METHODS OF MEASURING BLOOD FLOW 



!28 3 



occlusion technique, did not change. Even in states of 

 venous congestion leading to 2 per cent increase of 

 the limb volume, the blood flow was almost unaltered. 

 Less than 1 per cent increase of limb volume is usually 

 necessary in the application of the venous-occlusion 

 method. Considering all this, they offer several ex- 

 planations of Gaskell and Burton's findings. 



Greenfield and Patterson give an instructive dia- 

 gram of events during venous occlusion (fig. 6). In 

 the first phase of occlusion the plethysmographic 

 record shows a straight line increase indicating a 

 constant inflow of blood into the extremity. In the 

 second phase, the volume increase of the extremity 

 declines asymptotically, indicating that the inflow 

 of blood progressively decreases. This can be explained 

 by the decreasing arteriovenous pressure difference. 

 In a third phase the venous pressure reaches the oc- 

 clusion pressure. A new equilibrium obtains in which 

 there probably is a much lower blood flow through 

 the extremity. The volume increase in the occluded 

 region levels off. 



The "afterdrop" (a decrease in venous pressure 

 and limb volume which occurs on release of cuff pres- 

 sure if the veins are distended) can be considered as a 

 vasomotor phenomenon. It can also be explained on 

 mechanical grounds. The release of the pneumatic 

 cuff opens up an area of compressed veins thereby 

 acting like a muscle pump on the underlying veins 

 (2). [See also (1) and (82).] 



Pulse Plethysmog raphy 



According to Fick's suggestion it is generally ac- 

 cepted that the first differential quotient of volume 

 change in an extremity occurring during the course 

 of the arterial pulse equals the change in the rate of 

 arterial inflow, if the outflow is constant. Von Kries 

 (60) and later Frank (31) used tachographs and ple- 

 thysmographs on the forearm and measured changes 

 of volume and of the rate of arterial inflow during the 

 arterial pulse. A combination of pulse plethysmog- 

 raphy and venous-occlusion technique was used by 

 Burton (19, 27), Burch (16, 18), and others in order 

 to obtain absolute values for flow rates during the 

 time course of the arterial pulse in fingers and hands. 

 The plethysmographic devices (cylinders, cuffs, re- 

 cording systems) are adapted to the size of the ex- 

 tremities in question. The recording systems consist 

 of capsules covered with thin membranes, the bulging 

 of which corresponds to volume displacements and 

 are recorded optically. [For details see (17, 66).] 



Photoelectric Plethysmography 



Measurements of transparence and reflectance of 

 infrared light in skin areas furnish almost the same 

 values for blood volume changes as do the direct 

 mechanical methods (52-55). The calibration of such 

 instruments involving calorimetric or venous-occlu- 

 sion techniques cannot claim great accuracy. How- 

 ever, the simplicity of the experimental procedure 

 allows the use of instruments adapted to special pur- 

 poses not only in various skin areas but also on the 

 surfaces of organs such as the brain or kidney. The 

 latter, especially with its high blood content (about 

 23%), has been the object of blood flow studies uti- 

 lizing the light absorption properties of Hb in the red 

 and infrared regions. Procedures have been elabo- 

 rated (59) that allow measurement of blood content 

 in cortical and medullary areas of the kidneys, as well 

 as total blood flow using dye dilution and oxymetric 

 principles. 



THERMAL METHODS 



Thermostromuhr 



Thermal methods of measuring blood flow are 

 based on the principles of measurement of heat con- 

 duction. It is assumed that any condition leading to 

 loss or gain of heat in the blood stream would depend 

 among other variables on its volume flow. Gesell & 

 Bronk (37) cannulated the blood vessel and let the 

 blood pass through a tube surrounded by a concentric 

 water jacket which was flushed by a constant flow 

 of water at room temperature. The loss of heat from 

 the blood measured by the temperature increase in 

 the outflowing water was found to be inversely pro- 

 portional to the volume flow of blood. Corrections 

 were of course made for different blood temperatures. 

 The response to changes in flow is slow — of the order 

 of 1 min. 



A few years later H. Rein (71) constructed his 

 thermostromuhr, which was made for use on un- 

 opened blood vessels. This method was regarded as a 

 great improvement both as to lag time and conven- 

 ience. 



The original conception of the thermostromuhr 

 was based on the assumption that an alternating cur- 

 rent of high frequency applied to a blood vessel would 

 heat the blood radially. The temperature rise (A 7") 

 of this disc of blood would then be proportional to 

 the product of square of the current (T 2 ) and electrical 



