i 3 i8 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



fig. 30. Block diagram of the square-wave flowmeter circuits required to energize the magnet, 

 eliminate spurious emfs, amplify the flow signal, and convert it to direct current. [From Spencer & 

 Denison (126).] 



TYPE U 



TYPE C 



C 3 2 I A B 



8 cm 



H 



r- 



5 cm 



H 



fig. 31. Three types of magnet-sleeve units used in the square -wave flowmeter technique: Type U 

 (horseshoe) mainly employed in surgical measurements, type C for implantation about large vessels, 

 type I for small vessels. .4, B, C, magnet-coil terminals; I, 2, 3, electrode leads; 2, 3, split lead. The 

 units are imbedded in plastic cast. [From Spencer & Denison (126).] 



TYPE I 



375cm 



ULTRASONIC FLOWMETERS 



The measurement of blood velocity by recording 

 sound-transit times upstream and downstream within 

 a vessel segment of small length offers, in principle, 

 several important advantages. The device placed 

 around the vessel is very lightweight and simple in 

 construction. The vessel remains intact, and there is 

 no interference with the blood flow or pulse wave 

 except that effected by a short rigid sleeve causing a 

 moderate constriction. The calibration curve can be 

 made to be a straight line passing through the zero 

 point with equal slopes for forward and backward 

 flow. The signals obtained can follow the most rapid 

 changes of the instaneous blood velocity occurring in 

 the circulation. However, in contrast to the simple 

 device applied to the vessel itself, very involved 

 electronic equipment is required to detect and evalu- 

 ate the extremely small effects exerted by the flow on 



the sound transit times. Since sound velocity in blood 

 is about 1.5- 1 o 5 cm per sec, the time required for 

 traveling over a distance of 1 cm is about 7 n sec. If 

 the blood is moving at the velocity v along the direc- 

 tion of sound propagation, the apparent sound veloc- 

 ity measured between two quiescent points is (c — v) 

 or (c + v) for upstream or downstream sound direc- 

 tion, respectively. A flow velocity of 1 cm per sec will 

 therefore change the sound transit time (7 fisec/cm) 

 by about ±5'io~ n sec. Thus utmost precision is 

 necessary if differences of such a minute order of 

 magnitude are to be measured with sufficient accu- 

 racy, and the admirable advances made in this field 

 to date are based on very difficult and detailed work. 

 The development of ultrasonic blood flowmeters has 

 been carried out mainly by two groups using different 

 approaches. 



Haugen et al. (58) and Herrick & Anderson (59), 

 modifying the design of Kalmus, developed a phase- 



