594 
HEMODYNAMICS 
were instrumented in February 1970 (Group 
I), three animals were instrumented in March 
1971 (Group II). The two uninstrumented ani- 
mals were studied with the Group I animals. 
Eight-channel telemetry units were im- 
planted in both Group I instrumented animals ; 
two 5-channel units and an 8-channel unit were 
implanted in the Group II animals. The opera- 
tion of such multichannel units has been pre- 
viously described."'^^ Multichannel operation 
is provided by a time sharing multiplex scheme 
for switching between the various sensor in- 
puts; data are transmitted on a single carrier 
frequency using pulse interval modulation. One 
data channel in each unit is used to transmit a 
long duration pulse from a reference bridge to 
provide synchronization for the multiplex 
scheme. Battery power is conserved by the use 
of an implantable radio-frequency actuated 
switch. The manner of recording data from 
each animal in the immediate laboratory envi- 
ronment is schematically illustrated in Figure 1. 
A standard squeeze cage was utilized to house 
all animals. The perching bar of the cage was 
modified to serve as the receiving antenna for 
the implanted units. The movable wall of each 
RADIO FREQ SWITCH 
TURN UNIT ON 
COMMUTATED 
ANALOG 
SIGNALS 
SUB-CARRIER 
OSCILLATOR 
i 
PULSE INTERVAL 
MODULATION OF RF 
TRANSMITTER 
RECEIVER 
DEMODULATOR 
RECORDER 
RECEIVING 
ANTENNA 
Figure 1. — Manner of obtaining cardiovascular data by telemetry from a chimpanzee. 
