Methods for recording the Doppler shift of frequencies

The audiological method has this name, since the characteristics of the frequencies in the Doppler study are within the limits 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, synchronous signal with cardiac contractions is heard.
• The presence of stenosis differently changes the "melody" of the artery. Depending on the degree of narrowing, the signal becomes higher, jerky, sometimes whistling. With subtotal stenosis, sharp sounds can arise: "gull cry", vibration, "mur-mur" -phenomenon or weak blowing "damped" signal.

The flow signal through the veins has completely different audiological characteristics. It resembles either sea surf, or almost modulated blowing noise, almost unrelated to cardiac contractions, but rather dependent on respiratory excursions.

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

Nevertheless, the main method of registration is a graphical representation of the Doppler shift in time, which consists of two main components:

• The envelope curve is the linear velocity in the central layers of the flow;
• Doppler spectrum is a graphical characteristic of the ratio of erythrocytes moving at different rates within the control measuring volume.

In modern dopplerographs, both these components are recorded. They can be analyzed both separately and on a joint Doppler sonogram. The most important parameters of the Dopplergram are as follows.

• The maximum systolic, or peak, frequency of the linear velocity of blood flow, measured in kilohertz (or, more often, is translated into centimeters per second).
• Maximum diastolic frequency, which reflects the final velocity of blood flow at the end of the diastolic phase of the cardiac cycle.
• Average systolic frequency, which reflects the average weighted blood flow velocity along the entire diameter of the vessel. It is believed that it is the average systolic frequency that is most important for the objectivization of the linear velocity of the blood flow. It is calculated by the formula:

MFR = (MFR + 2MDCH) / 3 cm / s,

Where SSF is the mean systolic frequency; MSC - maximum systolic frequency; MDP - the maximum diastolic frequency.

• Power parameters are the frequency distribution of the spectrum intensity. The registration of these changes becomes possible, since not only the maximum speed changes but also the frequency distribution in the spectrum during the pulse cycle.

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

If the maximum systolic frequency depends on the volume of cardiac output, the diameter, the elasticity of the vessel, the viscosity of the blood, then the maximum diastolic frequency is associated exclusively with the level of resistance to the blood flow - the higher it is, the lower the diastolic component of the flow. In order to clarify the interdependence between these parameters of the Doppler sonogram 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 index of circulatory resistance is calculated by the formula:

DIC = (MFN-MEM) / MSC,

Where DIC is the index of circulatory resistance; MSC - maximum systolic frequency; MDP - the maximum diastolic frequency.

The index of circulatory resistance for the common carotid artery is normally 0.55-0.75, with stenosis becoming more than 0.75. The index of circulatory resistance also increases with increasing intracranial pressure. At the extreme manifestations of cerebral edema, the index becomes prohibitively high - more than 0.95. In such conditions, characteristic for the so-called tamponade of the brain, a pathological model of the reverberating flow of the "forward-backward" type is recorded along the internal carotid artery. The combination of such a flow variant with the termination of the registration of the signal from the orbital arteries, together with a sharp drop-cessation of circulation along the middle cerebral artery according to TCDD, are clear criteria for stopping intracerebral perfusion, i.e. Death of the brain. On the contrary, with such a pathological model of blood flow as arteriovenous malformation, the transfer of significant volumes of blood from one basin to another is accompanied by a decrease in the circulatory resistance index of less than 0.5.

The index of spectral expansion is calculated by the formula:

ISP = (MSF-SSH) / MSC,

Where ISR is the spectral expansion index; MSC - maximum systolic frequency; SSF is the average systolic frequency.

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

The majority of researchers are unanimous in that the attempt to standardize the indices of linear velocity of blood flow across different basins of the main arteries of the head is hardly advisable. This is due to a number of reasons: the inability to take into account the angle of the sensor's slope (see the Doppler frequency shift formula), which is necessary for the accurate calculation of the speed indicators; the uncertainty of the exact position of the measuring volume in the lumen of the vessel is the central position for the diameter or the "near-wall" position. At the same time, if for the carotid arteries these problems are completely overcome, then the location of the vertebral arteries is much more complicated. This is connected with the physiological asymmetry of the vertebral artery (the left one is usually 1-3 mm wider than the right one), and with the difficulty of finding the only available V3 segment for ultrasound dopplerography, and, most importantly, with much more frequent anomalies of the vertebrobasilar basin (hypoplasia, 15% of all patients). In addition, for the correct interpretation of Doppler sonograms, one should keep in mind the age features. As the physiological maturation, aging of a person, the parameters of blood flow along the main arteries of the head change regularly.

Taking into account the above-mentioned peculiarities prompts us to assume that the main diagnostic parameter is not the absolute magnitude of the linear velocity of the blood flow, but the degree of its asymmetry and direction change. Nevertheless, according to generalized data, the parameters of the linear velocity of blood flow along the main arteries of the head in healthy people aged from 20 to 60 years on average are: for the common carotid artery - 50 cm / s, for the internal carotid artery - 75 cm / on the vertebral artery - 25 cm / s, on the orbital artery - 15 cm / s.

[1], [2], [3], [4], [5], [6], [7]