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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



flowmeter, the deflections of which are detected either 

 by two coils acting as two arms of an inductance 

 bridge or by four coils forming a differential trans- 

 former. The coils are wound around the flow cannula 

 upstream and downstream from the pendulum. This 

 pendulum consists of a flexible ferromagnetic paddle 

 or of a spring-suspended ferromagnetic disk fixed on 

 the inner side of the tube wall. The authors empha- 

 sized improvement in stability achieved by using the 

 differential transformer. They implanted such devices 

 into the abdominal aorta of dogs under anesthesia 

 and obtained flow records some days later, the animals 

 being conscious. 



The most recent model of a pendulum flowmeter 

 with electromagnetic transmission was built by Pieper 

 (ioo). The flow-sensing element containing a differ- 

 ential transformer is arranged at the tip of a catheter 

 and can be introduced from a peripheral vessel into 

 an unopened central vessel, e.g., from the carotid 

 into the ascending aorta. The transformer coils are 

 wound around a longitudinal iron core. A ferromagne- 

 tic cylinder, surrounding the coils at a small distance 

 and covering about three fourths of their length, is 

 suspended by elastic springs so that it can be shifted 

 to and fro in its longitudinal direction. On its circum- 

 ference, the cylinder carries a small ring-shaped disk 

 which faces the blood stream. The force exerted on 

 the disk by the flow will shift the cylinder and thus 

 change the mutual inductances of the two transformer 

 parts in opposite directions. The natural frequency of 

 the elastically suspended cylinder is 120 cps per sec. 

 The frequencv response was found to be flat up to 20 

 cps. The probe is held centered in the vessel axis by an 

 umbrella-like arrangement. Rods surrounding the 

 probe are folded when the catheter is introduced, and 

 are spread by means of a mechanism actuated from 

 outside when the tip has reached its final position. 



The RCA 5734 transducer tube represents a new 

 and very useful means of electrical transmission in 

 pendulum and bristle flowmeters. This tube, which 

 was originally built for physical purposes, was em- 

 ployed in physiology for the measurement of small 

 forces, such as in the manometers and sphygmographs. 

 The essential characteristic of the 5734 triode is the 

 movable element consisting of the internal tube plate 

 (anode) and the external plate shaft (fig. 20). This 

 movable element extends through a thin and flexible 

 metal diaphragm, the center of which acts as a pivot 

 permitting small angular deflections of the plate 

 shaft so that the distance between the plate and the 

 fixed grid will be changed. This results in changes of 

 the plate current. Under the triode-operating condi- 



fig. 20. Mechano-electric transducer tube no. 5734 of the 

 Radio Corporation of America. Schematic sectional view. 

 [Redrawn from Muller (96).] K, cathode; Hi, H-_, heater 

 (filament) connections; 6', grid; .4, internal plate (anode); S, 

 external plate shaft; M, flexible metal diaphragm. .4, M, and S 

 are electrically connected to the tube's metal shell. Terminal 

 leads in clockwise order. Bottom view: heater, grid, heater, 

 cathode. Tube dimensions as indicated by RCA : maximal total 

 length (excluding leads), 1.3". Maximal diameter, 0.328". 

 Tube weight, 1.75 g. Rotational compliance of the diaphragm, 

 0.075 degree /g crn - Resonance frequency of plate shaft, 12,000 

 cps. The connection of the plate to the electric circuit is pro- 

 vided by the supporting clamp attached to the metal shell of 

 the tube. If there exists contact between tube shell and blood or 

 tissues, the electric circuit has to be designed in such a way 

 that the tube plate is grounded. The plane of deflection of the 

 plate shaft must coincide with the plane through terminal 

 lead of grid and tube axis. 



tions indicated by RCA (plate-supply voltage, 300 

 volts; grid voltage, o volts; load resistance, 75,000 

 ohms), the deflection sensitivity, i.e., the ratio of 

 change in output voltage to angular deflection of the 

 plate shaft, amounts to 40 volts per degree. Deflections 

 of more than ± 0.5 degree from the normal position 

 of the shaft may damage the diaphragm and the tube 

 electrodes. By virtue of its small weight and dimensions, 

 its high sensitivity and the low inertia of its moving 

 part, the 5734 is very appropriate to use in con- 

 structing a pendulum or bristle flowmeter. A further 

 advantage is its commercial availability. 



In 1953, the 5734 was first used for blood flow 

 measurement independently by Brecher and his co- 

 workers, and by Scher el a!., and in 1954 by Muller. 

 Fundamentally, all these designs were based on the 

 principles shown in figure 1 yc and e. The transducer 

 tube is placed in the side branch of a T-cannula, and 

 a needle or pendulum which protrudes into the 

 streaming blood is attached to the plate shaft. 



In the model of Scher et al. (1 18), the T-cannula is 

 made of stainless steel. Two types of obstacle to flow- 

 are used, the first being a flat paddle placed across 

 the stream, the second a streamlined rod or tube of 

 plastic. As can be expected, the sensitivity of the 

 paddle type is very high, and the deflection is ap- 

 proximately proportional to the square of flow- 

 velocity so that in such cases the second term of 



