1120 
MONITORING 
dynamics in chronic animal preparations are 
regional blood flows, ventricular and aortic 
pressures, and ventricular dimensions. In addi- 
tion, an interrogation system is carried by the 
animal to provide the capability for remote con- 
trol of power, selection of measuring sites, etc. 
Each of these subsystems will be described 
individually. 
TECHNIQUES 
Flow Measurement 
Regional blood flows are measured using the 
continuous wave ultrasonic Doppler shift blood 
flowmeter. The system utilizes the Doppler prin- 
ciple whereby sound reflected from a moving 
target will be shifted in frequency by an amount 
proportional to the target velocity. For blood 
flow measurement, 9 MHz sound is beamed 
through the unopened blood vessel wall into the 
bloodstream. The sound is scattered isotropi- 
cally, primarily by the erythrocytes, and a frac- 
tion of the backscattered sound travels through 
the blood vessel wall where it is sensed. The 
backscattered sound diff"ers in frequency from 
the incident sound by an amount proportional 
to the blood velocity, in accordance with the 
Doppler equation which states that the shift 
in frequency equals: 
2 Ft V cos a 
C 
where Ft = incident sound frequency 
V = erythrocyte velocity 
a = angle between the sound axis 
and the flow axis 
c = velocity of sound in blood 
For a practical transducer operating at 10 
MHz the Doppler shift in frequency will be 
nominally 100 Hz/cm /sec. The backscattered 
signal is mixed with the projected signal to pro- 
duce the sum and difference frequency signals. 
The frequency of the difference frequency sig- 
nal is recorded and calibrated in terms of in- 
stantaneous blood velocity. So long as the cuff- 
like transducer constrains the blood vessel to a 
constant diameter, the blood velocity as meas- 
ured by the Doppler flowmeter provides an ac- 
curate measure of volume flow. 
The component parts of the flowmeter are 
the transducer which is implanted around the 
blood vessel, an oscillator to provide 9 MHz 
power to excite the piezoelectric crystal within 
the transducer, and the receiver which amplifies 
and demodulates the backscattered sound signal 
sensed by the transducer. 
Transducers 
The implantable transducer has the form of 
two molded expandable polystyrene hemicylin- 
ders hinged together to form a cylinder which 
is clamped around the exposed blood vessel. A 
piezoelectric plate (LTZ-5, lead zirconate-titan- 
ate, 10 MHz) is glued within each half of the 
cylinder. The plates are angled at 45° with the 
flow axis so that the acoustical fields of the 
plates intersect within the lumen of the cylin- 
drical transducer, i.e., in the center of the blood- 
stream. PVC insulated, multistrand, copper 
wires in twisted pairs are connected to the elec- 
trodes of the crystals. The low impedance of the 
crystals (50-200 ohms at resonance) along with 
the inherent frequency modulation operating 
principle makes the Doppler flow system rela- 
tively insensitive to environmental noise and 
electrical shunting of tissue fluids. Thus, ex- 
treme electrical insulation and shielding are not 
required. 
At operation, the blood vessel is exposed and 
the transducer is tied in position around the 
blood vessel. The wires are passed subcutane- 
ously to the area between the scapulae and the 
wires brought through the skin. The transducer 
material prompts a transient inflammatory re- 
sponse with resultant growth of connective tis- 
sue from the blood vessel around the transducer 
and also a growth of tissue up into the cavity 
between the angled crystal plate and the wall 
of the blood vessel. The connective tissue in 
contact with the crystal forms an acoustical 
path for the sound to travel from the crystal 
into the blood vessel. After approximately a 
week the tissue is adherent to the transducer 
body, and the transducer can be expected to 
function routinely for three months. The most 
common failure mode is corrosion and breakage 
of the copper wires. Where experiments are to 
be conducted for longer periods of time, 
stranded stainless steel wire should be used in- 
stead of the copper wire. 
