W. S. TOPHAM 
1233 
fit snugly on the vessel, and care should be 
taken to make sure that there is good electrical 
contact between the electrodes and the vessel 
wall. Under these considerations, three com- 
mercially available flowmeters were evaluated 
by us. A sample of resulting waveforms ob- 
tained from the ascending aorta by these flow 
probes is shown in Figure 2. Trace 'a' shows an 
ECG breakthrough, and there is significant sig- 
nal due to the ECG with each heart beat. Trace 
'b', taken from a different manufacturer's flow- 
meter, shows a highly unstable baseline, and 
zero flow is not evident during diastole. Trace 
'c' is an accurate measurement of ascending 
aortic flow. The baseline is stable, zero flow is 
measured during diastole, there is no ECG 
noise, and the waveform appears correct. All of 
these tracings were taken in acute preparations 
and are representative of three experiments. 
All flowmeters were used during each experi- 
ment. 
PRESSURE MEASUREMENT 
In vivo pressure measurement is most often 
done by inserting a catheter into the blood- 
stream, and then connecting the end of the cath- 
eter to a transducer which is mounted near the 
animal. The transducer is sometimes taped to 
the side of the animal, particularly if he is to 
move around or exercise on a treadmill. This 
method of pressure measurement has some in- 
herent problems, such as the perturbations in- 
troduced on the pressure waveform due to the 
movement of the saline column within the cath- 
eter. The frequency response of the system is 
also reduced because of the added mass of the 
fluid in the catheter. The reduction in frequency 
response is generally not a serious problem, al- 
though it has been pointed out that if the rate 
of change of pressure with respect to time 
(dP/dT) is to be evaluated, the frequency re- 
sponse of the system must be carefully 
considered.^^ It has been reported that the fre- 
quency response of the system can be increased 
by flushing the catheter with CO2 immediately 
prior to its being filled with saline before use in 
the pressure measurement system.^* 
The ideal way to measure pressure would be 
to measure it at the point at which it occurs 
without an intervening transmission system 
such as the saline column in the catheter. This 
can be done with catheter tip transducers. 
These transducers have been available for many 
years, but have not gained wide acceptance for 
in vivo testing because of their inherent prob- 
lem of temperature drift and because there is 
no way to check the zero of the transducer after 
it has once been implanted. This problem has 
been solved by some pressure transducer manu- 
facturers by providing a lumen through the 
catheter on which the catheter tip transducer 
has been fixed. Thus, with a single catheter in 
the system, the transducer can be calibrated ex- 
ternally, and then the measurements during the 
experiment can be obtained from the catheter 
tip transducer. If the transducer could be cali- 
brated at the time of the implantation, this pro- 
cedure would not be necessary. However, if the 
medium surrounding the transducer changes 
temperature, a drift will occur. This drift is not 
as significant as that caused by the internal 
heating of the transducer from the excitation 
voltage. It can be shown that when the velocity 
of the flow increases across the transducers sen- 
sing diaphragm, then the heat that has been 
generated is dissipated at a higher rate.^^ This, 
in effect, reduces the temperature of the gauge 
and causes a false deflection, indicating a re- 
duced pressure. This makes the pressure trans- 
ducer sensitive to flow variations, and if the 
flow is to change during an experiment, it can 
be a serious problem. 
Venous Pressure 
One of the most difficult pressures to measure 
is venous pressure while the animal is exercis- 
ing on a treadmill. This pressure is so low it 
cannot be measured by using an external pres- 
sure transducer and catheter system because 
the motion artifact is so great as to completely 
destroy the accuracy of the venous pressure 
measurement. A catheter tip transducer must 
therefore be used, but the changes due to flow 
perturbations and temperature changes are 
again as large as any changes which would be 
observed in the venous pressure. In order to 
guarantee that meaningful pressure data is 
being obtained, these artifacts should be re- 
