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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



velocity from either the surface of a surgically exposed 

 vessel or by means of a catheter tip introduced into 

 the flow stream, but without causing significant dis- 

 tortion of the flow dynamics. 



All the blood flow recordings gathered by the 

 authors for this chapter, if not otherwise indicated, 

 have been made with a 240-cycle square-wave elec- 

 tromagnetic flowmeter (11) introduced in 1953 as the 

 first practical instrument for measuring blood flow in 

 any of the body's arteries and veins which have been 

 surgically exposed. As used here, this instrument has a 

 flat frequency response to 40 cps, and is down by 50 

 per cent at 100 cps, and, although the principle is 

 capable of an infinite frequency response, these 

 limitations are necessary in a practical instrument 

 primarily because of the carrier frequency residual 

 which would otherwise appear on the flow record. 

 The magnetic probes applied to the blood vessels 

 restrict pulsations and encroach on the lumen to the 

 extent of reduction in cross-sectional area by approxi- 

 mately 5 to 10 per cent. Such slight constriction 

 assures firm contact of the electrodes to the arterial 

 wall. Experimental testing showed that this amount of 

 constriction caused no perceptible change in the re- 

 corded flow pulse (45). 



Properties and Principles oj Flowmeters 



For recording of vascular flow velocity pulses an 

 ideal flowmeter should possess several properties: a 

 linear response to forward and backward flows, a 

 stable zero reference, and a frequency response ade- 

 quate to follow the phasic phenomena being re- 

 corded. It should also be unaffected by nonrelated 

 phenomena such as blood pressure, internal noise, 

 and muscle action potentials. Furthermore, its opera- 

 tion should not modify the phasic flow patterns, mean 

 flows, or blood pressure. To meet this last requirement 

 completely would mean that not only must the blood 

 vessel under consideration be unobstructed and non- 

 cannulated, but also that the recording be done 

 without anticoagulants or anesthesia and without 

 psychic trauma to the experimental subject (59). 



Obviously, a practical flow-recording system re- 

 quires some compromise with the above ideals; also, 

 such an elegant device would not be necessary for 

 most research work. If one knows the general charac- 

 ter of the quantities to be recorded, he may use with 

 confidence an equipment the characteristics of which 

 are considerably more restricted than the ideal. For 

 instance, a frequency response of zero to 50 cps is felt 

 to be adequate for cardiovascular work (6, 46); also, 



when recording ascending aorta flow, zero drift is not 

 serious since the flow can be taken to be zero at the 

 end of diastole, thus giving a continuously repeated 

 zero check. Therefore, an instrument used for cardiac 

 output measurements may have considerable drift of 

 zero and still be satisfactory for the purpose if it meets 

 the other requirements, although it might be unsatis- 

 factory for other situations where a stable zero refer- 

 ence is essential (14). 



Many different principles have been used for flow 

 recording; all have inherent potentialities for errors in 

 application or interpretation. These principles and 

 the instruments which embody them are discussed in 

 detail in Chapter 38. Here it will be necessary only to 

 list the different types of instrument and certain 

 references to the literature which are not found else- 

 where either in this chapter or in Chapter 38. 



a) Electromagnetic flowmeters (1, 3, 5, 8, 9, 16, 

 2 4> 53> 57> 63). b) Ultrasonic flowmeters (25). c) 

 Nuclear magnetic resonance (4, 23). d) Pendulum or 

 bristle flowmeters, e) Catheter tip pickups (36, 38). 

 /) Turbinometers (40, 41). g) Differential pressure 

 flowmeters (10, 17, 39). 



Cognate Phenomena 



LATERAL AND DIFFERENTIAL PRESSURES. Ill any Critical 



study of the relationship of the dynamics of pulsatile 

 flow it is necessary that pressure and flow be measured 

 simultaneously, and that the pressure be picked up 

 from a pressure tap the orifice perimeter of which is 

 in a plane parallel to the flow stream. One highly 

 practical system is to use a "clip needle" which by 

 means of a flexible clip holds the end of the needle 

 against the inside of the blood vessel wall (49). If a 

 Huber point is used on the clip needle, the recorded 

 pressure can be a true lateral pressure. 



The use of differential pressure measurements has 

 greatly enhanced our interpretation of the phenomena 

 occurring simultaneously within the vascular system. 

 Such a method usually takes the form of two pressure 

 taps conducted separately to either a differential 

 pressure transducer or to two individual pressure 

 transducers the amplified signals of which are elec- 

 trically subtracted from one another continuously. 

 The latter system has the advantage of being able to 

 view the individual pressures which make up the 

 differential pressure recording. These individual re- 

 cordings are useful in identifying artifacts which may 

 arise. 



computer techniques. For a proper understanding of 

 the hemodynamics of the cardiovascular system, a full 



