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HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



coil. This current is proportional to the rate at which 

 the turbine rotates and this in turn is proportional 

 to the flow. It suffers the disadvantage of a high re- 

 sistance to flow, positional instability, and the need 

 for heparinization. For these reasons it is useful only 

 in the open-chest animal under the immediate effects 

 of surgery. The adherence of small strands of fibrin 

 on the turbine and any backflow will stultify the 

 calibration. Its response is slow, and it cannot measure 

 phasic flows. 



In contrast to the above, the electromagnetic and 

 sonic flowmeters have been implanted on the aorta or 

 pulmonary artery and have been used to measure 

 flow after recovery from surgery and when the animal 

 was in a normal physiological condition. 



ELECTROMAGNETIC FLOWMETERS 



This device depends on the principle that when a 

 conductor moves at right angles to the lines of force of 

 a magnetic field an electrical potential is induced. In 

 this application of Faraday's law the conductor is a 

 stream of fluid (blood) passing between the poles of a 

 magnet. The induced current is led off" by electrodes 

 placed across the conduit of the stream. The current 

 is proportional to the velocity of the stream and its 

 polarity is determined by the direction of the stream. 



The general principle of electromagnetic induction 

 as a measure of flow is said (125) to have been sug- 

 gested by Faraday in 1832, who unsuccessfully at- 

 tempted to measure electric currents from the flow of 

 the Thames in the earth's magnetic field (see fig. 2). 



The application of this principle to blood flow in 

 living animals was independently developed by Kolin 

 (82) and Wetterer (141). Many authors have par- 

 ticipated in the development of the method and of 

 instruments to serve it (see 125). At first a constant 

 magnetic field was used which resulted in a unidirec- 

 tional pulsating current, necessitating bulky and un- 

 stable nonpolarizable electrodes. Difficulties in the 

 use of these electrodes led to the u.se of alternating 

 current, which avoided the need of nonpolarizable 

 electrodes but introduced "transformer" currents 

 which were independent of flow and were due to 

 fluctuations in the magnetic field. These transformer 

 currents could be eliminated in several ways, among 

 which was the use of a phase sensitive detector to 

 separate the transformer from the flow-induced po- 

 tential. Another method of silencing the transformer 

 current was by the use of a .square wave alternating 

 current to energize the magnet and a complex circuit 



to block the current induced during the instant when 

 the magnetic field was reversed (127). 



The pickup or probe developed by Spencer & 

 Denison (127) and by Kolin (83), and Kolin & Kado 

 (84), can be implanted in the body and used in chronic 

 experiments allowing a degree of freedom of move- 

 ment of the experimental subject. They can be ap- 

 plied to a human blood vessel during surgery and 

 promise a great deal of useful information in the near 

 future. A particularly interesting development is the 

 coreless electromagnetic pickup which can be used on 

 large blood vessels and which is minimal in bulk and is 

 easily implanted (83). 



A most important advantage of this method of re- 

 cording the blood flow is the fact that the detailed 

 phasic changes in the velocity and volume pulse can be 

 accurately recorded. This will no doubt prove of value 

 in arriving at an understanding of the cardiodynamics 

 of ejection under diff'erent circumstances and of the 

 nature and cause of oscillations (acceleration and 

 retardations of flow) in the aorta and its branches. 

 The fact that it will clearly plot the pattern of forward 

 and backflow is of considerable importance, differen- 

 tiating it from the thermostromuhr (50,98) which gives 

 confusing results because it does not differentiate 

 backflow. 



SONIC FLOWMETERS 



The velocity of the aortic stream can also be meas- 

 ured by the differential speed of sound going up and 

 downstream. This difference can be picked up as a 

 phase difference (77) or as a simple delay. A device 

 has been worked out in Rushmer's laboratory (2), 

 using the delay principle, which can be implanted on 

 the aorta or pulmonary artery and left in place with 

 wires passing through the body wall so that changes in 

 cardiac output can be assessed during normal activity 

 (38). High frequency electrical pulsations of \ery 

 short duration (0.2 Msec) were used to drive a piezo- 

 electric crystal of barium titanate at 3 mc (see fig. 3). 

 The crystal tranduces the electric pulses into ultra- 

 sonic vibrations. These are sent diagonally across the 

 aorta to a similar barium titanate crystal which acts 

 as receiver and is placed upstream. The current is 

 then switched mechanically to the receiving crystal 

 which then acts as a transmitter. The pulses are thus 

 alternately sent upstream and downstream 400 times 

 per sec, each crystal acting in turn as transmitter and 

 receiver. The path of the sound waves, being di- 

 agonally across the whole aortic stream, is affected 



