METHODS OF MEASURING BLOOD FLOW 



^99 



-P0« 



OP 



fig. 6. Principle of orifice flowmeter of Gregg and Green. 

 PO, connections to differential manometer; OP, orifice plate. 

 [From Green (50).] 



cannula lumen. Details of construction, as well as 

 the connection of the orifice-meter cannula to the 

 differential manometer, are shown in figure 7. The 

 rubber membrane of the manometer, to which the 

 mirror is attached, bulges under the action of pressure 

 differences between its two sides, while it is insensitive 

 to the absolute pressure. The steady-flow calibration 

 curve is virtually quadratic if saline solution is used. 

 When blood is used, the effect of term I is noticeable. 

 The natural frequency of the differential manometer 

 amounts to 50 to 70 cps or more (54). It is difficult to 

 judge the effect of term III on the records of pulsatile 

 flow; the coefficients of equation 8, including C 3 , 

 vary with the cannula and orifice diameter. Arterial 

 flow curves recorded with the orifice meter, particu- 

 larly those of the femoral, axillary, and carotid 

 arteries (120), might suggest that the contour of the 

 svstolic flow peak and the registered backflow phase 

 could be markedly affected by term III, although 

 arterial flow patterns are widely variable for physio- 

 logical reasons, as McDonald (93) discusses in detail. 

 The applicability of the orifice meter to veins is 

 limited because of its frictional pressure drop. 



Schroeder's differential - pressure flowmeter 

 ("Druckdifferentialstromuhr") (119) may be re- 

 garded as a further developmental stage, obtained by 

 new technical means, of Broemser's differential sphyg- 

 mograph (see below). Schroeder designed his instru- 

 ment, which is shown in figure 8, for application on 

 unopened carotid and femoral loops of conscious dogs. 

 In a special compartment (C), the artery (.4) is com- 

 pressed by a screw device (S) so that its wall is relaxed. 

 Two rubber diaphragms (Di , Z) 2 ), arranged at the 

 bottom of the compartment, are in direct contact with 

 the skin surrounding the vessel, one diaphragm is 

 placed at an upstream vessel segment, the other at a 

 downstream segment. Each diaphragm covers a 

 water-filled chamber [Chi , Ch ), and can transmit 

 the pressures from both vessel segments into these 

 chambers. Due to the compression of the vessel, its 

 skin and wall tissues are deformed so that the vessel 



lumen is narrowed toward the middle of the compart- 

 ment; the slight constriction gives rise to a flow-related 

 pressure difference between the two vessel segments, 

 which is detected by a thin metal membrane (.\/i ) 

 interposed between the chambers and is transferred 

 by a lever (L) to an air-pressure nozzle amplifier 

 (A.h ) for optical manometer registration. By two 

 other membranes (Mo , A/ 3 ) connected to similar air- 

 pressure amplifiers (NA2 , NAz ), the pressure of each 

 chamber is picked up, and the sum of both pressures 

 is optically recorded by an adding manometer. In 

 this way, the difference between the pressures (related 

 to the blood flow) and their sum (related to the blood 

 pressure) are recorded simultaneously. The steady- 

 flow calibration curve is almost quadratic; it is cor- 

 rected automatically by an optical linearizing device. 

 Distortions due to term III of equation 8 are reduced 

 by attaching an air chamber to the differential- 

 pressure air-transmission system. Although some me- 

 chanical functions of this design may require further 

 theoretical clarification, its records of pulsatile ar- 

 terial flow and pressure resemble to a surprisingly 

 high degree those obtained by other well-recognized 

 instruments. 



Two other devices mav be mentioned here for re- 



OP O PD 



fig. 7. Construction of orifice flowmeter and differential 

 manometer. OP, orifice plate; 0, orifice; PU and PD, upstream 

 and downstream connections to manometer. .?, shell; B, base; 

 C, cap of manometer. L, lens, carried by a ball. M, mirror 

 attached to rubber diaphragm of manometer [From Green 



(50).] 



