METHODS OF MEASURING BLOOD FLOW 130I 



j Fl G lg I 



I FIG. II I 



ML 



jm—_ 



> . 1 



- K 



fig. 10. Pitot meter with asymmetrical pressure taps, one facing upstream, the other arranged to 

 measure lateral pressure. [From Miiller (95).] 



fig. 11. Prandtl's tube, based on the Pitot principle. 1, upstream facing pressure tap. 2, lateral 

 pressure tap. L, distance between 1 and 2. [From Hardung (57).] 



fig. 12. Pitot meter with symmetrical pressure taps facing upstream and downstream. [From 

 Miiller (95).] 



within a wide range by changes of viscosity (water- 

 blood), by changes of the flow type (laminar-turbulent), 

 and even by changes of the velocity profile caused by 

 flow pulsations, provided the flow remains unidirec- 

 tional. Using his modification of the Pitot meter, he 

 constructed a catheter-tip cannula for recording the 

 coronary-sinus outflow as well as another cannula for 

 measuring the pump output in extracorporeal-circu- 

 lation devices. It seems possible to combine these 

 advantages with the arrangement described by Har- 

 dung (see below) which avoids distortions caused by 

 local acceleration. 



Besides the aforementioned Pitot types, there is an 

 older modification used by Cybulski (19). As shown in 

 figure 1 4, the sharp angle in the tube causes a sudden 

 change in direction of flow creating a hydrodynamic 



by Mixter (94). Brecher's device (fig. 13), designed for 

 introduction into the superior vena cava from the 

 jugular vein, consists of a rigid three-tube catheter at 

 the tip of which a streamlined lead "torpedo" is 

 attached. It contains the upstream and downstream 

 facing ends of the differential-manometer tubes U and 

 D while the longer tube A, serving to detect the 

 pressure in the right atrium, is connected to a separate 

 manometer. Tube A can be moved along U and D by 

 working an outside handle; in this way, springs are 

 expanded to form a basket (dashed lines in fig. 13) 

 around the torpedo in order to keep it centered in a 

 vessel of constant diameter. Mixter's torpedo is placed 

 in a metal tube for direct insertion into a large vein. 



A promising attempt to improve the performance 

 of the Pitot meter was recently made by Bretschneider 

 (13), who realized the difficulties involved in the 

 estimation of the average velocity from the velocity of 

 a small bundle of streamlines. He placed the opening 

 facing upstream at a point where the local fluid veloc- 

 ity equals the average velocity for both laminar and 

 turbulent flow. Theoretically, this point is situated at a 

 distance of about 0.7 R from the tube axis, or 0.3 R 

 from the wall, i.e., R/y/2. This arrangement, how- 

 ever, is impracticable if the lumen diameter is 6 mm 

 or less. In such cases a greater relative distance lrom 

 the wall (0.4-0.6 R) has to be chosen. Errors due to 

 such malposition are substantially reduced by using a 

 flow cannula with a conical inlet section which flattens 

 the velocity profile. Thus Bretschneider obtained an 

 average flow calibration curve which is unaffected 



DIFFERENTIAL 



PRESSURE 



MANOMETER 



CATHETER 

 FLOWMETER HEAD 



*;e 



S3r 



'— .D' 



RIGHT 

 ATRIUM 



-> SUP. CAVAL FLOW > 



ATRIAL 



PRESSURE 



MANOMETER 



fig. 13. Pitot "torpedo" of Brecher for recording superior 

 vena cava flow. For description see text. [From Brecher (8).] 



