i3*4 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



rate of i ml per sec will generate a flow-signal voltage 

 of about 0.25- io~ 3 volts (equation 14). In the ascend- 

 ing aorta or pulmonary artery trunk, signals of several 

 millivolts can be recorded at flow peaks. Non- 

 polarizable electrodes are indispensable. Zn-ZnS0 4 

 electrodes are useful; calomel half cells give still more 

 satisfactory results [Katz & Jochim (72); Jochim 

 (68)]. The electrodes are connected to the vessel wall 

 by wicks soaked in saline-agar solution or by saline- 

 agar filled glass tubes. Also Ag-AgCl electrodes are 

 recommended (33). Furthermore, the vessel's di- 

 ameter and cross-sectional area must be kept constant 

 throughout the measurements. The best way is to use 

 a rigid sleeve of insulating material (76). The size of 

 the sleeve should be carefully chosen so that the 

 vessel is narrowed down to that diameter which would 

 be reached if the blood pressure fell to the lowest 

 level expected during the experiment. This moderate 

 constriction will not essentially interfere with the 

 hemodvnamic conditions, nor will the rigid sleeve 

 give rise to pulse-wave reflections if it is not longer 

 than about 1 cm. The tips of the saline connections to 

 the electrodes are contained in two small holes placed 

 in the sleeve wall at right angles to the vessel axis and 

 to the lines of magnetic force. The sleeve also assures 

 a fixed position of the vessel relative to the magnet and 

 protects the exposed vessel from drying as well as from 

 undesired contact with neighboring tissues. To permit 

 introducing the vessel, either the sleeve has a small 

 longitudinal slot, or is composed of two halves which 

 are joined together around the vessel. If the flow- 

 signal voltage picked up by the electrodes is high 

 enough, it can be directly recorded by a string gal- 

 vanometer (75, 133). However, d-c amplifiers are 

 generally employed (33, 64, 68, 110, 128, 134), or 

 the input voltage is converted into alternating current 

 by a mechanical chopper (72, 76), and a capacitance- 

 coupled amplifier may then be used. The over-all fre- 

 quencv response of the amplifier system and recording 

 galvanometer should be uniform up to at least 50 cps. 

 The base line is assessed during the experiment 

 either bv clamping the vessel distal to the site of 

 measurement or by de-energizing the magnet. See 

 also (32). Calibration is performed by perfusing the 

 excised vessel or the vessel in situ with blood or saline 

 solution at known flow rates. Because of the strictly 

 linear calibration curve, it is sufficient to determine, 

 in addition to the zero point, only one point corre- 

 sponding to a flow rate near the upper limit of the 

 range under investigation. The calibration can also 

 be done using a nonsteady flow: a known quantity of 



fluid is injected into the vessel by means of a syringe, 

 and the course of the corresponding flow signal is re- 

 corded. Thus the mean flow rate and the mean flow 

 signal can be calculated from the known injected vol- 

 ume and the time and deflection as recorded on the 

 tracing (73). 



In spite of its theoretical simplicity, the d-c pro- 

 cedure has been widely abandoned because of several 

 practical drawbacks. The magnet and most types of 

 nonpolarizable electrodes are rather bulky. In the 

 case of flow measurements on small vessels, the flow- 

 signal voltage is very low so that high-gain d-c ampli- 

 fication with its inherent difficulties is required and 

 changes of the electrode potential will cause drift of 

 the base line. The results of Richards & Williams (1 10) 

 and of Inouye el al. (65) show that, in spite of utmost 

 care, such difficulties are present even in the applica- 

 tion of the d-c procedure to the dog's carotid and 

 femoral arteries. By improving the electrodes and 

 using modern stabilized d-c amplifiers, however, satis- 

 factory short-time recordings of the flow in the de- 

 scending aorta of the dog have been made possible 

 [Feder & Bay (33)]. As to vessels close to the heart, 

 cardiac action potentials may be picked up by the 

 electrodes in addition to the flow signal. 



The a-c modification [Kolin (76, 77)] is character- 

 ized by the use of an alternating magnetic field which 

 is generated by energizing the coils of the electro- 

 magnet with sinusoidal alternating current: 



B = B Q sin<ut 



(15) 



where B n = amplitude of magnetic flux density; co = 

 2 ir /;/ = frequency; t = time. The flow signal picked 

 up by the electrodes is, therefore, an a-c voltage which 

 is strictly in phase with the a-c magnetic-field strength 

 The amplitude of the flow-signal voltage is further 

 proportional to the average instantaneous flow veloc- 

 itv or flow rate: 



E'BDv.-IO 

 f A 



-8 



-- BDv • 10 ■ sin ut volts. (16) 

 o * 



This means that an amplitude-modulated flow signal 

 is delivered so that the well-known advantages of a 

 carrier-frequency operation result, especially with the 

 use of a-c amplifiers which permit higher gain and 

 greater stability than d-c amplifiers. A further ad- 

 vantage of the a-c modification is that the flow-signal 

 voltage can be picked up by simple metal electrodes, 

 such as platinum, gold, silver, or stainless steel. As 

 much higher gain is obtainable than by d-c amplifica- 



