W. S. TOPHAM 
1237 
the use of an A-C amplifier rather than using a 
D-C amplifier, which has its own inherent prob- 
lems of drift and zero stability. The resultant 
pressure signal was indistinguishable from that 
of the D-C excited system. 
To test the system, the gain of the amplifier 
was adjusted to produce a 0.2-volt output per 
cm/H20, and the transducer was immersed in a 
saline bath, which was kept at a constant tem- 
perature. A magnetic stirrer was then turned 
on and off to check the transducer sensitivity to 
flow dependent heat transfer. The transducer 
was first checked using the standard D-C exci- 
tation system, and the results are shown in Fig- 
ure 5, Part A. The same transducer, under the 
same conditions, with only a very slight change 
in the bath temperature, was again tested. It 
can be seen in Figure 5B that the zero drift was 
reduced to just over 0.5 cm/HoO, as compared 
to 7 cm/H20 under the D-C excitation condi- 
tions. This technique seems to solve the zero 
drift problem due to flow perturbations, but it 
was found that because of the dependence of the 
pulse on the capacity characteristics of the 
transducer, the transducer became more sensi- 
tive to the type of fluid inside the catheter be- 
cause it affects this capacitance. Thus, when 
calibrating the transducer, it is necessary to use 
the same fluid as is used during the experiment. 
Based on these in vitro tests and making a 
correction for any temperature change of the 
environment around the transducer, it was de- 
termined that this transducer had an accuracy 
of 0.5 to 1 cm/HsO. 
The purpose in developing the catheter was 
to facilitate accurate measurement of venous 
pressure and to record the relatively small 
changes in this pressure as a dog exercised on a 
treadmill. Under local anesthetic and with the 
dog fully awake and standing on the treadmill, 
two catheters were inserted into the right jugu- 
lar vein. One catheter was advanced into the 
right atrium, and the other advanced on into 
the inferior vena cava as was verified by fluoros- 
copy. Typical waveforms obtained during an ex- 
ercise run are shown in Figure 6. A typical 
right atrial pressure waveform is shown prior 
to exercise. The pulsations are greatly reduced 
at the inferior vena cava, and while there may 
be some artifact introduced on the pressure 
10 PcmH^O 
5 
O 
-5 
"L 
JL 
10 - 
5 - 
-5 
Pressure Transducer 
Amplifier Output 
Stirrer 
off 
on 
on 
off 
on 
-1 -"C 
+ 1 
-1 - 
Bath Temperature 
+ 1 
(A) 
D.C- Excitation 
(B) 
Pulse Excitation 
Figure 5. — Results of an in vitro test showing the zero drift due to temperature variation caused by flow changes. 
The standard D-C excitation and the new pulse excitation method are compared. 
