Electrical method and apparatus for non-invasively detecting abnormal flow in conduits

Method and apparatus are disclosed for obtaining flow information of a conductive fluid by detecting the streaming potential and analyzing the resultant signal. In a particular embodiment for detecting abnormal blood velocities in the arterial tree, the apparatus includes a flexible sensor having two fixedly spaced apart electrodes and a common mode signal electrode. Each sensor electrode is connected to an input of a differential amplifier and the common electrode is connected to the common input of the differential amplifier. The output signal from the differential amplifier, after filtering and amplification is provided to a recorder or oscilloscope so that it can be compared with the normal or average signal. A particular embodiment of the method is used to detect blood flow abnormalities, either too fast or too slow compared with most healthy individuals of the same age. In this method, a flexible sensor having two fixedly mounted electrodes is placed on the skin over the artery to be examined, for example the dorsalis pedis artery, so that each electrode is in electrical contact with a different part of the artery. The detected signal is processed and is compared to normal signals. These comparisons can be the wave shape or the time difference between the detection of the wave pulse and a pump timing signal such as the ECG signal.

FIELD OF THE INVENTION 
The present invention relates to obtaining flow information of a conductive 
fluid in a conduit. More particularly, the present invention relates to 
using passively detected electrical voltage generated by a pulsating 
liquid flow, known as streaming potentials, to obtain velocity 
information. In one particular adaptation, the present invention provides 
a means for determining the presence of abnormal blood flow between the 
heart and the particular artery being investigated. 
BACKGROUND OF THE INVENTION 
There has been a lot of recent activtity in the non-invasive study of the 
cardiovascular system. Traditional methods, such as angiography, usually 
relied on some invasion into the blood vessels. A disadvantage of invasive 
techniques is the risk of infection, increased mortality risks and 
increased morbidity risks. For example, there is a one to five percent 
mortality rate from angiography. Furthermore, those surviving patients 
often have traumatic reactions to the injected dye. In particular, 
increased attention has been focused on obtaining a diagnostic indicator 
of flow problems throughout the arterial tree. Unfortunately, most methods 
presently in use have significant detractions. One method recently gaining 
popularity is the use of ultrasound Doppler velocity meters. This process 
is described in articles such as the ones by Max Anliker, "Diagnostic 
Analysis of Arterial Flow Pulses In Man;" Cardiovascular System Dynamics, 
Beran et al, Eds. (1978) and Gosling and King, "Continuous Wave Ultrasound 
As An Alternative and Complement To X-rays In Vascular Examinations;" 
Cardiovascular Applications Of Ultrasound, R. Reneman Ed. (1974). The 
principal drawbacks in using ultrasound are the complex equipment that is 
needed, the trained technicians and diagnosticians that are required, the 
complexity of the procedure and the propensity for error based on 
incorrect vessel diameter and incorrect relative flow directions, and the 
inability to use the method as a quick screening test to determine 
potential problems. 
Another approach in the investigation of arterial flow is disclosed in the 
Findl et al. U.S. Pat. No. 4,166,455. This patent disclosed the 
theoretical basis for using the electrokinetic phenomenon known as 
streaming potential to determine reduced flow. That patent, which is 
incorporated in its entirety herein by reference, discloses a method and a 
sensor for detecting electrical voltages measured on the skin surface. 
While that method is satisfactory for locating certain lesions located in 
arteries near the skin surface, it does not provide a method for overall 
screening purposes to determine whether there is abnormal flow in the 
arterial tree. 
None of the prior art methods and apparatuses provide a simple, inexpensive 
screening technique for monitoring the changes in blood flow caused by 
disease stages, traumatic injuries and other factors. Thus, there is the 
need for inexpensive, uncomplicated, and easy to use apparatus for rapidly 
screening individuals to detect problems in the cardiovascular system. 
SUMMARY OF THE INVENTION 
The present invention provides a simple, non-invasive technique and 
apparatus for obtaining flow information about a conductive fluid flowing 
in a pulsatile manner through a conduit. Although any types of conductive 
fluids can be monitored, such as sea water, dissolved chemicals, reactants 
in chemical processes, and the like, one presently preferred use of the 
present invention is for detecting cardiovascular problems in animals, and 
in particular in human beings. The present invention provides a simple, 
inexpensive technique and apparatus for monitoring, measuring, and 
analyzing pertinent hemodynamic information. Abnormal blood velocity 
caused, for example, by lesions and arteriosclerosis located between the 
heart the the sampling portion or between two sampling points can quickly 
and easily be determined. 
The present invention is based upon the scientific concept of the 
electrokinetic phenomenon known as streaming potential. This phenomenon 
can be explained on the basis of Helmholtz' theory of an electrochemical 
double layer at all liquid-solid interfaces. The streaming potential 
phenomenon is believed to result by the mechanical perturbation that 
occurs when the solid, such as the walls of a conduit, is stationary and 
the liquid is moving. It appears that the fluid flow past the solid 
interface shears the mobile portion of the double layer in such a way that 
the predominant ions in the region are displaced in the direction of flow. 
It is this ion displacement that results in the streaming potential. In a 
fluid system in which there is pulsatile flow, the flow pulses originate 
at the pulsatile pump and are transmitted throughout the fluid system. The 
streaming potential related to the pulse of fluid that is pumped can be 
measured throughout the entire conduit system. In general, the magnitude 
of the streaming potential varies with the length of the conduit between 
the electrodes, with the dielectric constant and the zeta potential, and 
with the fluid velocity; and varies inversely with the conduit diameter 
and the electrolyte conductivity. The degree of variation of the streaming 
potential with each of the variables also depends upon whether the flow is 
laminar or turbulent. 
A particular embodiment of the present invention is connected with blood 
flow through the arterial tree. As an exemplary only embodiment of the 
present invention, the present invention will be discussed in this patent 
application with respect to blood flow in animals. 
The streaming potential measurements of animals were reported at least as 
early as 1975 by Sawyer et al, Coronary Artery Medicine and 
Surgery--Concepts and Controversies. This investigation utilized 
electrodes directly placed in contact with the blood flowing in the 
femoral arteries of dogs and used the streaming potential to determine the 
zeta potential of the arteries. However, such electrodes when experiencing 
flow at their surfaces do not accurately measure streaming potentials 
because of another electrokinetic phenomenon known as the motoelectric 
potential effect. This artifact is generally of the same order of 
magnitude as the streaming potential. Although measurements from the 
surface of the vessel are more difficult to obtain, they are more accurate 
and reproducible. One such result of that observation was the subject of 
the aforementioned Findl et al U.S. Pat. No. 4,166,455. That patent 
discussed the fact that the streaming potential increased when the 
electrodes straddled the site of a partial flow blockage at least for the 
reasons that the flow at the location was faster and perhaps because the 
flow was also turbulent. 
On the other hand, the present invention provides a technique for 
monitoring blood velocity with one application being a rapid method for 
detecting blockages between the heart and the location of the monitoring 
electrodes. Although the present invention does not give the exact 
location of the blockage, it does have the advantage of being a rapid, 
easy, and inexpensive method of screening for the existence and 
approximate location of such a partial blockage. 
Peripheral vascular diseases result in modifications of blood flow 
characteristics that can give an indication of the presence of these 
diseases. By detecting the streaming potential at various locations in the 
peripheral blood flow, the present procedure being denoted 
electroarteriography (EAG), rather subtle changes in the peripheral blood 
velocity profile can be detected. In arteriosclerosis (hardening of the 
arteries), the pressure pulse from the heart actually travels faster than 
it does in healthy individuals. In healthy individuals, the artery wall is 
pliable which results in a slower pressure pulse. In atherosclerosis 
(lesions in the arteries) the pressure pulse from the heart is slowed by 
the blockage. Under presently accepted theory, the contracting heart by 
forcing a pulse of blood into the arterial tree, generates a pressure 
pulse that causes the blood nearest the artery wall to begin flowing 
first. This flow generates streaming potentials that can be detected to 
provide the most accurate velocity waveform. 
A particular aspect of the present invention relates to a passive method 
for detecting the presence of abnormal flow of a conductive fluid in a 
conduit downstream from a pulsatile pumping source. The method comprises 
placing first and second, spaced apart, passive electrodes in electrical 
contact with respective portions of the conduit downstream from the 
pumping source, detecting the resultant electrical signal, and comparing a 
characteristic of the signal with a similarly detected signal from a 
conduit having normal flow. 
The apparatus according to another aspect of the present invention provides 
a means for non-invasively detecting abnormal blood flow downstream of the 
heart of an animal. The apparatus comprises means for detecting the 
streaming potential and producing a signal in response thereto, means for 
detecting the ECG signal; and means for permitting simultaneous comparison 
of the streaming potential and the ECG signal. 
A further apparatus embodiment of the present invention, also capable of 
being used for non-medical purposes, comprises a differential amplifier 
means, first and second passive electrodes connected to two inputs of the 
differential amplifier, and a third passive electrode for providing a 
common mode rejection signal to the "ground" or "common" connection of the 
differential amplifier. Further electrode means are provided for detecting 
a pumping signal from a pulsatile pump that is generated at the beginning 
of the pumping of a pulse of fluid. There is also provided means for 
permitting simultaneous comparison of the differential amplifier output 
signal and of the pumping signal. 
These and other objects and advantages of the present invention will be 
discussed in or become apparent from the detailed description of the 
presently preferred embodiment contained hereinbelow.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings in which like elements are represented by 
like numerals throughout the several views, and in particular with 
reference to FIG. 1, the electroarteriography (EAG) apparatus 10 is 
depicted. EAG apparatus 10 comprises an EAG sensor 12, an electronic 
processing circuit 14, and a signal analyzing means such as oscilloscope 
16. EAG apparatus 10 is shown connected to a test subject shown in the 
prone position. Attached to each arm of the subject are conventional 
electrocardiogram (ECG) electrodes 18 and 20. A conventional, air 
inflatable cuff 22, such as those used for taking blood pressure, is shown 
placed between the location of EAG sensor 12 and the patient's heart. Cuff 
22 has a conventional, air inflating device such as a rubber bulb 24 and 
an indicating pressure gauge 26. Thus, when cuff 22 is inflated by 
repeatedly compressing and releasing bulb 24, the amount of pressure can 
be read on gauge 26. Obviously, as cuff 22 is inflated to a higher and 
higher pressure, the amount of blood flow on the distal side of cuff 22 
from the heart will decrease. At pressures above systolic, an inflation 
pressure of approximately 150 millimeters of mercury, a complete cut-off 
of the blood flow beyond cuff 22 occurs and the EAG signal disappears. 
EAG sensor 12 is depicted in FIG. 1 as being connected for measurement of 
blood flowing through the dorsalis pedis artery. Also shown in FIG. 1 at 
12' is an alternate EAG sensor site to measure the blood flowing through 
the radial artery. Other locations for the EAG sensor can include the neck 
for monitoring the carotid artery; on the inside, lower side of the foot 
to monitor the posterior tibial artery; on the inside of the elbow to 
monitor the brachial artery; and on the anterior inner part of the thigh 
to monitor the femoral artery. The precise EAG sensor locations are 
illustrative only and are not meant to be either all inclusive or to be 
limiting. The EAG signal, described in greater detail hereinbelow, will 
vary in both the shape of the signal and the magnitude of the signal 
depending upon the particular location of EAG sensor 12. For illustrative 
purposes, the results of the EAG signal taken from the dorsalis pedis 
artery will be discussed and compared for various physiological conditions 
in the remainder of this application. 
EAG sensor 12 comprises an EAG electrode or pad 28, a means for attaching 
pad 28 to the body, such as an adjustable band 30, and a common mode 
signal electrode 32. With reference to FIG. 3, EAG pad 28 is comprised of 
a body 34, two electrode pads 36 and 38 housed in cavities in body 34, and 
silver wires 40 and 42 respectively adhered to the back of electrode pads 
36 and 38. Body 34 is preferably flexible to conform with the shape of the 
subject and is non-conductive. For example, body 34 can be molded from RTV 
or silicone rubber and have exemplary dimensions of 230 square 
millimeters. Electrode pads 36 and 38 are preferably molded from flexible 
silver filled silicone rubber and are located relatively closely to each 
other. Electrode pads 36 and 38 have exemplary dimensions of 3 millimeters 
by 14 millimeters and can be spaced apart from 6 millimeters to 15 
millimeters on center with a presently preferred spacing of 10 
millimeters. Electrode pads 36 and 38 are located in cavities in body 34 
and extend below the outer surface thereof. The space above the tops of 
electrode pads 36 and 38 is filled with a conventional electrode jelly 50. 
As clearly shown in FIG. 5, the inside surface of EAG pad 28, that is the 
side to be placed on the skin of the subject, has a plurality of isolation 
ridges to assure electrical isolation between electrode pads 36 and 38. 
Annular ridges 44 and 46 (see also FIG. 3) surround electrode pads 36 and 
38, respectively, and an elongate ridge 48 extending transversely on body 
34 between the inward parts of annular ridges 44 and 46 further isolates 
electrode pads 36 and 38 from each other. Because electrode jelly tends to 
migrate when heated by the body, ridges 44, 46 and 48 assure that such 
migration is contained and does not affect the mutual electrical isolation 
of electrode pads 36 and 38. Thus, EAG pad 28 is completely flexible and 
reusable and can easily conform to the particular shape of the body 
portion on which it is applied. Most importantly, EAG pad can be firmly 
mounted over an artery without occluding the artery. The electrode spacing 
is fixed to provide consistent results and the electrode size and 
electrical contact area with the skin is minimal to reduce electrical 
artifacts. 
Common mode signal electrode 32 can be either a conventional ECG electrode 
or can be used in combination with EAG pad 28, as shown in FIGS. 3 and 4 
and denoted 32' and 32" respectively. Electrode 32' and 32" are silver 
filled silicone rubber electrodes that have an "H" configuration, and an 
annular square configuration, respectively. Electrodes 32' and 32" have a 
size and configuration so as to permit electrode pads 36 and 38 to contact 
the skin of the subject. Other configurations, such as two parallel 
strips, can obviously be used. Electrode 32' as depicted in FIG. 3 has a 
size such that it will be contained within the perimeter of body 34, 
whereas the electrode 32" as depicted in FIG. 4 has a size such that its 
annular opening will receive body 34. In both cases, common mode signal 
electrode 32 is designed to be closely spaced to electrode pads 36 and 38. 
Alternatively, as depicted in FIG. 1, electrode 32 is placed on the 
subject at a location that is spaced from EAG pad 28. Such an electrode 32 
can be typically placed on a bony area (e.g., the patella or the fibula) 
to reduce the possibility of electrode 32 sensing an EAG signal. A wire 52 
is mounted on electrode 32 and can be easily connected to electronic 
processing circuit 14. 
As shown in FIG. 1, electronic processing circuit 14 includes a front end 
differential amplifier 54 having at least two signal inputs and a common 
input. Wires 40 and 42 are electrically connected to the signal inputs of 
differential amplifier 54 and wire 52 is electrically connected to the 
common input of differential amplifier 54. Although an ideal differential 
amplifier will not respond to the common difference between the input 
signals, a practical differential amplifier will in fact measure a 
difference between two symmetrical input signals and amplify that 
difference. The ability of a practical differential amplifier to reject 
the common signal between the two inputs is denoted the common mode 
rejection ratio (CMRR). Typical CMRR are 10.sup.6 which means that a 
common one volt signal will appear as a microvolt artifact. Because the 
EAG signal is of the same order of magnitude (e.g. 1 to 2 microvolts), 
this is a significant error. The common signal to both inputs is usually 
generated by the omnipresent 60 hertz noise as well as the ECG and EMG 
signals from the patient. Electrode 32 when placed so as not to measure an 
EAG signal and when connected to the common connection of differential 
amplifier 54 greatly enhances the performance of circuit 14. As a result, 
a hundred fold increase in EAG signal magnitude over the noise level was 
obtainable. 
Differential amplifier 54 can have a common mode rejection ratio of 100 db 
with a differential impedance of 10.sup.12 ohms, a common mode impedance 
of 10.sup.12 ohms, and a noise level of 5 microvolts RMS in a frequency 
range from 10 to 500 hertz. As a result of using the common mode signal 
electrode 32 connected to the ground connection of differential amplifier 
54, the overall common mode rejection of differential amplifier 54 is 
increased to 150 db. 
The output from differential amplifier 54 is fed through a filter 56 and a 
range select conventional amplifier 58 to a conventional isolation 
amplifier 60. Filter 56 can be a conventional two pole butterworthy 
band-pass filter in the 0.5 to 40 hertz band. The power for isolation 
amplifier 60 is provided from a power supply 62 fed through an isolator 
64. In this way, a signal with no ground currents can be obtained. The 
output from isolation amplifier 60 is connected through an output buffer 
66 to the output connection of circuit 14. 
As mentioned above, the input signal levels to circuit 14 are typically in 
the low microvolt range whereas the output signals therefrom are typically 
in the low millivolt range. Depending upon the setting of amplifier 58, 
the overall gain of circuit 14 can be 100, 1,000, or 10,000 and, as 
mentioned above, the overall common mode rejection ratio is 150 db at 60 
hertz. 
The outut from electronic processing circuit 14 is connected through a 
grounded coaxial cable 68 to the channel A input of oscilloscope 16. For 
the purposes of easy comparison, the output of circuit 14 is connected to 
oscilloscope 16 such that forward flow causes a negative signal. Although 
an oscilloscope 16 is disclosed in FIG. 1, it should be obvious that the 
same connections can be made to an ECG recorder or other signal recording 
or storage devices. 
As shown in FIG. 1, the performing of the method according to the present 
invention in the embodiment of electroarteriography comprises the 
placement of ECG electrodes on a subject that is prone on a bed or similar 
platform. Two ECG type electrodes are placed on one arm and one on the 
other arm. Depending upon the portion of the arterial tree to be screened, 
the artery of interest is located by either palpation or by use of a 
conventional sonic detector. The dorsalis pedis artery was selected for 
EAG pad 28 and the radial artery was selected for EAG pad 28'. Discussing 
only the signals from the dorsalis pedis artery, EAG pad 38 is placed on 
the skin of the subject directly over the artery with electrode pad 36 and 
38 placed so that it is upstream of the other pad 38 or 36. EAG pad 28 is 
held in place by an appropriate means such as an adjustable strap or band 
30 that is attached so as not to occlude the artery. Common mode signal 
electrode 32 is then attached over a bony portion of the foot of the 
subject and all three electrodes are connected, as discussed above, to 
circuit 14. Circuit 14 is then connected to the channel A input of 
oscilloscope 16. 
FIG. 1a shows the waveform obtained from both the EAG pad 28, at the top of 
the graph, and the ECG waveform from ECG electrodes 18 and 20 shown at the 
bottom of the graph. Reference is also made to the top two wave forms in 
FIG. 2 where the top waveform is the ECG signal and the bottom waveform is 
an EAG signal that has not been inverted. A typical ECG waveform consists 
of a series of complex pulses which are believed to be the result of 
electrical signals sent to stimulate the pumping action of the heart. A 
typical pumping action from the heart results in a first forward flow of 
flood through the arteries, a small reversal of that blood flow, and a 
subsequent forward flow of the blood. The particular blood flow is 
believed to be correlated with the ECG waveform. As shown in FIG. 2, the 
ECG complex wave has a small positive segment labelled "P", followed by a 
small negative part, denoted "Q", which immediately precedes a large 
spiked pulse denoted "R". The end of the R portion is a small negative 
portion, denoted "S" which slowly rises to a medium size positive pulse 
portion, denoted "T". Thus, the standard ECG waveform is comprised of a 
complex "PQRST" pulse. Because of the regular form of the ECG pulses, they 
cannot only be used as timing pulses, but they can also be used to trigger 
an oscilloscope in a known manner. In fact. the FIGS. 1a, 1b, and 1c are 
reproductions of actual signals obtained from human test subjects in which 
the oscilloscope was triggered by the R portion of the ECG. 
A principal discovery underlying the present invention is that the 
streaming potential produces a measurable signal having a complex waveform 
resulting from the forward-backward-forward flow of blood through the 
arteries. The shape of this waveform and the relative timing of the 
waveform with respect to the ECG waveform can determine whether the flow 
through the arteries is normal, is reduced because of some lesion, disease 
or traumatic injury is the arterial tree between the heart and the 
location of the EAG electrodes, or is too fast, being indicative of 
arteriosclerosis. 
With particular reference to FIG. 2, a normal dorsalis pedis EAG consists 
of a first, large positive peak, denoted 70, a second, small negative peak 
denoted 72, and a small positive peak denoted 74. The time between the R 
peak of the ECG signal and peak 70 of the EAG signal is denoted the 
T.sub.2 time and is a function of the time delay between the contraction 
of the heart and the subsequent pulse of blood flowing past the particular 
EAG electrode. The further the placement of the EAG electrode from the 
heart, the greater the T.sub.2 time. Similarly, the T.sub.2 time will be 
increased if the blood flow is uphill, such as with the leg raised above 
the heart, than with the blood flow that is downhill, such as in a 
standing subject. Other time portions of the EAG signal is the time from 
peak 70 to peak 72, the T.sub.3 time, the time from peak 72 to peak 74, 
the T.sub.4 time, and the pulse width of peak 70, the T.sub.5 time. The 
T.sub.1 time, shown in the upper graph in FIG. 2, is the time between 
heartbeats and is measured between the two R peaks of the ECG signal. 
Other usable characteristics of the EAG signal is the height of peak 70, 
denoted P.sub.1 ; the height of peak 72, denoted P.sub.2 ; and the height 
of peak 74, denoted P.sub.3. More particularly, it has been found that 
while the absolute value of the amplitude of these peaks may vary from day 
to day as a result of artifacts, the ratio of the peak amplitudes 
apparently remains constant. 
In order to simulate the effects of different amounts of restrictions to 
the blood flow, a blood pressure cuff 22 was placed on the thigh of the 
subject and inflated to various pressures from 0 through systolic pressure 
for the individual subject to a cut-off pressure. As mentioned above, FIG. 
1a shows the result with no pressure in cuff 22, FIG. 1b shows the results 
of 40 millimeters (Hg) of pressure in cuff 22, and FIG. 1c shows the 
result with 80 millimeters (Hg) of pressure in cuff 22. Other studies 
performed on a subject with a known traumatic injury to the left foot 
resulting in permanent inflammatory edema confirmed that there is a 
dramatic difference in the EAG signal obtained from the normal foot than 
from the pathologic foot. The normal foot contained the typical EAG 
waveform as shown in FIG. 1a or at the second curve from the top in FIG. 
2, whereas the pathological foot had an EAG signal that was very broad, 
lacking well defined flow reversal, and that began late in the cycle as 
shown in FIG. 2. 
A study of various EAG signals indicates that the signals vary with a 
number of factors. These factors include the location at which the signal 
is obtained, the heartbeat of the subject (e.g. taken while the subject is 
at complete rest or after the subject has walked on a treadmill), and the 
location and extent of blockage of blood flow. In addition, possibly 
because of inconsistent electrode placement techniques, the absolute 
amplitude of the EAG signal may vary from test to test. Reliable 
modifications of an EAG signal from an average signal that reflect 
predicted hemodynamic changes include the broadening of the pulse width 
(T.sub.5 greater), lack of flow reversal (P.sub.2 equals zero), a decrease 
in ratios of signal amplitude (P.sub.2 /P.sub.1), and the length of time 
between the peak of the ECG QRS wave and the peak of the subsequent EAG 
wave (T.sub.2 time). In addition, possibly because of inconsistent 
electrode pressures, the absolute amplitude of the EAG wave parts may vary 
from test to test. 
A review of FIG. 1b, 1c and the middle waveform in FIG. 2, shows that with 
increasing pressure in cuff 22, P.sub.1 decreases, pulses 72 and 74 become 
less defined, and T.sub.2 increases. In fact, the middle wave in FIG. 2 is 
so distorted that it could simply be due to background noise. The bottom 
two waveforms in FIG. 2 show other effects on the EAG signal from reduced 
blood flow. 
With reference now to FIG. 6, the T.sub.2 transit times for non-symptomatic 
"normal" subjects obtained from the dorsalis pedis artery are depicted as 
falling within a normal band of values shown by the two dashed lines. The 
T.sub.2 times were found to decrease with age and this is believed to be 
the result of the tendency of arteries to harden with age and the tendency 
of blood pressure to increase with age. Hardening of the arteries 
(arteriosclerosis) lowered the transit time by decreasing the ability of 
the arterial wall to flex, making the artery more like a rigid tube. The 
lowered wall compliance translates into a faster blood flow and a 
decreased T.sub.2. Not shown in FIG. 6 is the effect of the height of the 
person, or rather the distance from the heart to the EAG sensor. It has 
been found that there is an increase in T.sub.2 time on the order of 20 to 
40 milliseconds for a distance of one foot. 
Although there are numerous factors which result in the variation of the 
T.sub.2 time, it has been found that whenever the T.sub.2 time falls 
outside of the "normal" range, (i.e., is too large or too small based on 
the age of the subject) there is probably an abnormality in the blood 
velocity and further examination is warranted. Such further examination 
can be a detailed analysis of the EAG waveform or the use of other means 
to confirm the presence of atherosclerosis or arteriosclerosis, or some 
other possible damage to the arterial tree. Two clinical tests shown in 
Table I below illustrate the utility of the T.sub.2 time. 
TABLE I 
______________________________________ 
Patient Info. 
T.sub.2 time (msec) 
Age Sex R. Leg L. Leg 
Diagnosis 
______________________________________ 
90 M 230 360 Atherosclerosis present 
in left leg. (Confirmed 
by angiography.) Right 
leg borderline normal. 
Arteriosclerosis in 
excess of normal for 
age suspected. 
68 M 780 890 Severe atherosclerosis 
in both legs. Blood 
flow seriously reduced. 
(Confirmed by angiography.) 
______________________________________ 
From experimental results, 86% of the trials indicated waveforms as 
depicted in FIGS. 1a, 1b, and 1c. A 10 to 20% increase in T.sub.2 time 
usually accompanied cuff pressures at or below the diastolic pressure of 
the subject. In fact, a 10 to 20% increase in the T.sub.2 time was 
obtained with an applied cuff pressure of 40 millimeters of mercury. 
Although the present invention does not necessarily provide a 
determination of the extent of an occlusion or the exact location of the 
occlusion, it does provide an easily applied test with immediate results 
which can be used to determine whether further tests should be done. 
Application of the present invention, obviously, can be in the preliminary 
screening for occlusive cardiovascular disease. Other uses, in a 
non-medical field could be the monitoring of pulsatile flow of a 
conductive liquid in chemical experiments, in manufacturing processes, and 
in liquid transportation systems. In fact, the present invention can be 
used in any fluid transport system in which there is a possibility of an 
occlusion in the conduit of the system. 
As described above, the present invention provides a screening process 
requiring no special signal processing and having a high correlation with 
the actual blood velocity profile and between the transit time through the 
vessel and the degree of occlusion in the vessel. The present invention 
utilizes passive electrodes of a very simple construction. By passive 
electrodes it is meant an electrode that only receives an electrical 
signal and is not used to transmit either voltage or current to the vessel 
being monitored. In addition, a passive electrode does not involve a 
chemical reaction and is substantially unaltered by the monitoring 
process. 
Although one aspect of the present invention utilizing the T.sub.2 time as 
the measured signal characteristic employs the ECG signal, it is obvious 
that in both animal and non-animal applications of the present invention a 
separately generated pumping signal can be used or a second set of 
electrodes can be placed closer to the pump outlet to provide an initial 
signal to which the distally detected signal can be compared. Non-animal 
applications require a conduit that is a poor conductor in the axial 
direction and conductive in the radial direction (e.g., porous glass). No 
streaming potentials are generated in axially conductive conduits (e.g. 
metal) and none can be detected non-invasively in radially non-conductive 
conduits (e.g. plastic). 
With respect to the medical applications of the present invention, the 
present invention provides a reliable, simple and non-invasive blood 
velocity measurement technique that has substantial value in today's 
clinical environment. The EAG waveform is remarkably similar to the blood 
velocity profiles that have been obtained using ultrasound techniques and, 
as mentioned above, are believed to be more accurate. Because blood is a 
conductive fluid and because the blood vessel is immersed within a bulk 
conductor, the streaming potentials from the blood flow through certain 
arteries can be detected along the skin surface above that artery. 
The apparatus according to the present invention provides an effective way 
of measuring and comparing velocity dependent voltage signals produced by 
streaming potentials. The use of a common mode signal electrode connected 
to the common input of a differential amplifier permits the low voltage 
streaming potentials to be detected among the much larger noise signals. 
The above invention has been described in detail with respect to specific 
embodiments thereof. However, obvious modifications should be apparent to 
those skilled in the art.