1122 
MONITORING 
to monitor the condition of the pressure gauge. 
The drift rate of the gauges when implanted is 
significant for chronic applications. A small 
fraction of the gauges drift insignificantly over 
St period of a month while another small frac- 
tion drift as much as 50% of full scale over a 
period of a month. The drift rate is different 
after implantation than when they are stored. 
The typical zero baseline drift rate after im- 
plantation is on the order of 10 mm Hg/month. 
The sensitivity of the gauges also decreases 
with time, typically on the order of 5% /month. 
For these reasons in vivo calibration using a 
catheter is essential for accurate pressure moni- 
toring. The frequency response of the pressure 
gauges is well above that required for repro- 
duction of the high frequency components of the 
pressure waveforms. Clotting caused by the 
gauge is a rare but real problem. Occasional 
massive embolization will occur. Necrosis of 
the tissue around the pressure gauge and the as- 
sociated sutures with resultant rupture of the 
aorta or ventricle is an occasional problem. With 
all of these problems, implantation of the minia- 
ture gauge is still probably the best compromise 
for provision of a high fidelity recording of con- 
tinuous blood pressure in a form suitable for 
radiotelemetry. 
Dimension Measurement 
The transit time of sound travelling between 
two crystals sutured in opposition across the 
left ventricle is taken as a measure of the diam- 
eter of the left ventricle. The transducer con- 
sists of piezoelectric discs (LTZ-2, 5 MHz, 3 
mm X 0.5 mm approx.) with a styrene diverging 
lens glued to one side of the disc. The crystals 
are backed with a dacron patch epoxied to the 
crystal so that sutures can pass through the 
dacron to fasten the transducer to the epicar- 
dium. PVC insulated, twisted pairs of multi- 
strand copper wire are soldered to the elec- 
trodes on the crystals. For measurement of 
ventricular diameter the two crystals are su- 
tured to the epicardium in the area of the inter- 
ventricular sulci opposing each other across a 
diameter of the left ventricle. The wires are 
passed subcutaneously to the area of the scap- 
ulae where they are brought through the skin. 
A brief (.2 fxsec) electrical pulse is applied 
to one crystal. It responds by vibrating tran- 
siently and this acoustical pulse is coupled into 
the epicardium and travels across the ventricle 
to strike the other crystal. The electrical signal 
occurring across the receiver crystal when the 
sound strikes it is amplified to form an electrical 
pulse delayed in time from the initial pulse by 
an amount equal to the transit time of the 
sound. A rectangular waveform of voltage is 
generated which begins when the sound begins 
its transit and which is terminated by the sound 
arriving at the second crystal. This process is 
repeated 2000 times/sec. The resultant train 
of rectangular voltage waveforms is filtered 
(averaged) to form a voltage proportional to 
the transit time of sound across the ventricle. 
This voltage is amplified to a level appropriate 
for driving the telemetry apparatus. The fre- 
quency response of the system is limited elec- 
tronically to 100 Hz. The minimum separation 
that can be measured is on the order of 5 mm. 
The stability of the system is typically 0.03 
mm/hr drift but this drift rate can be elimi- 
nated by direct calibration on an oscilloscope 
against a crystal oscillator. The advantages of 
this approach include a light-weight, essentially 
inertialess transducing system for provision of 
an extremely accurate measurement of dimen- 
sion. The primary difficulty involved in the 
practical application of this approach involves 
the orientation of the transducers. The sound 
beams are relatively directional and although a 
diverging lens is used on the crystals, some ex- 
perience is required to place the crystals such 
that each will lie in the sound field of the other. 
If the crystals are not oriented into each other 
properly the received signal will be sufficiently 
low or will change so drastically during the 
cardiac cycle as to preclude an accurate meas- 
urement. This is quite apparent when the re- 
ceived signal is viewed on the oscilloscope. 
Radiotelemetry 
The radiotelemetry system is a straightfor- 
wa^n adaptation of aerospace FM/FM tech- 
niques, i.e., frequency modulated subcarriers 
are summed and the composite signal modu- 
lates the frequency of a VHF radio transmitter. 
