1284 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



resistance (/?) and inversely proportional to the blood 

 flow (V) and specific heat (c). Measurements of blood 

 flow using a device with two thermojunctions placed 

 on each side of a pair of heating electrodes seemed 

 to justify the above assumptions, and permit using 

 the following equation : 



1/-C-" 



AT 



0.239 



(I) 



According to this equation, the calibration curve is 

 hyperbolic. This type of curve has actually been 

 found in all thermostromuhr devices. However, quan- 

 titative measurements of AT (3) show values about 

 ten times higher than expected. This finding indicates 

 that the assumption of a uniformly heated cross sec- 

 tion of the blood vessel is not valid. The error in de- 

 termining AT is found to result from heating the 

 vessel wall much more than the blood. Due to the 

 complicated arrangement of electrical resistances to 

 high-frequency current in the wall, the liberation of 

 heat in the blood column amounts to only 10 per cent 

 in arteries and 20 to 40 per cent in veins (83). The 

 original assumption, therefore, must be revised: the 

 radial heat gradient is directed from outside to inside 

 the vessel and not, as suggested by Rein, from inside 

 to outside. The basic principle by which the stromuhr 

 measures flow is the change in T in the vessel wall 

 with blood flow, because of cooling it by the blood 

 stream. Findings based on this assumption are in good 

 agreement with the earlier results obtained with the 

 direct current method, showing that there is no basic 

 difference between the methods (4, 5, 74, 77). 



Further studies (3) on heat dissipation in the wall 

 of the vessel and in the blood stream have revealed 

 a temperature profile of complex nature. 



The temperature gradients are directed from out- 

 side to inside the vessel radially and also along the 

 length of the wall both upstream and downstream 

 with highest temperature underneath the heating 

 electrodes. 



This temperature profile, however, is not sym- 

 metrical for two reasons: first, since heating electrodes 

 are attached to a segment of the wall, the tempera- 

 tures measured in the plane of the heating electrodes 

 are higher than in a plane at an angle to it; second, 

 since the blood stream cools the upstream wall sec- 

 tion more than the downstream section, the tempera- 

 ture profile is lengthened in the downstream direction. 



The temperature profile changes with blood flow. 

 The asymmetry of temperature distribution along 

 the wall of the vessel increases with decreasing blood 

 flow. The temperature of the upstream section changes 



less than that of the downstream section. It is this 

 fact which makes the device a flowmeter. 



From Gregg's investigations (43, 74) on direct cur- 

 rent stromuhrs it was expected that pulsations of the 

 blood stream should distort the temperature profile 

 in an unpredictable manner. Wever & Aschoff (84), 

 working with a stream having large pulsations, found 

 that thermojunctions arranged at an angle of 90° 

 to the heating electrodes yield false readings which 

 are opposite to those obtained at an angle of 0°. The 

 practical application of these studies has led to the 

 construction of a device using ring electrodes, by 

 which temperatures of the complex profile are aver- 

 aged, and errors due to pulsation are avoided. These 

 electrodes also compensate for errors resulting from 

 nonlinearity of the calibration curve (fig. 7). Since 

 the highest temperature exists on the outside of the 

 vessel wall, any uncontrolled heat dissipation to the 

 outside of the unit would lead to an undetectable 

 error of measurement. In the new models (3), a 

 double wall including air for thermoisolation is in- 

 troduced. 



Where backflow occurs, the deviation in the meas- 

 urements is always in the direction of increasing flow. 

 The effect of backflow can be diminished by means 

 of asymmetrical placement of the thermojunctions 

 (77). When the downstream thermojunction is placed 

 close to the heating electrodes and the upstream junc- 

 tion is farther off, the backflowing blood heated during 

 its passage through the hot vessel wall will reach the 

 upper junction later and will have less influence on 

 the measurement. 



Although methods based on the thermostromuhr 

 principle have been abandoned during the last dec- 

 ades because of inherent inaccuracies (7, 25, 26, 43), 

 the new analysis given by Aschoff and Wever has 

 revived interest in the matter. 



Skin Blood Flow Measurement Based on Thermal 

 Conductance Measurement (20, 32, 48-51, 85) 



Since the heat produced in animals and humans is 

 transported mainly by blood flow, the heat flow of a 

 defined area of the skin is related to blood flow through 

 it. However, it is obvious that any change of tempera- 

 ture gradient, such as that induced by changes of the 

 surrounding temperature, will influence the heat flow 

 and therefore invalidate the measurement of blood 

 flow. The best values are obtained with devices which 

 measure heat flow and temperature gradient simul- 

 taneously. 



The following equation gives a measure of blood 



