBF was different<br />
when s T = 38 o C than when s T = 32 o C. The algorithm described below is faithful to<br />
underlying physiology and provides reasonable results when used in a new two-dimensional<br />
human thermal model.<br />
RESULTS<br />
The principal conclusion is that the local cutaneous perfusion rate can be represented as the<br />
product of three factors; that is,<br />
SkBF = DILAT(T , T , V&<br />
C s O , r ) x CVCM( Ts<br />
) x CVCL(Ts<br />
) . 2 (1)<br />
, T , V&<br />
, ) is the centrally mediated drive for active vasodilation, T )<br />
DILAT(TC s O2<br />
r<br />
460<br />
CVCM( s<br />
accounts for the reflex effect of T s on cutaneous vascular conductance, and ) CVCL(Ts accounts for the effect of Ts on cutaneous vascular constriction.<br />
Active vasodilation increases linearly with Tc above a threshold temperature (Tc,th) that<br />
depends on T s , O r , & and posture.<br />
V 2
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