55^ 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



KINETIC ENERGY FLOWMETERS 



One of the early efforts to describe the flow in an 

 artery was that of Marey and Chauveau. In the 

 hands of Chauveau and his pupils (92) an ingenious 

 apparatus was developed which enabled one to 

 record the pulsatile changes in the velocity of arterial 

 blood. The fundamental principle is to arrange that 

 the force of the stream moves an obstructing vane, 

 the movement of which is proportional to the force 

 or velocity of the stream. The movement actuates a 

 tambour-air transmission recorder. The records made 

 by this instrument are very nice technically and would 

 serve if quantitatively calibrated as indications of 

 the volume flow through the artery. The published 

 records, however, which are easily available are not 

 quantitatively calibrated (see fig. i). 



Later workers using the same principle employed 

 electrical signals made by movements of the vane or 

 bristle which was set to make changes in capacity, 

 resistance, or electromagnetic inductance. Such a 

 flowmeter, which has been exploited in both venous 

 and arterial pulsatile flows, is the bristle flowmeter 

 of Brecher (10). It is based on the mechanoelectric 

 transducer RCA 5734, a subminiature vacuum tube 

 that can be bought on the open market. 



Pendulum or bristle flowmeters are very useful 

 for recording pulsatile changes in arterial or venous 

 velocity. These can be quantitated by careful calibra- 

 tion and the volume flow measured if the size of the 

 vessel is known and held constant. For measuring the 

 cardiac output it must be placed on the pulmonary 

 artery. This sort of meter has not been used in the 

 closed-chest animal, since it must be clamped motion- 

 less and with the bristle centered in the vessel. It is 

 hard to see how it may become useful to measure 

 total flow under physiological conditions. 



Other methods which make use of the kinetic 

 energy of the flowing stream take advantage of the 

 differential pressure produced by registering a 

 flow-dependent pressure difference between two 

 points in a flowing stream. The pressure difference 

 is measured by a differential manometer (see Volume 

 II). Among devices of this sort are the venturimeter 

 (10), the orifice flowmeter (49), and the Pitot meter 

 (loi). These instruments have been widely applied 

 in the measurement of regional blood flow and will 

 be described in detail in \'olume II. The\ are 

 mentioned in passing here not because they have 

 been the means of measuring the cardiac output 

 definitively but because they offer that possibility if 

 technical ditliculties mav be overcome. 



These difficulties are in the surgical procedures 

 needed to insert them, the blood lo.ss and trauma 

 involved, and the fact that it is difficult to close the 

 chest about them. Catheter tip Pitot meters (loi) 

 can probably be inserted into the pulmonary artery 

 and the cardiac output measured. The same may be 

 said of inductance or possibly of strain gauge flow- 

 meters, both of which have been made for insertion 

 by catheter (10). Since the mechanical effect of the 

 flow is transduced into an electrical signal within 

 the (e.g.) pulmonary artery, the large mass of con- 

 ducting columns of fluid is avoided so that there is 

 the possibility that the transducer may be of high 

 frequency and hence the record may be of high 

 fidelity. These instruments, however, have not been 

 widely used to measure the cardiac output. 



Another related approach has been introduced by 

 Fry (43). It is based on a detailed analysis of the 

 hydrodynamics of flow through tubes (42). The 

 basic data used in computing the velocity (44) are 

 the pressure differences between lateral taps from a 

 double lumen catheter placed in the axial stream of 

 the aorta, or the pulmonary artery. These pressure 

 differences are those that overcome the inertia and 

 friction of the moving column of blood between the 

 taps and are sensed by a differential manometer. 

 They are minimal and just about at tlie threshold of 

 resolution of a\'ailablc pressure recorders. The 

 equation which relates these pressure differences to 

 velocity is quite complex and can be computed only 

 by means of an electrical analogue. In view of the 

 obvious artifacts of the published catheter tip pressure 

 records from which the computations were made, 

 it is amazing that the plot of the computation has 

 the same appearance as the classical records of 

 Chauveau (fig. i), as well as records from such 

 sophisticated instruments as the electromagnetic 

 and sonic flowmeters. Whereas this approach has 

 yielded considerations of great theoretical interest it 

 is seriously to be doubted that it will compete effec- 

 tively with other instantaneous methods for measuring 

 the cardiac output. 



The rotameter has been widely used to measure 

 regional flows but has also been used to measure the 

 total cardiac output on a few occasions (91). The 

 principle of the instrument is to direct the stream 

 upward through a widening tapered tube. A "float" 

 heavier than the blood is placed in the tube. The 

 tendency of the float to sink through the blood is 

 counteracted by the upward force of the stream. 

 Since the upper part of the tube is wider than the 

 lower part, tlie blood pas.ses the float in greater 



