METHODS OF MEASURING BLOOD FLOW 



12 79 



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Water 



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Rotating 

 plate 



A-\sB 



fig. 2. Schematic drawings of 

 direct recording flowmeters de- 

 rived from Volkmann's and 

 Ludwig's principles. [From 

 Dawes et al. (22).] a: Volkmann 

 (1850). Open 7"i, close 7\, and 

 time movement of blood through 

 U-tube. b: Ludwig (Dogiel, 

 1867). Time movement of blood 

 through one chamber and then 

 reverse chambers by hand, c: 

 Pavlov (1887). Time movement 

 of blood through one chamber 

 and reverse direction of flow 

 automatically by opening elec- 

 tromagnetic tap Ti and closing 

 T-.. d: Dawes et al. (22) Close T 

 and time movement of blood be- 

 tween electrodes A and B, restore 

 blood levels by opening T. 



METHODS BASED ON LUDWIG S PRINCIPLE 



Volkmann, (81 ) in 1850, was the first to measure 

 blood flow per unit time in arteries (fig. 2a). His device 

 consisted of a U-tube inserted in an artery. The U- 

 tube could be bypassed by two 3-way stopcocks. When 

 measurements were taken, the U-tube was filled with 

 saline. After turning both stopcocks simultaneously 

 the blood flowed through the U-tube, the time was 

 measured between the moments when the blood 

 entered and left the U-tube. 



This method obviously did not allow continuous 

 measurement of blood flow. Another drawback was 

 the repeated infusions of saline with each measure- 

 ment of flow. Therefore Ludwig and colleagues 

 (see 24) modified Volkmann's device (fig. 2b). In their 

 version, the upstream limb of the U-tube was filled 

 with oil and the downstream one with blood. The 

 tube itself could be turned by hand through 180 , 

 to connect the two limbs alternately to the distal and 

 proximal ends of the artery. The blood was allowed to 

 enter the upstream limb and the tube had to be turned 

 when the oil content reached the entrance to the 

 distal arterial connection. Each turn was marked on 

 a smoked drum, thereby recording blood flow con- 

 tinuously. Apparatus based on Ludwig's principle 

 have been constructed with many modifications and 

 are still in use (6, 12, 69). The directness of the meas- 

 urement of flow can be regarded as the main reason 

 for its popularity. In Ludwig's laboratory Pavlov 



(70) developed in 1887, a self-recording flowmeter 

 based on the same principle. To avoid manipulations 

 for reversing the direction of flow, he designed his 

 meter so that blood could be made to enter alternately 

 either limb of the U-tube by opening and closing 

 electromagnetic taps, (see fig. 2c). The taps were 

 automatically operated by means of floats in both 

 limbs moving with the direction of flow and closing 

 contacts in the electromagnetic circuit when the rising 

 float reached the top of its limb. 



The Pavlov type flowmeter has been used in numer- 

 ous modifications. The U-tube can be made very 

 small for low flow rates. To avoid electrical contacts 

 within the blood stream the electromagnetic taps 

 can be controlled by photoelectric relays (63). 



An ingenious device based on Ludwig's principle 

 has recently been described by Dawes et al. (22) (fig. 

 2d). The upper part of the U-tube is filled with silicone 

 oil, the lower part of both limbs with blood. The by- 

 pass can be closed by an electromagnetic tap. In the 

 inflow limb of the U-tube, two electrodes which oper- 

 ate a relay for opening and closing the electromag- 

 netic tap of the bypass are inserted with a distance 

 between them, such that about 1.5 ml of fluid is 

 enough to cover both contacts. When blood enters 

 the proximal limb it touches one electrode. After 

 1.5 ml more have entered, the other electrode is con- 

 nected with the first, thereby closing an electric cir- 

 cuit and setting a relay causing the tap to open. This 

 allows the blood which entered the proximal limb 



