Methods of recording Doppler frequency shift

The audiological method is so named because the frequency characteristics in Doppler research are within the range perceived by the human ear - from 20 to 22,000 Hz.

• In unchanged arteries, where blood elements have a high linear velocity, a clear, “singing” pulsating signal, synchronous with heart contractions, is heard.
• The presence of stenosis changes the "melody" of the artery in different ways. Depending on the degree of narrowing, the signal becomes higher-pitched, abrupt, sometimes whistling. With subtotal stenosis, sharp sounds may occur: "seagull cry", vibration, "purr-purr" phenomenon or a weak blowing "damped" signal.

The venous flow signal has completely different audiological characteristics. It resembles either the surf or an almost modulated blowing noise, practically unrelated to heart contractions, but very dependent on respiratory excursions.

Such purely audiological analysis of the Doppler shift produced by a portable pocket device can be very useful in emergency medical care and screening studies.

However, the main method of registration is the graphical display of the Doppler shift in time, which consists of two main components:

• envelope curve - linear velocity in the central layers of the flow;
• Doppler spectrum - a graphical characteristic of the ratio of erythrocytes moving at different speeds within the control measurement volume.

Modern Dopplerographs record both of these components. They can be analyzed separately or on a combined Doppler sonogram. The most important parameters of the Dopplerogram are as follows.

• The maximum systolic, or peak, frequency of the linear velocity of blood flow, measured in kilohertz (or, more commonly, converted to centimeters per second).
• Maximum diastolic heart rate, reflecting the terminal blood flow velocity at the end of the diastolic phase of the cardiac cycle.
• The average systolic frequency, reflecting the average weighted blood flow velocity across the entire cross-section of the vessel. It is believed that it is the average systolic frequency that has the greatest significance for objectifying the linear blood flow velocity. It is calculated using the formula:

SSC = (MSC + 2MDC) / 3 cm/s,

Where ASR is the average systolic heart rate; MSHR is the maximum systolic heart rate; MDR is the maximum diastolic heart rate.

• Power parameters - frequency distribution of spectrum color intensity. Registration of the specified changes becomes possible because during the pulse cycle not only the maximum speed changes, but also the frequency distribution in the spectrum.

In the systolic peak phase, the linear blood flow velocity profile flattens, the Doppler shift maximum moves toward high frequencies, and the spectrum width decreases, appearing as an "empty" zone (the so-called window) under the systolic peak. In the diastolic phase, the spectrum approaches parabolic, the frequency distribution becomes more uniform, the spectral line is flatter, so that the "empty" zone near the zero line is filled.

If the maximum systolic frequency depends on the volume of cardiac output, diameter, elasticity of the vessel, blood viscosity, then the maximum diastolic frequency is associated exclusively with the level of resistance to blood flow - the higher it is, the lower the diastolic component of the flow. In order to clarify the relationship between the specified Doppler sonogram parameters and various degrees of arteriovenous discirculation, a number of indices and functional tests have been proposed, the most common of which are listed below.

The circulatory resistance index is calculated using the formula:

ICS = (MSCh - MDC) / MSCh,

Where CRI is the circulatory resistance index; MSHR is the maximum systolic heart rate; MDR is the maximum diastolic heart rate.

The circulatory resistance index for the common carotid artery is normally 0.55-0.75, and becomes more than 0.75 in case of stenosis. The circulatory resistance index also increases as intracranial pressure increases. In extreme cases of cerebral edema, the index becomes extremely high - more than 0.95. In such conditions, typical of the so-called cerebral tamponade, a pathological model of a reverberating flow of the "forward-backward" type is recorded in the internal carotid artery. The combination of such a flow variant with the cessation of signal recording from the ophthalmic arteries, coupled with a sharp drop-cessation of circulation in the middle cerebral artery according to TCD data, are clear criteria for the cessation of intracerebral perfusion, i.e. brain death. On the contrary, in such a pathological model of blood flow as arteriovenous malformation, the movement of significant volumes of blood from one pool to another is accompanied by a decrease in the circulatory resistance index to less than 0.5.

The spectral broadening index is calculated using the formula:

ISR = (MSCH - ASC) / MSCH,

Where SBI is the spectral broadening index; MSF is the maximum systolic frequency; ASF is the average systolic frequency.

Normally, the spectral expansion index in the common carotid artery is 32-55%. When the carotid artery is narrowed, it can increase to 80%.

Most researchers are unanimous in the opinion that an attempt to standardize the linear blood flow velocity indices in different basins of the main arteries of the head is hardly advisable. This is due to a number of reasons: the impossibility of taking into account the sensor tilt angle (see the Doppler frequency shift formula), which is necessary for accurate calculation of the velocity indices; the uncertainty of the exact position of the measuring volume in the vessel lumen - the central position by diameter or "parietal". Moreover, if for the carotid arteries the above-mentioned problems are quite surmountable, then the location of the vertebral arteries is much more difficult. This is due to the physiological asymmetry of the vertebral artery (the left one is usually 1-3 mm wider than the right one), and the difficulties of finding the only segment V3 accessible for ultrasound Dopplerography, and, most importantly, with significantly more frequent anomalies of the vertebrobasilar basin (hypoplasia, tortuosity - up to 15% of all patients). In addition, for the correct interpretation of Doppler sonograms, one should remember about age-related features. As a person physiologically matures and ages, the parameters of blood flow in the main arteries of the head naturally change.

Taking into account the above-mentioned features, we suggest that the main diagnostic parameter is not the absolute value of the linear velocity of blood flow, but the degree of its asymmetry and change in direction. Nevertheless, according to generalized data, the linear velocity of blood flow in the main arteries of the head in healthy people aged 20 to 60 years on average is: in the common carotid artery - 50 cm/s, in the internal carotid artery - 75 cm/s, in the vertebral artery - 25 cm/s, in the ophthalmic artery - 15 cm/s.

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