W. S. TOPHAM 
1231 
flowmeter had been reduced in size and could be 
used to measure flow in arteries as small as 1.5 
i mm in diameter." The development of the Dop- 
pler ultrasonic flowmeter gave similar measure- 
ment capability.'^ 
These two methods of measuring flow are 
widely accepted. They both require the place- 
ment of a flow probe around the vessel in which 
the flow is to be measured, and they have the ca- 
pability of measuring instantaneous flow in al- 
most any main vessel in the cardiovascular sys- 
tem. Comparison tests of the two methods 
showed that they both gave nearly identical 
waveforms, but each had a limitation.^ It was 
very difficult to obtain a reliable electrical zero 
in the electromagnetic flowmeter, and the Dop- 
pler ultrasonic flowmeter did not give a reverse 
flow measurement. Other reports indicate solu- 
tion of both of these problems. Folts demon- 
strated that a reliable zero for the electromag- 
netic probe could be obtained in animals with 
implantations of up to twenty-eight days.° Re- 
cent comparisons of a directional-sensitive Dop- 
pler ultrasonic flowmeter with an electromag- 
netic flowmeter have shown no differences in 
the waveforms, although the Doppler flowme- 
ter does tend to be more sensitive to disturbance 
in velocity flow profile than does the electro- 
magnetic flowmeter. Another major difference 
between these two methods of measuring flow is 
the amount of power required to drive the 
probe. The Dopper flow probe uses very little 
power and has thus been used in extensive stud- 
ies in which telemetry has been used to te- 
lemeter the information from free-roving ani- 
mals to a central recording station. This is very 
difficult with the use of electromagnetic probes 
because of the power necessary to generate the 
magnetic field across the vessel. 
The electromagnetic fiow probe has become a 
standard method of measuring fiow, and when 
compared to other methods in i7i vitro tests, has 
proven to be a very reliable and accurate 
method of fiow measurement." One of the 
major problems encountered when using the 
electromagnetic flowmeter has been that of cali- 
bration. It has been necessary to perform an 
elaborate external calibration procedure or cali- 
brate internally using a direct flow measure- 
ment method such as the indicator dilution or 
the Fick method, and these methods may not be 
as accurate as previously thought. Electromag- 
netic flow probes are now available which have 
been previously calibrated and are guaranteed 
accurate within ±5 per cent, provided they are 
applied properly to the vessel.^^ T^jg applica- 
tion includes a thorough cleaning of the elec- 
trodes and, in acute preparations, a slight con- 
striction of the vessel (10 to 20 per cent) will 
guarantee good electrical contact between the 
vessel and the electrodes. In chronic implanta- 
tion, the probe should be slightly larger than 
the vessel, and measurements should not be 
made until the adventitia has grown and made 
good contact with the electrodes — about two 
weeks later. Another important improvement in 
electromagnetic fiow measurement is the elec- 
trical or non-occlusive zero. This feature allows 
zero fiow to be observed any time the fiowmeter 
is being used by turning a switch on the flowme- 
ter; thus, zero flow from chronic implants is 
readily available and direct flow readings can 
be made using the precalibrated probes once 
zero flow is known. Tests of the electrical zero 
against an occluded zero are found to be very 
accurate. The results of these tests using sev- 
eral different flow measurements are shown in 
Figure 1. In all cases, the electrical zero was set 
prior to the occluded zero being measured. 
These two features, electrical zero and pre-cali- 
brated probes, have improved the flow measure- 
ment capability of the electromagnetic fiowme- 
ter significantly and have made it a valuable 
direct fiow-reading method. 
There are many commercial fiowmeters avail- 
able ; however, the cost ranges from $2,000 to 
$3,000 per channel, and before purchase is 
made, a careful evaluation of the instrument 
should be made. While in vitro tests are good to 
measure the linearity and some flow character- 
istics of the flowmeter, in vivo tests should be 
conducted for a complete evaluation. Such items 
should be considered as the stability of the base- 
line, the reproducibility of the flow measure- 
ment by removing the probe from the vessel 
and replacing it, the sensitivity of the flow 
probe to orientation on the vessel, and the 
amount of other electrical noise, such as ECG, 
which is picked up by the flow probe. When the 
tests are being conducted, the flow probe should 
