FRANKLIN, VATNER, HIGGINS, PATRICK, KEMPER AND VAN CITTERS 
1125 
The system for measurement and telemetry 
of blood flow using the ultrasonic Doppler telem- 
etry flowmeter has also been described in de- 
tail. ^'^-'^ In brief, the Doppler flow signal with 
frequency components from 200 Hz to 15 kHz, 
is linearly summed with the voltage-controlled 
oscillator signal representing pressure to form a 
frequency-multiplexed video signal. This signal 
is applied to the input of a frequency-modulated, 
radio frequency transmitter operating in the 
88-108 MHz range. The power output, approxi- 
mately 10 mw, coupled to a shortened, induc- 
tively, loaded stub antenna, is adequate to trans- 
mit the signals over a several hundred foot 
range to the mobile recording station. 
The mobile recording unit consists of an FM 
communications receiver, subcarrier discrimi- 
nator, signal processing electronics, and a tape 
recorder. A commercial 5-element FM antenna, 
mounted on the roof of the van, serves to re- 
ceive the telemetered signals from the animal. 
Power for the equipment is obtained from a DC 
to AC inverter (Heathkit Model MP-14, Heath- 
kit, Benton Harbor, Mich.) which is powered 
by a 12-volt automobile battery. 
The FM tuner (Heathkit Model AJ-15), modi- 
fied to disable the FM deemphasis network and 
multiplex circuitry, provides video response 
sufficient to recover both the Doppler flow signal 
(200 Hz-15 kHz) and the 37-43 kHz pressure- 
modulated subcarrier signal. The output of the 
FM tuner is processed by a 15 kHz low-pass 
filter to extract the Doppler flow signal which is 
then recorded on one channel of the tape re- 
corder (Honeywell Model 5600, Honeywell, 
Minneapolis, Min.). The output of the tuner is 
also applied directly to the input of a subcarrier 
frequency discriminator (Airpax Model FDS- 
30, Airpax Electronics, Inc.) which converts 
the 37 to 43 kHz frequency modulation signal 
to a DC voltage proportional to pressure, which 
is coupled to another channel of the tape 
recorder. 
Exercise studies conducted in the field with 
dogs running spontaneously at speeds of greater 
than 20 miles/hour are superior to treadmill 
exercise not only because they yield a purer 
form of exercise with less components of ex- 
citement, but also the level of exercise attained 
is greater. Thus, control mechanisms which can 
only be elicited with severe exertion might not 
be evident with the moderate exercise attained 
on the treadmill. An example of this is the left 
ventricular response to exertion. Studies con- 
ducted in dogs running on the treadmill indi- 
cated that the left ventricular response to exer- 
tion occurred almost entirely through increases 
in heart rate;^''' whereas, the contribution of 
the Frank-Starling mechanism, a means by 
which the heart can augment its output by in- 
creasing initial myocardial fiber length, was not 
evident.^'^ We recently re-examined this prob- 
lem in conscious untethered dogs running spon- 
taneously in the field at speeds of greater than 
20 miles/hour. In this study, measurements of 
left ventricular dimensions and pressure were 
telemetered to a mobile recording unit. The re- 
sults of this study indicated that during severe 
exertion near maximal increases in heart rate 
(over 300 beats/min) and contractility (dP/dt 
over 12,000 mm Hg/sec) occurred, while in all 
eight animals studied a significant increase in 
left ventricular end diastolic dimensions could 
be identified. Thus, by utilizing radiotelemetry 
techniques to study severe exercise, it was found 
that an important cardiovascular control mech- 
anism (Frank-Starling) is utilized only in sit- 
uations of extreme stress. 
We have applied the techniques for radiotelem- 
etry of simultaneous measurements of blood 
flow and arterial pressure to describe the nor- 
mal and abnormal peripheral circulatory ad- 
justments to severe exercise. It is traditionally 
held that exercise causes sufficient increases in 
visceral vascular resistance to reduce visceral 
flow in order to provide additional flow to the 
active musculature.""^^ However, treadmill ex- 
ercise in dogs failed to produce significant reduc- 
tions in visceral flow."'-" It was demonstrated 
that treadmill exercise is not sufficiently severe 
to elicit this circulatory compensatory mecha- 
nism. Accordingly, measurements of visceral 
flows and arterial pressure were telemetered 
from Alaska sled dogs running great distances 
in sub-zero weather and also from mongrel 
dogs running spontaneously in the field at 
speeds greater than 20 miles/hour." This level 
of exercise was sufficiently severe to elevate 
heart rate to 300/min in some dogs and to in- 
crease mean arterial pressure from 89 to 140 
