1286 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



linear relationship exists between blood flow and the 

 apparent increment of X. Direct readings of X may 

 be recorded (42) by keeping T constant through 

 variation of 7. It should, however, be borne in mind 

 that only relative values for blood flow can be ob- 

 tained. The principle can be applied either to surfaces 

 or to inner regions of organs. For these various ap- 

 plications several types of instruments have been 

 developed. As an example, for measurements of blood 

 flow through deep layers, heat source and temperature 

 measuring units are contained in a needle (38, 39, 

 42, 47, 68). 



Hensel's modification shown in figure 8 contains 

 both thermojunctions within the needle, one at the 

 tip together with the heating wire, and the other at 

 the middle of the needle. According to Graf & Rosell 

 (40), reliable measurements are obtained only when 

 the tip of the needle is placed in close proximity to a 

 vessel, either an artery or vein. This condition is 

 checked by comparison of X-values obtained in the 

 ischemic and normal state which should be in the 

 order of 1 X io~ 4 cal per sec cm C. Lower values 



Connections 



to healing sou/re 



nj^—to galvanometer 



Cross 

 section 



-Cannula 

 -Heating wire 



r hermojunction 

 (hot) 



fig. 8. Schematic drawing of longitudinal and cross sections 

 of Hensel's needle for measuring thermal conductivity in tissue. 

 [From Hensel el at. (47)-] 



T her moj unction 

 ( cold J 



□ V2A steel 

 f~1 Copper 

 ^ Iron 

 ■I Constantan 

 r~"l Solder metal 

 I I Glass 



50 



150 



250 



350 



fig. 9. Relation between thermal conductivity increments 

 (AX) and blood How in muscles of the cat's hind limb. Changes 

 in blood flow were induced by intra-aortic infusions of adrena- 

 line and acetylcholine and by hypothalamic stimulation. Four 

 experiments marked with different signs. [From Graf & Rosell 

 (40).] 



indicate that the tip is placed too far from the vessel. 

 This remote position would also result in a slow re- 

 sponse to blood flow changes. A plotting of direct 

 recorded values of blood flow through a hind limb 

 of a cat and values obtained with Hensel's needle 

 show that the percentage changes of X accord well 

 with the percentage changes of direct measurements. 

 However, the above-mentioned condition of a con- 

 ductivity increment of at least 1.0 X io" 4 cal per 

 sec cm C was fulfilled (fig. 9). The advantage of the 

 device lies in the fact that it can be used in humans 

 without interfering with the normal state of blood 

 flow. 



To record blood flow on organ surfaces, as on vkin, 

 brain, or eye, several modifications of a "surface 

 thermostromuhr" have been constructed. A. C. Bur- 

 ton has utilized the principle of electrical resistance 

 changes with temperature. "Two flat resistance coils, 

 the smaller a central disc and the larger a ring about 

 it, with an insulating gap between, form the two arms 

 of a Wheatstone bridge. These coils are of wire which 

 has a high temperature coefficient of resistance, while 

 the other two coils of the bridge are of constant re- 

 sistance. The battery supplying the bridge is arranged 

 to drive current through the two 'skin' coils in parallel. 

 Since the central coil is of lower resistance, greater 

 heat is generated since the current is greater in it, 

 and as a result it will reach, in the thermal steadv 



