METHODS OF MEASURING BLOOD FLOW 



I28l 



length which is inserted into the blood stream from 

 an artery. Near the proximal end of the tube an air 

 bubble of such a size as to completely fill a short sec- 

 tion of the tube is injected and time required for the 

 bubble to pass the length of the tube is recorded. Near 

 the distal end of the tube the bubble is caught in a 

 trap. The rate of flow is calculated from the ratio of 

 volume and time, as is done for all the foregoing re- 

 corders. 



To utilize this principle for continuous recording 

 of blood flow, an automatic injector for air bubbles, 

 automatic removal of the bubbles after they pass the 

 tube, and a recorder of time required for passage of 

 each bubble are necessary. Several solutions of the 

 problem have been proposed (13, 15, 33, 57, 64, 65, 



87). 



A recorder for the passage time of the bubble which 

 uses photoelectric signals caused by the bubble when 

 passing a light source and phototube was introduced 

 by Selkurt (73). Baumgartner et al. (11) added an 

 automatic bubble injector, the operation of which is 

 timed by the passage of the bubble past the photocell 

 detector. A schematic drawing of a recent model (72) 

 is seen in figure 4. They also studied the over-all prop- 

 erties of the principle. They could not confirm the 

 assumption that blood and bubble velocity are equal. 

 Rather, they found that at low flows the bubble ve- 

 locity is less and at more rapid flows it is greater than 

 the blood velocity. The reasons for these deviations 

 are complex. At high flows the bubble seems to lose 

 contact with the wall and to move in the faster axial 

 stream of the blood. Viscosity influences the bubble 

 velocity somewhat but not seriously. Maximal devia- 

 tions are not greater than ±5 per cent. However, if 

 an accuracy within 1 to 2 per cent is desired, they 

 suggest calibration of the apparatus with blood of the 

 animal. Pulsation is without influence on the calibra- 

 tion curve. They used a tube 3.5 mm in diameter 

 and 35 cm in length. The resistance to flow in such 

 a tube is low in comparison to that of the peripheral 

 vascular beds. The maximal flow they studied 

 amounted to 300 ml per min. At this rate the pressure 

 gradient was not more than 4 cm of H.O. This value 

 is comparable with other methods used on opened 

 vessels. The diameter of 3.5 mm cannot be much in- 

 creased because at diameters of more than 4.5 mm 

 the air bubble will not fill the flowmeter tube. Length- 

 ening the tube increases the sensitivity of the meas- 

 urement, but also increases resistance to flow. This 

 fact is to be considered mainly in measurements of 

 venous flow. 



VENOUS-OCCLUSION METHODS 



The principle of the venous-occlusion method con- 

 sists in temporarily blocking the venous outflow from 

 an organ which is enclosed in a plethysmograph. The 

 blood that enters the organ via the artery is thereby 

 retained and indicated as a volume increase by the 

 plethysmograph. 



In this way the method is an almost direct volume 

 measurement per unit time and thus comparable in 

 principle to those described in the foregoing para- 

 graphs. Brodie & Russell (14), who first described the 

 venous-occlusion principle, were aware of the main 

 conditions to be fulfilled: ". . . It is obviously essential 

 that the blockage of the vein must not be maintained 

 so long as to impede the flow through the capillaries. 

 Under all ordinary conditions the veins are never 

 completely filled, so that it is possible to store up in 

 them a small extra quantity of blood without checking 

 the inflow into them from the capillaries." As long 

 as the volume recorder indicates a uniform increase, 

 the inflow is not impeded. Brodie's method was 

 adapted by Hewlett & von Zwaluwenburg (56) to 

 measure blood flow in extremities in man. A plethys- 

 mograph similar in construction to that of Mosso 

 was used : a glass cylinder wide enough to enclose 

 the hand and forearm, from which a rubber tube of 

 small dimensions leads to the recorder. The whole 

 system is filled with water to avoid volume errors due 

 to temperature changes. The veins are blocked by 

 applying pressure of 50 mm Hg into a pneumatic cuff" 

 placed on the upper arm. 



Several modifications (79) of the original device 

 have been described (8-io, 46). H. Barcroft's as- 

 sembly is now most commonlv in use (fig. 5). Mosso's 

 glass cylinder is replaced by a conic metal tube. The 

 hand is covered with a large surgical rubber glove 

 which is fixed outside the plethysmograph to avoid 

 leakage of water. Since any movement of the forearm 

 will change the volume of water inside the plethys- 

 mograph, the circumference of the glove is stiffened 

 by a diaphragm J/4-inch thick. The diaphragm is 

 bolted to a 2-inch-wide flange on the end of the ple- 

 thysmograph by means of metal plates and wing 

 nuts. (For further details see the original paper.) The 

 pneumatic cuff is connected through a three-way tap 

 with a reservoir of compressed air at 60 to 70 mm Hg. 

 The three-way tap allows inflation of the cuff from 

 the reservoir and deflation when it is opened to room 

 air. Two or even four measurements can be taken in 

 1 min, if blood flow is high. At this rate of measure- 

 ment the cuff is inflated for only 5 sec. It is found that 



