In cardiac surgeries it is frequently necessary to carry out interventions in internal heart structures, and where the blood circulation and oxygenation are made by artificial ways, out of the patient's body, in a procedure known as extracorporeal circulation (EC). During this procedure, one of the most important parameters, and that demands constant monitoring, is the blood flow. In this work, an ultrasonic pulsed Doppler blood flowmeter, to be used in an extracorporeal circulation system, was developed. It was used a 2 MHz ultrasonic transducer, measuring flows from 0 to 5 liters/min, coupled externally to the EC arterial line destined to adults perfusion (diameter of 9.53 mm). The experimental results using the developed flowmeter indicated a maximum deviation of 3.5% of full scale, whilst the blood flow estimator based in the rotation speed of the peristaltic pump presented deviations greater than 20% of full scale. This ultrasonic flowmeter supplies the results in a continuous and trustworthy way, and it does not present the limitations found in those flowmeters based in other transduction methods. Moreover, due to the fact of not being in contact with the blood, it is not disposable and it does not need sterilization, reducing operational costs and facilitating its use.397844845

Button Vera L.S.N.

Costa Eduardo T.

Dantas Ricardo G.

Maia Joaquim M.

English

Repositorio da Producao Cientifica e Intelectual da Unicamp

Ultrasonic Doppler blood flowmeter for extracorporeal circulation
Ricardo G. Dantas*; Eduardo 1. Costab;Joaquim M. Maia"; Vera L. S. N. Buttond
de Engenharia Biomédica da Faculdade de Engenharia Elétrica e de Computaço
(DEB/FEEC) e Centro de Engenharia Biomédica (CEB) da Universidade Estadual de Campinas
(UNTCAMP - SP- BRASIL)
ABSTRACT
In cardiac surgeries it is frequently necessary to carry out intewentions in internal heart structures, and where the blood
circulation and oxygenation are made by artificial ways, out of the patient's body, in a procedure known as extracorporeal
circulation (BC). During this procedure, one of the most important parameters, and that demands constant monitorin& is the
blood flow. In this work, an ultrasonic pulsed Doppler blood flowmeter, to be used in an extmcorporeal circulation system,
was developed. It was used a 2MHz ultrasonic transducer, measuring flows from 0 to 5liters/mm, coupled externally to the
EC arterial line destined to adults perfusion (diameter of 9.53mm). The experimental results using the developed flowmeter
indicated a maximum deviation of 3.5% of full scale, whilst the blood flow estimator based in the rotation speed of the
peristaltic pump presented desations greater than 20% of full scale. This ultrasonic flowmeter supplies the results in a
continuous and trustworthy way, and it does not present the limitations found in those flowmeters based in other transduction
methods. Moreover, due to the fact of not being in contact with the blood, it is not disposable and it does not need
sterilization, reducing operational costs and facilitating its use.
Keywords: ultrasound, Doppler, flowmeter, extracorporeal circulation
1. INTRODUCTION
In cardiac surgeries it is frequently necessary to carry out intewentions in internal heart structures, as valves substitutions,
wall surgeries and others. This is only possible when the heart is disconnected from the circulation system, making the blood
circulation and oxygenation by artificial ways, out of the patient's body, in a procedure known as extracorporeal circulation
(EC) or heart-lung bypass.
In an EC procedure the heart pump function is substituted by an external mechanical pump, and the lungs oxygenation
functions by an external oxygenator. Figure 1 shows a simplified scheme of the EC system. The venous blood, that comes
from the SSC (superior systemic circulation) and the ISC (inferior systemic circulation), instead of going into the nght
atrium, is deviated to the oxygenator, through PVC flexible tubes. The oxygenator is an external patient device that changes
the venous to arterial blood. This arterial blood is pumped back to the patient, going to the SSC and ISC, restarting the cycle1.
One of the most important parameters, and that demands constant monitorin& is the blood flow. Quite often, blood flow
estimators based in the pump rotation speed (for peristaltic pumps) or those using the electromagnetic principle (for
peristaltic or centrifugal pumps) are used. The technology available for these methods is relatively well known, but not very
reliable for EC systems due to their intrinsic characteristics. In this work, an ultrasonic pulsed Doppler blood flowmeter was
developed. It was used a 2MHz ultrasonic transducer, measuring flows from 0 to 5 liters/mm, coupled externally to the EC
arterial line destined to adults perfusion. This flowmeter is also showed in figure 1, placed in the arterial line, just after the
pump.
There are two major kinds of mechanical pumps used in EC systems: the centhiugal and the roller pump. The roller pump is
the older model, where an U shape flexible tube is pressed by two rollers, pumping the blood in a peristaltic manner. It
generates a negative pressure in the tube input and a positive pressure in its output In this kind ofpump, it is common to find
a flow estimator based on the pump speed rotation. This flow estimator, depending on the flow rate and the characteristics of
the hydraulic circuit, can lead to wrong results, as it sll be shown ahead. The centrifugal pump pumps the blood by
*
Correspondence: E-mail: bedUbp; Cjoaqujceb.unicampbr, 'vera(ceb.unicamp.br,
Address Caixa Postal 6040 — Canipinas — SP — CEP: 13083-970 — Brazil; Telephone: +55-19-7887895; Fax: +55-19-2893346.
In Medical Imaging 2000: Physiology and Function from Multidimensional Images, Chin-Tu Chen,
448 Anne V. Clough, Editors, Proceedings of SPIE Vol. 3978 (2000) • 1605-7422/00/$15.00
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increasing its kinetic energy, through a high speed moving gmup of palettes1'2. The flow estimator based on the pump speed
rotation can no longer be used in centrifugal pumps, and there is the need of a real blood flowmeter. It is common to find an
electromagnetic flowmeter coupled to this kind of pump, but, in our opinion, this is not the better choice, due to its
characteristics, such as its hematocrit dependency3'4'5.
ULTRASONIC
LOWMETER
Figure 1. The extracorporeal circulation scheme.
The choice of using the ultrasonic method is justified due to the fact that the ultrasound is considered harmless to the
patient's health and because there is no contact between the transducer and the blood, resulting that it is not disposable and it
does not need sterilization, reducing operational costs and facilitating its use6'7'89.
The ultrasonic flowmeter was developed using conventional pulsed Doppler techniqUeS (see figure 2). The ultrasonic
transducer emits acoustical pulses that are scattered by the red blood cells and detected by the same transducer,working as a
receiver. According to the Doppler effect, as the red blood cells are travelling through the tube in a velocity V. there isa
frequency shift between the transmitted (fT) and the received frequency (fJ known as the Doppler shift (fD =fT- fR).
.
Ultrasonic
TransducerV///////Z///7777
Tube
Walls
//)3(,(///T\ /////Z/Z//////y/z////////Af
V
Vcos 0
Figure 2. The ultrasonic pulsed Doppler technique.
449
OXYGENATOR
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450
The Doppler shift is related to the blood velocity according to the following equation10:
fD C (1)
where c is the sound velocity in the blood (approximated 1540m/s) and the signal means that fT can be greater or smaller
than fR, dP11dffig on the direction ofthe blood flow
2. MATERIAL AND METHODS
In this worlç, the ultrasonic pulsed Doppler blood flowmeter, to be used in an extracorporeal circulation system, was
developed using a 2MHz ultrasonic transducer, measuring flows from 0 to 5liters/mm, coupled externally to the EC arterial
line destined to adults perfusion (diameter of 9.53mm). The transducer (KB-Aerotech) is represented in figure 3. Figure 4
shows how the transducer is coupled to the EC tube. As it can be seen, the EC tube does not need to be cut.
45
Figure 5 shows the block diagram of the ultrasonic flowmeter. As there is only one ultrasonic transducer, it is used as a
transmitter and a receiver. During a brief period of time, the transducer acts as a transmitter, sending a burst of 10 pulses in a
2MHz frequency. These pulses are scattered by the red blood cells, going back to the transducer, that now acts as a receiver.
The burst generation circuit generates a 10-pulse 2MHz burst with a 15.625kHz (64s) rate. These pulses are amplified till
12V by the transmitter circuit, using an IGBT transistor, and applied to the ultrasonic transducer.
17,5mm
Figure3. The ultrasonic transducer.
Figure 4. The ultrasonic transducer coupled to the EC tube.
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As the transducer acts asa transmiuer and as a receiver, the transmitter circuit is electrically connected to the receiver circuit.
In other to avoid the receiver amplifier sathration. there is a protection circuit that cuts the voltage in The receiver
circuit amplifies the received signals with an AUC (automatic gain control) RF amplifier. This amplified signal is
demodulated in frequency with a 2MHz carrier. This demodulated signal contains the Doppler shift of the reflections at
different depths, including the tube walls. There is a lOps sampling channel that enables the sample and hold circuit, allowing
the signal processing of only the scattering of ultrasound waves from the tube central regioit In the signal processing circuit
unit, a zero crossing detector converts the Doppler shift to a pulsed signal, whose frequency is measured by a frequency-to-
voltage converter. According to equation 1, the Doppler shift frequency is proportional to the blood flow velocity. A
liiiearization circuit to correct small deviations from the measured to the expected (desired) values has been added The
linearization circuit converts the analog signal into an 8 bit resolution digital signal. We have used an EPROM as a look up
table linearization circuit. The digital signal is used as the EPROM's address (table input), and the EPROM's data (table
output) corresponds to the linearized digital signal. The lineariZed flow value is showed in a two digit numeric display.
Figure 6 illustrates the 10-pulse 2MHz burst and the sampling channel. As can be seen, the bursts are repeated at each 64ps,
avoiding interferences between the actual transmitted pulse and the reflections caused by the last transmitted pulse. The
sampling channel starts 17.5s after the beginning of the burst transniissioit During the flowmeter development, instead of
using blood for the tests procedures, it was used a solution of water with corn starch. The solution was used with different
concentrations in order to simulate the hematocrit variation caused by hemodilution.
0
(I)
>
>
C4
I,
•0
=
0.
E
Time (js)
70.0 80.0
451
Figure 5. The ultrasonicfiowmeter block diagram.
64 ts
Burst
0.0 10.0 20.0 30.0 40.0 50.0 60.0
Figure 6. The transmitted burst and the sampling channel.
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3. TESTS AND RESULTS
Most of the ultrasonic Doppler systems use sinusoithi lransmitted pulses. In order to simplify the electronic circuit and
reduce its cost, we decided to use an asymmetric square pulse. The transducer works as a filter for frequencies greater than its
resonance frequency, attenuating the high order harmonics. In figure 7-a, it is shown a l2Vpp sinusoidal burst and its echo
geneiated by reflections in a stainless steel flat disc. In figure 7-b, it is shown the same test, but with a l2Vp square burst.
I
: z1zz: :i Iiiri*V_ Iiiiiiiiii
1 2 3 4
a) b) Time (its)
Figure 7. The sinusoidal (a) and the square (b) bursts used on tests.
In figure 8-a and 8-b, it is shown the Fast Fourier Transform (FFT) of the received echoes. The graph of figure 8-a
corresponds to the echo of the sinusoidal burst, and the graph offigure 8-b to the echo of the square burst. It can be seen that
the central frequencies are almost the same: 1.96MHz for the sinusoidal burst and 1.95M}lz for the square burst. The
bandwidth, measured at -3dB line, is 240kHz for both transmitted pulses.
0.9 0.9
0.8 08
w 07 - a,
-C • •0 - -
O.6 r O.6 r r
E E
0.5 , , < 05 ,
-C -C
a, a,
- 0.4 ' O.4
I
- 1 2 3 — - 5 6
a) Frequency (MHz) b) Frequency (MHz)
Figure8. The Fast Fourier Transform ofthe received echoes generated by the sinusoidalpulse (a) and the squarepulse (b,).
In order to calibrate the developed Ultrasonic flowmeter, it was used an hydraulic test circuit. Figure 9 shows this
experimental setup. The tank #1 has two infra-red level sensors, a low and a high level sensor. During the tests, the pump
speed rotation was set to a fixed value, and the test solution pumped from tank #2 to tank #1. When the test solution reaches
the low level sensor, it triggers a timer, stopping it when the solution reaches the high level sensor. The flow value do not
452
Time (ps)
: -3dB
3
-3dB
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High-level
sensor
Low-level
sensor
change during each test, and can be calculated knowing the volume between the level sensors and the time spent to fill this
volume. This flow reference value was compared to the flow indicated by the ultrasonic flowmeter and by the rollerpumpflow estimator.
Tank #1
Roller pump with
flow estimator
95cm
Ultrasonic
flowmeter
Figure 9. The hydraulic test circuit.
Figure 10 shows, for each pump speed rotation, the reference flow, the measured flow by the ultrasonic flowmeter and the
estimated flow by the roller pump. As it can be seen, the flow indicated by the roller pump is a straight line according to its
pump speed, as expected due to the estimation of the flow based on the pump speed. The developed ultrasonic flowmeter
showed a better result, with its indicated values veiy close to the reference values. The tests were carried out 10 times for
each pump speed, showing a maximum error of 3.5% of full scale between the ultrasonic flowmeter and the reference flow,
whilst the blood flow estimator based in the mtation speed of the roller pump presented deviations greater than 2O% of full
scale.
7.0
6.0.
2 ::
.
3.0 • ;;;A 11111
— Reference flow0 2.0 .,.'
_._;- __-__ Ultrasonic fiowmeter
1.0 - ------
------
______i
Roller pump
0.0. I I
0 25 50 75 100 125 150 175
Pump speed (RPM)
Figure 10. The referenceflow, the measuredflow by the ultrasonicflow,neter and the estimatedfiow by the rollerpump.
Figure 1 1 uses the same data used in figure 10, showing the ultrasonic flowmeter data versus the reference flow. There is a
great linear pattern through the points, with a correlation factor very close to 1 (R2 0.9993).
453
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Reference flow (I/rn in)
Figure 11. The ultrascinicflowmeter calibration curve.
The values of the reference flow, the ultrasonic flowmeter and the roller pump flow estimator, used in the graphs of figures
10 and 1 1 are shown in table 1.
Table 1. Values ofthe referenceflow, the ultrasonicfioineter and the rollerpumpflow estimator.
454
s_
rotation
(RPM)
Reference
Flowmeter
(1/mm)
Standard
deviation
(11mm)
Ultrasonic
flowmeter
(11mm)
Standard
Deviation
(1/mm)
Rolier
pump
(1/mm)
Standard
Deviation
(11mm)
0 0.00 0.000 0.00 0.000 0.00 0.000
13 0.44 0.030 0.41 0.014 0.50 0.000
25 0.90 0.048 082 0.017 1.00 0.000
38 1.39 0.040 1.30 0.019 1.50 0.000
50 1.81 0.052 1.78 0.027 2.00 0.000
63 2.39 0.054 2.33 0.031 2.50 0.000
75 2.84 0.092 2.82 0.018 3.00 0.000
8g 3.18 0.056 3.15 0.017 3.50 0.000
100 3.54 0.084 3.57 0.029 4.00 0.000
113 3.95 0.072 3.89 0.036 4.50 0.000
125 4.27 0.095 4.26 0.032 5.00 0.000
138 4.57 0.086 4.47 0.026 5.50 0.000
150 4.85 0.116 4.84 0.033 6.00 0.000
5.0
.E 4.0
E
' 3.0
E
i
.0
R2 0.9993
0.0 1.0 2.0 3.0 4.0 5.0
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4. DISCUSSION AND CONCLUSIONS
Since the beginning of this project, the major goal was the development of an equipment that could be really useful in an
extracorporeal circulation system, improving its performance by giving measurements in a trustworthy way, and with a low
final cost. We have reached the desired results with our developed blood flowmeter. It is simple to be used, due to the fact
that there is no need to cut the EC tube for its coupling, avoiding it to be disposable nor needing sterilization. Instead of using
the roller pump estimator, that can present wrong results, the use of our ultrasonic flowmeter is a good alternative. Its use is
also practicable for BC systems that use centrifugal pumps. This ultrasonic flowmeter can also be used as a standalone
device.
From different available techniques, we have chosen the simplest and cheapest electronic circuits11 carrying out. One
example is the signal processing circuit. Instead of using a fast analog to digital converter and a DSP (digital signal
processor) to digitize the Doppler shift signal and measure its frequency through a FFT algoritht&213"4'15'16, we have decided
to do the signal processing unit with low cost analog devices, using the zero crossing detector" and the frequency to voltage
converter, as well as the linearization circuit, that has the linearization table recorded in an EPROM, usingjust a few and low
cost devices.
The flowmeter can improve the safety of the extracorporeal circulation procedure, by showing to the perfusionists a
trustworthy flow measurement. In a future work, the flowmeter can be joined to others sensors, like an oxygenator blood
level and a blood gas saturation sensor, in order to make an automatic pump speed control'8.
ACKNOWLEDGMENTS
We would like to grant the CAIES for the financial support.
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Ultrasonic Doppler Blood Flowmeter For Extracorporeal Circulation

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Ultrasonic Doppler Blood Flowmeter For Extracorporeal Circulation

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