[2g8 HANDBOOK OF PHYSIOLOGY <-" CIRCULATION II 



UPO 



fig. 3. Venturimeter of original type. [From Green (50).] 



manometer connections were very distant from each 

 other. The aortic flow records obtained with this 

 cannula therefore represented acceleration curves 

 rather than velocity curves. A detailed polemic was 

 carried out on this point by Frank (41, 42) on the 

 one side and by Broemser (16) and Ranke (107) on 

 the other. To minimize the distortions, the distance 

 between both manometer connections (length L in 

 equation 9) must be as small as possible. In contrast 

 to a widely held opinion, the distorting effect of term 

 III cannot be detected by comparing the directly 

 measured mean flow with mean pulsatile flow deter- 

 mined by planimetering the recorded curves. This is 

 true because when areas are determined by the 

 planimeter, the distortions generated during flow- 

 acceleration may be compensated for by opposite 

 distortions generated during flow deceleration. An 

 analytical correction of the records would be feasible, 

 but very difficult. The best way, therefore, is to keep 

 Cz minimal by appropriate construction of the flow- 

 meter. Similar considerations apply also to Pitot 

 tubes (see below). 



Venturi tubes like those of figure 3 are used rela- 

 tivelv seldom. The cannulae of de Burgh Daly (20) 

 and of Lauber (87) may be mentioned here. Lawson 

 & Holt (88) modified Daly's method. 



The Venturi principle is applicable also to other 

 designs. Figure 4 shows the effect of an inflection of 

 the tube wall on the streamlines. In case of such an 

 inflection of small length, the point at which the 

 streamlines run closest to each other is not situated 

 at the tip of the inflection, but somewhat downstream 

 from it. This means that the fluid's linear velocity 

 is higher and the lateral pressure is lower on the 

 downstream side than on the upstream side of an 

 inflection or constriction. Thus a pressure difference 

 corresponding to term II is generated between two 

 points situated upstream and downstream from a 

 nearby constriction even if the tube's cross sections 

 are equal at both points. 



Broemser (15) and Reissinger (109) in 1928, making 

 use of this effect, constructed an instrument which 

 proved appropriate for recording pulsatile flow in the 

 ascending aorta (fig. 5). The advantages of this can- 

 nula consist in the very short distance between the 



fig. 4. Deviation of streamlines caused by an inflection of 

 the wall. [From Reissinger (109).] 



fig. 5. Cannula of Broemser 

 and Reissinger. [From Reissinger 



(109).] 



lateral openings and in their symmetrical arrangement 

 which provides equal sensitivity to forward and back- 

 ward flow. The optimal inflection angle between tube 

 axis and wall was found to be 7 to 8°; by using this 

 angle, sufficient sensitivity is achieved and no eddies 

 occur even at the highest physiological flow velocities. 

 Since the planes of the lateral openings are not par- 

 allel with the vessel axis, an additional Pitot effect 

 (see below) may be involved. 



Nilsson & Kramer (97) in 1954 developed a Venturi 

 meter according to the aforementioned principles for 

 the registration of the pulsatile flow in the intra- 

 thoracic vena cava. Steady and oscillatory flow 

 calibrations showed that, for this device, the terms 

 I and III are of subordinate significance. 



The orifice flowmeter of Gregg & Green (55) 

 [cf Green (50) and Gregg (54)] is also based on the 

 Venturi principle. The pressure difference is generated 

 by an opening ( = orifice) in a thin disk placed across 

 the stream (see fig. 6). Lateral manometer connec- 

 tions are arranged upstream and downstream from 

 the disk at distances equal to the tube radius. As seen 

 from figure 6, the streamlines converge downstream 

 from the orifice so that there is a point at which the 

 pressure reaches a minimum as described above. Due 

 to its symmetrical arrangement, this device has equal 

 sensitivities to forward and backward flow. The size 

 of the orifice can be adjusted during the experiments 

 either by substituting disks of different orifice diam- 

 eters through a slot in the cannula wall or, according 

 to a modification devised by Shipley et al. (120), it 

 can be altered from the outside by means of a stud 

 screw, the rounded end of which protrudes into the 



