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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



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fig. 8. Differential-pressure flowmeter of Schroeder (119) 

 for application on skin-coated arterial loops. For description 

 see text. 



cording mean flow. A Yenturi cannula in connection 

 with a differential water manometer, for application 

 to abdominal arteries, was used by Wagoner & 

 Livingston (131). YVretlind (138), modifying a plan 

 by Stephenson (1948), carefully designed a meter for 

 mean flow in the ascending aorta of the cat. As figure 

 9 shows, the blood streaming from A to B passes a 

 constriction (£>) of small length which causes a pres- 

 sure drop of a few mm Hg. Pulsations in the differen- 

 tial manometer (G) records are eliminated by expand- 

 ing chambers (C,E) at the upstream and downstream 

 side of the constriction; within these chambers, 

 pulsating blood columns rise to different mean levels 

 corresponding to the pressure difference which is 

 necessary to drive the mean blood flow through the 

 constriction. The tops of the two chambers are con- 

 nected to each other by an air-filled tube (F) which 

 acts as an elastic bypass transmitting a part of flow 

 pulsations from the central to the peripheral end of 

 the meter. 



The principle of Pitot flowmeters (1 728-1732) 

 consists in the measurement of the hydrodynamic 

 increment in pressure which is generated by the 

 locally circumscribed stagnation of a small part of 

 the streaming fluid. For this purpose, a thin tube, the 

 opening of which faces upstream, is placed in the 

 fluid. The difference between the pressure exerted on 

 that opening ("end" or "total" pressure) and the 

 "lateral" (or •'static") pressure is indicated by a 

 differential manometer. The opening which picks up 

 the lateral pressure may be placed in the wall (fig. 

 10) or near the opening facing upstream (fig. 1 1). In 

 other devices, two thin tubes are inserted, with one 



fig. 9. Flowmeter of YVretlind 

 for ascending aorta of cat. For 

 description see text. [From 

 YVretlind (138).] 



opening upstream and the other downstream (fig. 12); 

 the pressure difference is greater with this design 

 because suction is effected at the downstream opening 

 by eddy formation. This arrangement also offers the 

 advantage that almost equal physical conditions can 

 be provided to measure forward and reverse flow. If 

 equation 8 is applied to Pitot meters, v of term II is 

 not the average velocity of the fluid, but the velocity 

 of that small bundle of streamlines which hits the 

 opening facing upstream. This is an advantage be- 

 cause it offers the possibility of using Pitot meters like 

 those illustrated in figures 10 to 12 as probes which 

 can be shifted along the tube radius in order to 

 measure, point by point, the hydrodynamic pressure 

 distribution between the axis and the wall. Thus, the 

 velocity profile is determinable for hydraulic investi- 

 gations of theoretical and practical interest [Miiller 

 (95)]. If, on the other hand, the average flow velocity 

 is to be detected by Pitot meters, errors resulting from 

 changes of the velocity profile have to be taken into 

 account. If the flow is pulsatile, term III of equation 8 

 requires special consideration. 



Prandtl's tube (fig. 1 1 ) is a modification of the Pitot 

 meter; the openings lie at the surface of a probe which 

 is placed in the streaming fluid. This device minimizes 

 eddy formation. 



The construction of most Pitot meters applied in 

 cardiovascular physiology can be deduced from one of 

 the types shown in figure 10 to 12. Aortic flow was re- 

 corded in 1899 with Frank's (36) double-lumen cathe- 

 ter which was introduced through the carotid artery. 

 Other Pitot devices were used by Baxter & Pearce (4) 

 and by Jameson (67) for recording the pulmonary 

 artery flow, by Johnson & Wiggers (70) for recording 

 the coronary sinus outflow, and by Eckstein et al. (26) 

 for recording the vena cava flow. 



"Torpedo"-shaped Pitot meters offering low re- 

 sistance to flow were built in 1953 by Brecher (8) and 



