[320 



HANDBOOK OF PHYSIOLOGY -^ CIRCULATION II 



fig. 32. Simplified diagram of the 

 pulsed ultrasonic flowmeter. For de- 

 scription see text. [From Franklin el al. 

 (44)-] 



UPSTREAM- DOWNSTREAM 

 VOLTAGE COMPARATOR 



1- 



SYNCHRONOUS 

 DETECTOR 



RECORDER 



AORTIC FLOW 



JUUjUX 



H-lsec* 



velocity v R averaged over the vessel's diameter or 

 radius, and that therefore the calibration in terms of 

 the flow rate varies with the velocity profile. On the 

 other hand, corrections might be brought about by 

 additional effects, such as some flattening of the 

 velocity profile by the slight constriction of the vessel 

 caused by the sleeve, and the fact the ultrasound 

 beam reaching the receiver is not in the form of a line, 

 but of a band. Rushmer describes the calibration of 

 his ultrasonic flowmeter as being independent of the 

 velocity profile within ±5 per cent (44). With respect 

 to the growing importance of this flowmeter type, the 

 problem should be reconsidered both theoretically 

 and practically. 



TRAVELING MARKERS 



Estimations of blood velocity can be made by 

 observing, continuously photographing, or filming 

 the movement of any substance which acts as a dis- 

 tinguishable marker traveling with the blood stream. 

 In most cases, such a procedure will allow only single 

 short-time recordings which can be repeated at 

 intervals. The marker may be represented by dye, by 

 a drop of fluid nonmiscible with the blood [see (54, 

 p. 60) (50, p. 116)], by radiopaque material for 

 cineradiography (2, 7, 24, 90, 106), or by a gas bubble. 



Even the blood's own corpuscles can be used as 

 markers (63), and the progress of blood columns 

 differing in oxygen saturation may be assessed photo- 

 electrically (85) (cf Chapter 18, vol. I, of this Hand- 

 book). Foreign substances are usually injected into a 

 side branch and then observed through the wall of 

 the vessel under investigation. In contradistinction 

 to flowmeters in the proper sense, the use of traveling 

 markers does not give the volume flow at a fixed site; 

 it rather gives a function of time and space since the 

 mark changes position during the measurement. In 

 case of relatively small displacements, the change in 

 the site of measurement may be neglected. Dyes are 

 particularly useful for the study of the flow course in 

 small vessels. Valuable results were obtained with 

 China ink and high-speed cinematography on pulmo- 

 nary capillaries (130) and on very small arteries of 

 the rabbit ear (136). 



In a carefully elaborated procedure, McDonald (91- 

 93) studied the flow pulse in the rabbit aorta and in 

 peripheral arteries of the dog by filming the move- 

 ment of injected gas bubbles through the translucent 

 vessel wall. Gas embolism was avoided by using pure 

 oxygen instead of air. An injected gas bubble travels 

 at a velocity quite near to the average blood velocity 

 v A if the bubble is spherical and just fills the lumen 

 completely. Smaller spherical or larger cylindrical 

 bubbles will run faster. High-speed cinematography 



