ELECTROMAGNETIC BLOOD FLOWMETER

I. N. Kraev, V. N. Titov

(Presented by the scientific seminar of the Faculty of Physics and Engineering)

One of the most important questions in experimental medicine is the study of blood circulation and, in particular, the accounting of the amount of blood flowing through the organs of an animal under the influence of various factors.

However, for several decades the dynamics of the cardiovascular system were studied mainly by blood pressure, owing to the absence of blood flowmeters.

Modern measuring technology possesses a very large number of methods for measuring the flow rate of liquids in pipelines. Many of these methods can also be successfully applied to the study of blood circulation in living organisms.

However, the conditions of experiments and their purposes are highly variable. Therefore, in each particular case, in selecting a sensor one proceeds from the physiological conditions of the experiment or clinic. The sensor of a blood flowmeter must measure minimum velocities down to 1 cm/sec, be small in size and weight, withstand the corrosive action of liquids, and not stimulate excessive development of the surgical scar. It is necessary that the flowmeter have the greatest accuracy at high sensitivity and linear calibration; create insignificant dynamic resistance to the blood flow and introduce no vortices into the flow.

The experimental model of an electromagnetic blood flowmeter developed in the laboratory of the Tomsk Polytechnic Institute basically satisfies these requirements.

The flowmeter is intended for continuous automatic measurement of blood-flow velocity in blood vessels under the conditions of a chronic experiment.

The operating principle of the instrument sensor, Fig. 1, is based on measuring the e.m.f. induced between the electrodes when the blood flow moves in a magnetic field.

The magnitude of the induced e.m.f. is determined by the expression

\[ e=\left[(\mu H_0\sin\omega t)\,dv-(A_0\cos\omega t)\right]\cdot 10^{-8}\ \text{volts}, \]

where \((\mu H_0\sin\omega t)\,dv\) is the voltage in phase with the magnetic field, proportional to the flow velocity.

\(A_0 \cos \omega t\) is the interference voltage, independent of the flow and shifted in phase by \(90^\circ\).

The interference voltage (transformer emf) arises because, in the alternating magnetic field, there is a loop formed by the two electrodes, the column of liquid between them, and the leads coming from the electrodes.

Fig. 1. Diagram of an electromagnetic flowmeter.
1—pipeline; 2—magnet; 3—measuring electrodes; 4—recording instrument.

The magnitude of the transformer emf depends on the area of the loop penetrated by the magnetic flux, the intensity of the magnetic field, and the frequency of its variation. Usually the magnitude of the transformer emf is comparable with the magnitude of the useful signal and amounts to several \(\mu\text{V}\).

In the flowmeter whose block diagram is shown in Fig. 2, compensation of the transformer emf is achieved by using a rectangular current waveform to supply the sensor magnet. Such a current waveform, as shown in [1], induces in the measuring circuit a rectangular signal proportional to the velocity of the blood motion.

The action of the transformer emf is manifested in the appearance of sharp pulses at the instant when the polarity of the current feeding the magnet is reversed. This difference in waveform makes it possible to separate the flow signal more effectively from the transformer emf, Fig. 3. The sharp peaks caused by the transformer emf can be suppressed by a strobing circuit. For this purpose the amplifier is strobed at the instant when the polarity of the current feeding the sensor magnet is reversed. The emf is measured only during those intervals of time when the field is maximal and constant. Thus, the time derivative of the magnetic flux and, accordingly, the signal produced by the transformer emf are equal to zero.

Fig. 2. Block diagram of the instrument.
1—rectangular-oscillation generator; 2—power amplifier; 3—strobing circuit; 4—gating-pulse amplifier; 5—rectifier; 6—recording instrument.

A flowmeter with a rectangular waveform of the supply current makes use of the advantages and eliminates the shortcomings of this type of instrument

with a permanent magnet and with the magnet powered by an alternating sinusoidal current.

The flowmeter set consists of a sensor, an electronic unit, and a recording instrument.

The sensor of the instrument consists of a C-shaped magnetic core, assembled from sheets of electrical steel, excitation coils, and a cuff with platinum electrodes, which are applied to the blood vessel.

Fig. 3. Signal diagrams explaining the operation of the circuit.

Measurements of the distribution of magnetic induction in the air gap of three types of sensors showed that the field nonuniformity is 3%. Therefore, the dependence of the instrument readings on the velocity profile is insignificant.

The use of a cannula with rigidly soldered electrodes made it possible to calibrate the instrument using a 1% saline solution.

The instrument was calibrated each time when one form of magnetic core was replaced by another. Numerous measurements showed that the calibration curves are linear and remain constant for a given form of magnetic core and magnet power supply.

The amount of saline solution flowing through the cross section in 1 sec was determined by measuring the volume of liquid that flowed through this section over a specified time interval.

Figure 4 presents the dependence of the sensor output signal on the magnitude of the flow rate.

When measurements are made near the heart, especially on the ascending aorta, the action currents of the heart interfere.

To eliminate errors caused by these currents, Wetterer [3] recommends placing the electrodes at equipotential points. This method is inconvenient and complicated, since, when the pulsating blood flow moves, the points at which the electrodes are attached will constantly change.

Rigid fastening of the electrodes is not recommended because of the considerable distortion of the flow.

The influence of cardiac potentials can be eliminated when the instrument circuit operates at a carrier frequency that differs substantially from the frequency of biocurrents.

The frequency spectrum of biocurrents, which is rich in harmonics, lies in the range from 0 to 100 Hz [4].

Thus, by selecting the supply frequency of the sensor magnet above the frequency of biocurrents (in the flowmeter described, the supply frequency of the sensor magnet is 400 Hz), it is possible to avoid the spectral region containing large noise voltages.

The laboratory tests carried out on the electromagnetic flowmeter made it possible to determine its principal technical characteristics and demonstrated the possibility of using it to measure blood flow.

Fig. 4. Dependence of the readings of the recording instrument on the flow rate.

Technical Characteristics

1. Lower limit of measurement: 0.5 l/min.
2. Power consumed by the instrument: not more than 100 W.
3. Supply frequency of the magnet: 400 Hz.
4. Measurement error: ±3%.
5. Overall dimensions:
   a) sensor: 70×40×30 mm.
   b) secondary instrument: 300×250×200 mm.
6. Weight:
   a) sensor: 0.5 kg.
   b) secondary instrument: 5 kg.

References

1. A. B. Denison, J. M. P. Spencer and A. D. Green, A Square-wave electromagnetic flowmeter for application to intact blood vessels, Circulation Research, vol. 3, pp. 39, 1955.
2. H. W. Shirer, R. B. Shackelford, K. B. Jochim. A magnetic flowmeter for recording cardiac output, Proceedings of the J. R. E., pp. 153, 1959.
3. E. Wetterer. Zschr. Biol., vol. 98, pp. 26, 1937.
4. B. Bayer. Biophysics. Foreign Literature Publishing House, Moscow, 1962.