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HANDBOOK OF PHYSIOI.O(;Y 



circulation ii 



fig. 27. A-C flowmeter for application on exposed arteries 

 (Kolin). Left: cross section of sleeve £. E\, E2, electrodes, Pi, 

 Pi, location of magnet pole pieces; U\, Ws, braided electrode- 

 lead wires; Si, slot for insertion of vessel. Right: total view with 

 artery A. C, magnet coils; 5, sleeve; R, rubber sheet for 

 insulation of tissues from magnet core. [From Kolin 182 1. | 



and sampling the amplified flow signal at the peaks 

 of B(B = B„; dB dt = o) have been employed (3, 83, 

 132) and recommended in combination with mode e. 

 Another way, that of phase detection at the points 

 (B = o; dB/dt = maximum) was described by 

 Olmstead & Aldrich (99) and found to yield a stable 

 base line, g) Abandoning the sine-wave type and 

 using rectangular wave shape led to the square-wave 

 flowmeter which will be described below. 



Cancellation of the transformer emf E, is closely re- 

 lated to the assessment and stability of the base line. 

 Only if E t is eliminated or compensated for, does 

 switching off the magnet current render a true zero- 

 flow reading as is obtained by occluding the vessel 

 while the magnet is energized (82). 



Although there is the possibility that zero changes 

 occur as a result of changes in the position or con- 

 ductivitv of the vessel wall at the site of measurement 

 (23), such difficulties are not a necessary accompani- 

 ment of the application of the a-c procedure to intact 

 vessels as shown by recent reports (82). 



Numerous models of a-c flowmeters have been built, 

 some of which may, as examples, be mentioned here. 

 Figure 27 shows an early a-c device (Kolin) applicable 

 to exposed and unopened arteries. The simplicity of 

 sleeve and electrodes as compared with d-c devices is 

 obvious. Special sleeves with imbedded electrodes 

 have been used for chronic implantation (80, 81). 

 After recovery from the operation, the animal is 



placed in the field of an external a-c magnet. Auxiliary 

 coils attached to the sleeve deliver induced a-c sig- 

 nals which are used for calibration, orientation, and 

 compensation. The further development (see 81, 82) 

 produced miniaturized devices consisting of magnet- 

 sleeve assemblies constructed in compact units and 

 cast in acrylic plastic. These provide a fixed orienta- 

 tion between vessel and magnet in chronic-implanta- 

 tion experiments. Units weighing 5 to 10 g have been 

 successfully built which reach flux-density peaks (Bo) 

 of 200 to 500 gauss at a carrier frequency of 400 cps. 

 For chronic implantation around small arteries of 

 diameters down to 1.5 mm or less, subminiature units 

 of still lower weight are described. As to the skillful 

 manufacturing of these designs see (82) and figure 28. 



The miniaturization of the a-c flowmeter is made 

 possible by specially tuned amplifiers (400 cps) char- 

 acterized by extremely high gain and low noise (82, 

 83). Input signals of 1 nv are measurable at a noise 

 level of less than 0.2 fiv. The high amplifier gain allows 

 the use of coreless coil-sleeve units for implantation 

 around large vessels of diameters greater than 1 cm, 

 e.g., aorta of dog (81 , 82). A very low magnetic field 

 strength (B = 10 gauss) is sufficient in these cases. 

 Core magnets are also used in a-c devices implantable 

 about the aorta of dogs (98). 



As already mentioned, one of the ways ot 

 eliminating the transformer component consists in 

 avoiding the sinusoidal wave shape in a-c flowmeters. 

 Thus, a group of "signal-separating" a-c flowmeters 

 (126) has been created, the most important model 

 being the square-wave type developed by Denison 

 et al. (22, 23, 126). The magnet is energized by an 

 alternating current following a rectangular time course 



fig. 28. Subminiature flowmeter (Kolin). Dimensions in 

 mm. 7, iron cores; C\, d, magnet coils, wy-Wi, coil-terminal 

 wires; S, sleeve; C, sleeve channel for vessel; Si, slot, E\, E?, 

 electrodes. Leads and coating of the unit with acrylic plastic 

 are not shown. [From Kolin (82).] 



