1200 
MONITORING 
HERMETIC METAL CASE 
WITH SILASTIC COATING 
ELECTRONICS 
BATTERY 
ANTENNA WITH 
FERRITE CORE 
ELECTRODES 
liiMiiiiiiii^ 
Figure 2. — Single channel implantable ECG unit. 
Studies of cardiovascular phenomena in 
awake animals have demonstrated the need for 
monitoring of more than one or two variables. 
Although multiple single channel units may be 
used for such purposes, a system which uses 
only a single RF link and therefore only one re- 
ceiver greatly reduces the complexity of such 
studies. A system capable of transmitting 3 to 
10 channels of data on a single RF link has re- 
cently been constructed and tested in our labo- 
ratories and proved to be reliable and reasona- 
ble to use.i^" The pulse generation and RF 
transmitter circuitry are similar to that for sin- 
gle channel units. By careful choice of circuit 
design, power consumption can still be main- 
tained at very low levels. Multichannel opera- 
tion is provided by a time sharing multiplex 
scheme for switching between various sensors. 
Sampling rate is 0.7 msec per channel. A typical 
8-channel system, illustrated in Figure 4, con- 
sists of three pressure sensors, two ECG leads 
and a three-axis accelerometer. For four pres- 
sure transducers and one set of ECG electrodes 
the average current drain is 2 ma from two 
mercury cells (1.35 V each). By the use of an 
implantable switch 500 hours of continuous op- 
eration has been extended to over one year. 
Multichannel capability has also been devel- 
oped to detect flow and distribution of flow 
using Doppler and/or electromagnetic flow 
(EMF) transducers. ^^'^^ The development of 
a multichannel battery operated EMF system 
has been of considerable interest because of its 
accuracy and wide acceptance for cardiovascu- 
lar research. Provision has been made in the de- 
sign to handle as many as four simultaneous 
flow measurements. Batteries and electronics 
for a four-channel system weigh approximately 
