METHODS OF MEASURING BLOOD FLOW 



1287 



001 3r 



0012 -: 



•0011 



0010 - 



0009 



0008 - 



0007 



fig. 10. The relationship between effective thermal conductivity of the skin and blood flow in the 

 lingers. The three curves are for different subjects. The points for zero How (circled) were obtained 

 by occlusion of the flow by a cuff on the proximal phalanx, while the other data for low flows were 

 obtained by vasoconstriction in response to cold. [From Burton ( 2 1 ).] 



state, a higher temperature above the skin than the 

 other, 'ring' coil. The difference of temperature be- 

 tween the two coils is registered directly by the de- 

 flection of the galvanometer of the bridge." [Burton 



(2.).] 



Hensel's device consists of a round plexiglass plate 

 on which the hot and cold thermojunctions are placed 

 at a distance of a few centimeters. The units can be 

 applied to finger tips and the conductivity increment 

 obtained with changes of blood flow can be checked 

 by the venous occlusion technique. Burton's results 

 on three subjects show linear relationships (fig. 10). 

 However, in the ischemic state X varies significantly 

 from subject to subject so that a general calibration 

 cannot be used. 



A similar device to apply on the brain surface in 

 animal experiments has been designed by Kanzow 

 (58a). In his unit heat is directly applied by diathermy 

 to the brain tissue, and the temperature difference 

 between the heated area and the unheated control 

 area is measured by thermocouples. This device 



shows that a linear relationship between X and blood 

 flow exists, providing the electrical resistance of the 

 tissue does not change. Kanzow claims that X-changes 

 occurring with changes in blood content within the 

 tissue can be detected from readings of tissue resist- 

 ance, thereby helping to avoid errors in blood flow 

 estimations. 



Thermocouples can be replaced by thermistors 

 which are placed in Cournand catheter tips or in 

 glass cannulas used for blood flow measurements in 

 larger vessels (23, 28, 30, 36, 61, 67, 76, 78, 84, 8g). 

 The chief advantage of such units is their small size. 

 However, this type of device does not distinguish 

 backward from forward flow, and quantitative meas- 

 urement of blood flow is not possible with it since 

 the diameter of the vessel is not controlled. Even 

 with measurements on arteries cannulated with glass 

 tubes with imbedded thermistors the results are 

 doubtful, since the velocity profile of the blood is not 

 uniform. Also, recordings of flow pulses are distorted 

 because of the low frequency response of such units. 



REFERENCES 



I. Abramson, D. I., H. Zazeela, and J. Marrus. Plethysmo- 

 graphy studies of peripheral blood flow in man. I. Criteria 



for obtaining accurate plcthysmographic data. Am, Heart 

 J. iT. 194-205, 1939. 



