1280 
MONITORING 
to flush the skin which improved the electrical 
signal. Standard spot electrodes were covered 
with electrode paste and applied to the skin 
with circular adhesive pads. This technique per- 
mitted us to obtain a good ECG for several 
hours while the dog was active and exercising 
on a treadmill. The slight irritation to the skin 
seemed to be particularly important to reduce 
the impedence between the animal and the elec- 
trodes. If the impedence were not reduced in 
this manner, then noise was introduced on the 
signal because of the motion of the wires as the 
dog exercised. 
If the dog is anesthetized or otherwise im- 
mobile, a good ECG signal can be obtained by 
coarsely shaving the upper part of the two 
front legs, washing the area with alcohol, 
applying large amounts of electrode paste, and 
taping spot electrodes over the paste. The 
ground electrode is generally connected to the 
abdomen or hind leg in a similar manner. The 
ECG signals obtained are very adequate for 
triggering and rhythm determinations and have 
been found to be more reliable than needle 
electrodes. 
FLOW MEASUREMENT 
Measurement of in vivo flow has always been 
difficult. The two standard methods referred to 
in almost every physiological textbook,^ - the 
indicator dilution method and the Pick method, 
use equations from which a direct measurement 
of flow can be obtained. For the measurement 
of cardiac output, both methods require the in- 
sertion of at least one catheter in the venous 
system and a catheter or needle in the arterial 
system. In addition, the Pick method requires 
that the total amount of gas breathed by the an- 
imal be collected in order that the oxygen-con- 
sumption-per-unit time can be determined. This 
procedure is very difficult in an animal which is 
awake, but for an anesthetized animal, the en- 
dotrachial tube provides a convenient method 
for collection of the respiratory gas. A sample 
of thoroughly mixed venous blood must be 
taken to guarantee accuracy of the measure- 
ment, and the placement of the catheter should 
be in the pulmonary artery. The indicator dilu- 
tion method requires thorough mixing of the 
dye and the blood between the point at which 
the dye is injected and the sampling site where 
the concentration of dye is measured. In the 
body the heart and lungs provides the most 
thorough and complete mixing chamber, and the 
dye is injected into the right heart. Its concen- 
tration is sampled at an arterial site in the 
systemic circulation. Since the total cardiac 
output passes through the injection site, direct 
measurement of cardiac output is obtained. 
More recently, thermal dilution curves have 
been used to measure cardiac output and are 
being used more and more because they require 
the insertion of only one catheter, which serves 
as the injection site for a temperature-con- 
trolled saline solution, as well as the sampling 
site.'' The saline is quickly dispersed within the 
body and does not offer any of the problems of 
recirculation or the buildup of dye concentra- 
tion as is seen with the indicator dilution curve 
using indocyanine green dye. 
All three of the above methods are an averag- 
ing process and give an average measure of flow 
over several seconds. They cannot measure in- 
stantaneous flow. In addition, they are most 
successful in measuring total cardiac output. 
The dilution method could be used to measure 
regional or organ flows, providing adequate 
mixing of the indicator could take place be- 
tween injection and sampling sites. However, 
this method and the Fick principle are awk- 
ward for these special flow measurements ; and 
it is often desirable to observe the change in 
flow during a heart cycle or at least obtain a 
mean flow measurement with each heart beat, 
such as, stroke volume. Even though stroke vol- 
ume can be determined by the above measure- 
ments by dividing the cardiac output by heart 
rate, only the average stroke volume can be ob- 
tained, and transient beat-to-beat changes can- 
not be detected if present. 
Electromagnetic Flowmeters 
These devices were developed in 1936 by 
Kolin,*'^ but did not become popular because 
of their bulky size and awkwardness in use. It 
was necessary to use large D-C magnets and ex- 
ternalize the artery to make a measurement 
using the electromagnetic method. By 1959, the 
