I 3 IO 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



fig. 23. Schematic diagram of 

 the transducer-tube flowmeter of 

 Miiller. T, 5734; E, micrometer 

 screw. A', resistance body. [From 

 Mailer (96).] 



artery's diameter is kept constant by the metal band 

 N placed around the vessel wall. As in the standard 

 model, the "zeroing" cylinder G is used for the 

 determination of the base line of flow during the 

 records, and the side tube A' for the removal of air 

 bubbles and for manometer connection. 



As to the additional electrical equipment, Brecher 

 uses a load resistor of 500,000 ohms for the transducer 

 tube which gives higher sensitivity and better d-c 

 stability than the 75,000-ohm resistor recommended 

 by RCA. The load resistor is connected to the cathode 

 while the plate is grounded in all cases. The changes 

 in plate current due to the deflections of the plate 

 shaft cause proportional variations of the cathode 

 potential which are amplified either by battery- 

 operated or main-fed d-c amplifiers. Both types were 

 designed by Praglin and are described by Brecher 

 (g). It should be noted that the greatest plate-shaft 

 deflection ever observed in Brecher's blood-flow 

 experiments is 3 min of arc. This deflection results in 

 a potential change of 4 volts at the tube's cathode, 

 corresponding to a sensitivity of 80 volts per degree. 

 Thus the deflections caused by the blood flow remain 

 within the mechanically safe range, which extends to 

 30 min of arc on either side. 



The transducer-tube flowmeter developed by 

 Miiller (96) is shown in figure 23. It was built for 

 flow measurement in blood vessels as well as for 

 investigations of more general hydrodynamic interest, 

 particularly for the study of the forces exerted by 

 streaming fluids of various Reynolds numbers on 

 resistance bodies of different shapes. An accurate 

 calibration in terms of force is therefore needed and 

 can be performed by the micrometer screw E which 

 causes a small spring to press on the tube's plate shaft. 

 The transducer tube is arranged in a bridge circuit, 

 the adjacent limb of which contains a second triode 

 of similar properties which compensates for fluctua- 

 tions of operating voltage, etc. The bridge output is 

 connected to a push-pull d-c amplifier. A special 

 model for coronary-artery flow recording was de- 

 signed by Laszt & Miiller (86) (fig. 24). The hori- 



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M 



fig. ->4- Transducer-tube flowmeter of Laszt and Miiller. 

 ^ r ' 5734; A, plate shaft; M, manometer; A\ vertical limb of 

 T-cannula; S, extension rod of bristle. Si, movable cylinder 

 around R, Wk, cylindrical resistance body at the bristle tip; A', 

 horizontal limb of T-cannula. [From Laszt & Miiller (86).] 



zontal cannula A' is inserted into the vessel without 

 interruption of the flow and without using ligatures 

 around the vessel. The vessel wall is pierced by the 

 sharp edge, and the cannula is tilted and moved 

 slightly until A slips into the vessel. The cylinder St 

 is then pressed downward to immobilize the vessel 

 wall around the incision. 



Critical remarks on the bristle flowmeter technique 

 are based on experimental data and on theoretical 

 considerations. The main practical advantages 

 enumerated by Brecher (9) are as follows: negligible 

 resistance to and interference with the flow; equal and 

 opposite response to forward and backward flow; 

 high sensitivity and frequency response. The main 

 practical disadvantages are: necessity for opening the 

 vessel and using anticoagulants; gravitational effects 

 on the bristle when the position of the cannula is 

 altered; and the nonlinearity of the calibration 

 curve. This latter drawback can be overcome by 

 electrical linearization (9, 103). Since, for this purpose, 

 the amplification of low flow signals is made much 

 greater than that of high ones, the device becomes 

 very sensitive to minute shifts of the base line so that 

 an exceedingly stable base line is required. The 



