1124 
MONITORING 
ent sites of interest. These transducers can be 
connected through electromechanical relays to 
a single flow meter. By switching among the 
three flow transducers one can sequentially 
sample 3 different flows without approaching 
the animal for resoldering of wires. This has 
proved to be a considerable time-saving con- 
venience, particularly when intractable animals, 
such as baboons, are being studied. In addition, 
the remote control feature provides a capability 
for electrical stimulation or infusion of cardio- 
active drugs. 
The receiving station consists of an appro- 
priate receiving antenna, FM radio receiver, 
appropriate discriminators and a magnetic tape 
recorder for storage of the information. For 
field operation this equipment is housed within 
a truck to serve as a mobile receiving station. 
This approach is particularly effective for stud- 
ies of exercising dogs where the animal runs 
spontaneously behind the moving recording 
vehicle. 
The receiving antenna is a standard enter- 
tainment quality FM 100 MHz 5 element Yagi 
antenna. 
The 100 MHz signal from the antenna is 
coupled to an entertainment quality FM radio. 
The deemphasis network is bypassed in the 
radio receiver to provide a linear frequency 
response. The output of the radio receiver is 
coupled to appropriate frequency discrimina- 
tors. Where voltage control oscillators are used 
on the animal these are standard commercially 
available frequency-to-voltage converters. The 
Doppler flow signal requires a special kind of 
discriminator. While in principle it is similar 
to the standard discriminators, the frequency 
deviation is so wide in the Doppler flow signal 
that a special discriminator is required. The 
practical instrument currently in use provides 
a voltage proportional to the average number 
of zero crossings per unit time of the input 
signal over a frequency range of from 200 Hz 
to 20 kHz. The output frequency response of 
the discriminators is limited electronically to 
100 Hz. 
APPLICATIONS 
While the capability for measurement of 
blood flows, pressures and cardiac dimensions in 
conscious animals has extended the scope of 
cardiovascular physiological investigation, the 
ability to monitor these variables remotely has 
further extended the capabilities of the physiol- 
ogist. There are certain studies for which telem- 
etry is essential, for instance, studies con- 
ducted outside the confines of the laboratory 
and especially the study of animals in their 
natural habitat. Other studies, which involve 
non docile animals, such as the larger primates, 
are also made feasible through the use of telem- 
etry techniques. In addition it is apparent that 
any study in conscious animals is infiuenced to 
an unknown extent and may even be limited by 
the laboratory environment. Several of these 
studies have been demonstrated to be improved 
substantially by applications of telemetry tech- 
niques. For instance, the study of maximal 
exercise in dogs is difficult if not impossible 
utilizing "directly wired" techniques and tread- 
mill exercise. However, it is not uncommon for 
dogs to run spontaneously at speeds of greater 
than 20 miles/hour in the field. The capability 
for telemetry provides a means by which a de- 
scription of the cardiovascular response to se- 
vere exercise can be accomplished. 
The system developed for radiotelemetry of 
left ventricular dinlensions and pressure has 
been described in more detail previously.^'^ In 
brief, an improved ultrasonic dimension gauge 
was used, which measures the transit time of 
acoustic impulses travelling at the sonic velocity 
of approximately 1.5 X 10^ mm/sec between the 
two 5-MHz piezoelectric crystals sutured in op- 
position on the epicardial surfaces of the left 
ventricle. A voltage proportional to transit time 
is recorded and calibrated in terms of crystal 
separation and thus provides a continuous meas- 
urement of ventricular size. The signals from 
the diameter gauges and pressure implant sys- 
tem are transmitted from the animals via a 
two-channel FM-FM telemetry system to the 
mobile recording unit. A measure of left ven- 
tricular pressure is provided by the miniature 
pressure gauge ^ implanted within the left ven- 
tricle. A voltage proportional to pressure is ob- 
tained, which after appropriate amplification, 
can be telemetered along with a measure of 
dimensions using separate voltage-controlled 
oscillators. 
