1198 
MONITORING 
helmets, saddles or submersible canisters. The 
use of these latter systems requires much more 
careful surveillance and training of such instru- 
mented animals than when using implanted 
units. These problems markedly increase in 
complexity as the number of monitored parame- 
ters is increased. Experience in this laboratory 
has also shown that the use of backpack units 
becomes very difficult or impossible when un- 
cooperative animals are utilized. As advances 
are being made in technology and as monitoring 
needs are increasing in complexity, it is becom- 
ing evident that implantable systems have an 
important and indispensable role in physiologic 
investigation. 
Various investigators have reported implant- 
able systems for measuring temperature,^ vas- 
cular and intraocular pressures,^'^ brain 
activity,^ ECG^ and acceleration.*' The number 
and nature of the systems utilized in a particu- 
lar experiment are now largely determined by 
the demands of the experiment, the size of the 
animal and the nature of the transducer. It is 
the purpose of this article to discuss the present 
state of development and use of implant teleme- 
try systems for study of cardiovascular func- 
tion. 
DESIGN REQUIREMENTS 
The implant package must be sufficiently 
small so as not to disturb the normal physiologi- 
cal functioning of the animal. The tolerable size 
obviously depends upon the size of the animal 
and the location of the implant package in the 
body. Multichannel systems, weighing 30 grams 
and with a volume of 25 cm^ have been used 
frequently in 15- to 20 Kg dogs. Although tech- 
nological advances in microelectronics and inte- 
grated circuit techniques make it possible to de- 
velop very sophisticated electronic systems in 
extremely small packages, such methods do not 
solve the power supply problem or the need for 
micropower operation if long duration of opera- 
tion is required. Since the implant system must 
contain not only the electronics, but also the 
power supply, the use of the smallest possible 
battery consistent with the required operating 
life is essential. The magnitude of the power 
drain then becomes an important criterion, 
since it effects the overall size of the implanted 
unit through the battery requirements. 
A power requirement of a few microwatts to 
a few milliwatts has been achieved in the single 
channel unit designs. Sensors vary considerably 
in their power requirements depending upon 
the kind of measurements being made. High 
impedance thermistors require only a fraction 
of a microwatt for temperature measurements. 
About Vi milliwatt is required to operate the 
solid state, strain-gage type cells presently used 
for pressure measurement, while the transduc- 
ers for flow measurement require considerably 
more power. 
The other major part of an implanted teleme- 
try system is the radio-frequency (RF) trans- 
mitter. Power requirements depend upon the 
operating range, antenna efficiency, informa- 
tion rate, and encoding scheme used. To reliably 
sustain oscillation in the transmitter state, a 
2N918 type transistor has been used in the cir- 
cuit designs in this laboratory and requires a 
minimum current of 0.5 to 0.8 ma or more, 
using a 1.35-volt mercury cell. This power level 
will typically provide an operating range of 50 
to 150 feet which is more than adequate for 
most experiments. In order to reduce the aver- 
age transmitter power demand to the microwatt 
power level, a pulse modulated RF is used in- 
stead of a continuous wave (CW) carrier. This 
results in a reduction of range to 5 to 10 feet be- 
cause of susceptibility to spurious noise, but 
provides continuous transmission of data and is 
adequate for use with small caged animals, such 
as rats. 
The use of glass, metal, or ceramic to provide 
hermetically sealed cases for all the electronics 
is the only reliable method for protection of a 
totally implanted transmitter. Metal with 
glass-to-metal seals is the most versatile con- 
struction technique, but special precautions 
must be observed to obtain effective RF radia- 
tion. Ceramic and glass will not interfere with 
the RF, but the high sealing temperatures can 
cause difficulties with the electronics and bat- 
teries. 
METHODS AND RESULTS 
All parameters required for description of 
cardiovascular performance in hydraulic or me- 
