1170 
MONITORING 
Figure 2. — Schematic diagram of CW transducers and beam patterns including condition for measurement of blood 
velocity at the center of the lumen (aD<<D) and optimum transducer design (a = 35° and d = V3xD)''' for 
meeting this condition." 
The scattered energy, which is Doppler shifted 
in frequency, is detected by the same transducer 
to provide a measure of blood velocity as a func- 
tion of location within the lumen. The coordi- 
nates of each location are derived from the 
round trip transit time of the burst of ultra- 
sonic waves from the transducer to the location 
and return. Therefore, a blood velocity profile 
can be measured with this instrument. In addi- 
tion, ultrasonic energy scattered by the walls 
of the vessel provides the basis for a direct 
measurement of its lumen diameter. Conse- 
quently, the pulsed Doppler flowmeter provides 
more than sufficient information for an accurate 
direct measurement of instantaneous volume of 
flow. Its performance with respect to measure- 
ment ambiguities, electronic precalibration, 
zero drift and scale factor changes is largely 
equivalent to that of the CW Doppler velocity 
flowmeter; it is not subject to any inherent 
shortcomings. 
Comparative Features of Flow Telemetry 
Comparing the various types of flowmeters, 
which have been discussed with respect to meas- 
urement ambiguities, electronic precalibration, 
zero stability and scale factor constancy it ap- 
pears that the CW Doppler velocity flowmeter 
and ' the pulsed Doppler velocity profile or 
volume flowmeter offer the highest levels of 
performance. The capabilities of these two in- 
struments although generally desirable are 
especially attractive for reliable chronically im- 
plantable telemetry systems. 
Considering size and power drain, the prin- 
cipal difference between the flowmeters under 
discussion is a consequence of the large (e.g., 
500 ma) field currents which are needed in the 
electromagnetic flow transducer. Typical power 
requirements for electromagnetic flowmeters 
exceed 500 mw while those of the implantable 
ultrasonic flowmeters described hereafter are 
10-20 mw. As a consequence of their large field 
current requirements electromagnetic flovmiet- 
