The paper describes the design and performance of the smallest iron core electromagnetic flow transducer built so far. The design is miniaturized as to the total volume of the device as well as to the size of the artery it accommodates. The use of this transducer for recording of blood flow in the smallest species of animal employed until now in blood flow research is described. Illustrations of pharmacological observations in anesthetized rats are presented

Arora, H. L.

Cox, P.

Kolin, A.

Caltech Authors - Main

IEEE TRANSACTIONS ON BIO-MEDICAL ELECTRONICS
Electromagnetic Determination of Carotid Blood
Flow in the Anesthetized Rat*
P. COXt, H. ARORAtt AND A. KOLINt
Summary-The paper describes the design and performance of
the smallest iron core electromagnetic flow transducer built so far.
The design is miniaturized as to the total volume of the device as
well as to the size of the artery it accommodates. The use of this trans-
ducer for recording of blood flow in the smallest species of animal
employed until now in blood flow research is described. Illustrations
of pharmacological observations in anesthetized rats are presented.
rT HE PROGRESSIVE miniaturization of the elec-
t tromagnetic flow meter in recent years has enabled
blood flow to be measured in the conscious cat in
arteries as small as 1.5 mm in diameter.' There is, how-
ever, need for even smaller flow meters which would not
only allow the measurement of blood flow to small or-
gans or parts of larger organs but would also permit
smaller, inexpensive and more readily available labora-
tory animals to be used for circulation research. The
difficulties of reducing the size of the flow transducer
increase very steeply with decreasing artery size. Even
the step from a 1.5 mm to a 1 mm artery lumen presents
considerable problems. The difficulty is due to the great
care and precision with which the wiring has to be done
to avoid excessive error signals at zero flow and to the
need for adequate space for a slot through which the
artery is to be inserted and to the smallness of the shut-
ter which seals this slot.
The present paper describes a miniature flow meter
for arteries 1 mm in diameter which has overcome this
problem and has been used successfully to record carotid
blood flow in anesthetized rats. Fig. 1 shows a photo-
graph of the transducer comparing it to the size of the
animal and of an aspirin tablet. Fig. 2(a) shows the
magnet core (terminating in the pole pieces Pi, P2) the
sleeve S and the wiring (85 turns of #31 gauge Formvar
insulated wire) in cross section. Fig. 2(b) shows a per-
pendicular section of the magnet and of the cylindrical
sleeve S (with electrodes El and E2) into which a slot
[SL of Fig. 2(a)] is cut after potting. Essential details of
the wiring and shielding are also shown as well as the
slot SL and shutter Sh which closes the slot SL when
inserted. The EMF induced in the blood as it streams
transversely between the poles P, and P2 of the open-
toroid magnet is picked up by the platinized platinum
* Received August 29, 1963. The work reported in this paper was
supported by Grant #H-3092 of the U. S. Public Health Service,
and a Grant from the Office of Naval Research, Washington, D. C.
t Department of Biophysics and Nuclear Medicine, University ofCalifornia, Los Angeles, Calif.
tt California Institute of Technology, Pasadena, Calif.
' A. Kolin and R. T. Kado, "Miniaturization of the electro-
magnetic blood flow meter," Proc. NatL. Acad. Sci. USA, vol. 45,
pp. 1312-1321; August, 1959.
electrodes (El and E2) (0.4 mm in diameter) and con-
veyed through the twisted leads L,, L2 to the gated sine-
wave amplifying and recording system of the type de-
scribed in Kolin and Kado.1 The magnet is excited by a
400 cps alternating current of 0.6 a rms. Great care is
taken in construction of the flow probe to eliminate, or
Fig. I-View of the miniature flow transducer and comparison of its
dimensions with those of an aspirin tablet and of the animal.
6
5
. 4
_, 3
i 2
0
-PI
Q E,
Sh
(a) (b)
Fig. 2-(a) Sections of the flow transducer perpendicular to the flow
velocity vector and (b) parallel to the flow velocity vector. Wi,
W2: current supply leads to coil energizing the magnet. Li, L2:
Electrode lead wires, B: grounded wire braid shielding the elec-
trode leads. G: ground lead. El, E2: electrodes. Pl, P2: magnet
poles. S: plastic sleeve. SI: slot for admission of artery. Sh:
shutter.
1963 171
IEEE TRANSACTIONS ON BIO-MEDICAL ELECTRONICS
greatly reduce, the quadrature EMF disturbance as
described by Kolin and Wisshaupt.2 The platinum elec-
trodes are platinized using the procedure referred to in
Kolin and Wisshaupt.2 The plastic cylinder ("sleeve" S)
in which the electrodes are mounted is made of epoxy
(Hysol #2038 rod) and the entire assembly of magnet
and electrodes is potted in Hysol #2038 epoxy so that the
polyvinyl chloride tubing (not shown in Fig. 2) encasing
the wires shown in Fig. 2(a) and (b) is potted over a
length of at least 2.5 mm in the solid epoxy resin. The
plug is made by molding of the same plastic material
according to a design previously described2 in connec-
tion with flow meters of a different type. The animal is
grounded at a point close to the flow transducer by a
separate ground lead. The magnetic field obtained with
such flow transducers at a magnet current of 1 a is ap-
proximately 290 gauss. Calibration and phase adjust-
ment procedures are described in Kolin and Wisshaupt,2
Ryan,3 and Glasser.4
Fig. 3(a) shows the contour of the blood flow through
the left acrotid artery of a 450-gm rat taken at high
speed of the recording paper. The subsequent records
of Fig. 3, taken at a slower speed, show respiratory
variations in blood flow (measured in the left carotid
artery) and blood pressure which was recorded in the
right carotid artery by a "Sanborn" physiological pres-
sure transducer. A 20-,uf condenser in parallel to the
recorder input makes it possible to record average values
of flow as well as pressure. Fig. 3(b) illustrates the tran-
sition from recording of instantaneous values to average
values at points P1 and P2.
Fig. 3(b)-(e) give examples of performance of the
miniature flow transducers in observing carotid blood
flow in physiological experiments on rats. Blood pressure
is recorded at the same time. In Fig. 3(b) the effect of
inhalation of amyl nitrite for 7 seconds upon average
carotid blood flow and blood pressure is shown. The
blood flow drops to 40 per cent of the original value
while the blood pressure is reduced to 74 per cent of its
value before inhalation. The effect upon the blood flow is
thus more pronounced than the effect upon blood pres-
sure.
Fig. 3(c) shows the effect of amyl nitrite inhalation for
10 seconds upon instantaneous values of carotid blood
flow and blood pressure.
The next two records illustrate the effect of an intra-
venously injected drug. Fig. 3(d) shows the effect of
0.125 mg of acetyl choline upon the average values of
carotid blood flow and blood pressure. The blood flow
drops markedly to 11 per cent of its initial value while
2 A. Kolin and R. Wisshaupt, "Single-coil coreless electromagnetic
blood-flow meters,' IEEE TRANS. ON BIo-MEDIcAL ELECTRONICS,
vol. BME-10, pp. 60-67; April, 1963.
3 D. P. Ryan, "A low cost, portable electromagnetic blood flow
meter system, utilizing commercially available devices," IEEE
TRANS. ON Bio-MEDIcAL ELECTRONICS, vol. BME-10, pp. 57-60;
April, 1963.
4 0. Glasser, "Determination of blood flow by electromagnetic
induction," in 'Medical Physics," Year Book Medical Publishers,
Inc., Chicago, Ill., vol. 3, p. 141; 1960.
ra e -
(a)
Il 0 :41; i
"50°
* Si S-H~
SEC.
(b)
t 100
575o
a L50
I SEC.
(C)
ql
o.toE
500
5o
_-
I25-
51 [52
I SEC
(d)
It
t o
7 575o
h 50-
-I 1.
I SEC.
(e)
Fig. 3-Performance of the miniature blood flow transducer in re-
cording blood flow in a rat's left carotid artery 1 mm in diameter.
(a) Tracing of instantaneous blood flow. (b) Upper tracing: blood
flow in the left carotid artery. Lower tracing: blood pressure taken
in the right carotid artery. The initial portions of the tracings
show instantaneous flow and pressure. Points P, and P2 mark the
transition from observation of phasic values to average values.
During the time interval S1-S2 the animal inhales amyl nitrate,
which leads to pronounced drops in average blood flow and blood
pressure. (c) Effect of inhalation (during time interval S1-S2) of
amyl nitrate upon instantaneous carotid blood flow and blood
pressure. (d) Effect of 0.125 mg of acetyl choline (injected intra-
venously between Si and S2) upon average values of carotid blood
flow and blood pressure. (e) Effect of 0.250 mg of acetyl choline
(injected intravenously between SI and S2) upon instantaneous
carotid blood flow and blood pressure. This record reveals not
only the changes in blood flow and blood pressure but also those
in heart rate.
October172
-1 '2
Davis et al.: Transducer for Arterial Pulse Waves
the blood pressure is decreased to 49 per cent of its value
at the outset of the experiment. Again the effect upon
blood flow is much more pronounced than upon blood
pressure. Fig. 3(e) shows the effect of twice the previous
intravenous dose (0.25 mg) upon the instantaneous
values of blood flow and pressure revealing the initial
effects upon heart rate as well as changes in the pulse
volume. The initial diminution in blood flow and blood
pressure is followed by a marked subsequent increase.
To our knowledge, the rat is the smallest animal in
which blood flow has been recorded by the electromag-
netic method and the arterial diameter as well as rate of
blood flow are the smallest recorded so far.
The work on futher miniaturization of the flow trans-
ducer with the ultimate objective of measuring blood
flow in small arterial branches of conscious freely mov-
ing animals is in progress and will be reported in greater
detail elsewhere.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the able techni-
cal assistance of S. Austin in the performance of the
physiological experiments and of D. P. Ryan in connec-
tion with the calibrations and operation of the electronic
system. Thanks are also due to Dr. Gordon Ross for a
critical review of the inanuscript.
Improved Transducer for Externa Recording of
Arterial Pulse Waves*
M. DAVISt, B. GILMORE$, AND E. FREIStI
Summary-This paper describes a new instrument with excellent
low frequency response for recording arterial pulse wave forms. It
uses a fluid filled chamber and a stiff diaphragm to which a set of
semiconductor strain gages are cemented.
J\AjTOST METHODS for recording arterial pulsa-
tions externally in man depend upon the appli-
cation of a sensing device to the skin over a
large artery. The periodic pulsation of the arterial seg-
ment produces proportionate pressure changes which
activate some form of mechanical or electrical trans-
ducer.
To determine whether the pulse waves so obtained
have diagnostic value, particularly with regard to
changes in the elasticity of the arterial walls associated
with aging and atherosclerosis, we considered it neces-
sary to develop a method of recording which would not
introduce significant instrumental artifacts. The char-
acteristics of an acceptable transducer should include 1)
an essentially flat frequency response between approxi-
mately one and 20 cps, 2) sufficient sensitivity to record
weak pulsations, 3) compactness and ease of application,
and 4) some means for determining the amount of force
being applied to the skin overlying the artery.
A recording system was desired which would be re-
sponsive to steady state as well as pulsatile pressure
fluctuations. This was necessary because the major
characteristics of the arterial pulse contour are deter-
* Received April 25, 1963; revised manuscript received August
29, 1963.
t National Bureau of Standards, WNashington, D. C.
t Veterans Administration Hospital, Washington, D. C.
mined by its low frequency components.
Preliminary trials with stiff diaphragms applied di-
rectly to the skin gave unsatisfactory results. This was
due in part to the difficulty in centering directly over the
artery which led to uneven flexing of the diaphragm, and
in part to irregularities in skin contours overlying cer-
tain arteries. To obviate these difficulties, it was decided
to use a fluid system between the skin and the dia-
phragm of the transducer; the fluid was contained in a
housing which was rigid except at the surface applied to
the skin where it would be sealed by a compliant mem-
brane. Although valuable clinical information was ob-
tained with such capsules connected to commercial
strain gage manometers, this combination was rather
bulky and did not lend itself to convenient and accurate
manipulation. The necessary restriction in the connec-
tion between the capsule and the pressure gage also was
found to result in phase shifts between input and output
signals at frequencies above 12 cps.
CONSTRUCTION OF INSTRUMENT
Fig. 1 shows a crosssectional view of the instrument
designed to fulfill the requirements outlined above. The
rigid housing is partitioned into two compartments by a
thin metal diaphragm to which two semi-conductor
strain gages1 are cemented to the upper surface with
epoxy. The lower compartment is water-filled and sealed
below by a plastic membrane (Saran Wrap@R) and above
by a metal diaphragm. The air-filled upper compart-
' Micro Systems, Inc.
1963 173


Electromagnetic Determination of Carotid Blood Flow in the Anesthetized Rat

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