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Information and didactic part

Rheography-method of investigation of blood circulation, based on registration of vibration resistance of body tissues (impedance) of alternating current of high frequency (40 - 500 kHz) and low power (up to 10mA).

Electrical conductivity of tissue is determined mainly by the charge transfer ions of dissolved salts, so it is considered primarily as an ion conductor. When passing a current through the living tissue, it behaves as a complex impedance, which includes the ohmic and capacitive components.

The active component of tissue is determined primarily by ionic conductivity, capacitive conductivity due to occurrence of polarization capacity at the time of current flow due to inhomogeneity of tissue and a large number of cell membranes!

Living tissue composed of cells, washed tissue fluid. The cytoplasm of cells and tissue fluid are electrolytes, separated by a poorly conducting cell membrane. This scheme has the static and polarization capacity.

According to modern concepts, the living tissues do not have inductivity and their resistivity has only capacitive and active components. Complete resistivity is called Impedance Z. In the most general case it is equal to:

(1)

Vasculature, with moving in it blood, rapidly changes its volume after each systole (while other tissues or no change in volume or change slightly). Accordingly, at the time of rise of systolic pulse wave electrical resistivity decreases during the diastolic rise of its increase (or decrease in conductivity).

Using a special generator in the rheograph created harmless to an organism the currents that supplied through current electrodes. Simultaneously, the body appear potential (or potentiometric) electrodes, which detect a flowing current.

Depending on the specific clinical of the problem changes the investigation zone and accordingly the place of electrodes. Therefore, distinguished: the central rheography (rheography aorta, pulmonary artery) - rheography of vessels of the brain (rheoencephalography), and rheography of limb vessels (rheovasography, etc.).

Below the figure of skull

Rheovasography- method for studying intensity of peripheral circulation, assessment of vascular tone, venous system.

Figure of limbs

In order to determine the systolic (and, hence, cardiac output) is used so-called integral tetrapolar rheography. It records rheogram and differential rheogram. In what follows calculate promlems, allowing with high degree of accuracy to determine the value of systolic volume (CO). In particular, one of the formulas is as follows:

СО=() _,

where: - specific resistance blood = 135 Ом м, the distance between the potentiometric electrodes, см, basic resistance between the electrodes,Ом, А_дифф.- he amplitude of the differential rheogram, Т_изгн. – time of expulsion of blood, с.; it is determined by the differential rheogram (from its beginning up to the top of the negative prong).

Overall rheography has been widely used in many fields of clinical medicine in surgery (for the diagnosis of vascular permeability) in therapy (to determine the CO, the IOC and other indicators), in obstetrics to estimate utero-placental blood flow during pregnancy.

During the passage an alternating current through the living tissue observed conductivity dispersion: tissue impedance increases with decreasing frequency current to a certain maximum value Zmax and tends to a minimum value Zmin with increasing frequency.

As alternating current is called current that has instantaneous values ​​which vary periodically in magnitude and direction. In general, an alternating current can have any complex shape of oscillations, the most common technique is sinusoidal, which is usually understood as "alternating current". In this case, the instantaneous values ​​of voltage u current I determined by the following expressions:

U=U sin( t) и i=I sin( t),

where U and I - maximum (peak) voltage and current values, - the circular frequency.

The resistance R like metal conductors (or an electrolyte solution) in the AC circuit is called active, since it is an irreversible energy loss. In the circuit with active resistance and Ohm's law holds oscillations of the current i occur in phase with the voltage fluctuations U. (Figure 1.)

Fig.1.

In circuit with the inductor L is formed electromotive force inductance = - Ldi/dt. The instantaneous values ​​of current are determined from the condition of mutual balancing of instantaneous values applied voltage U =U sin ( t) -Ldi/dt=0, where di/dt= sin ( t). Integrating this equation, we obtain i=- cos( t)=-I cos( t)=I sin( t- /2), where I =U / L.

The equation shows that the current in the circuit, like laid voltage is sinusoidal, but in phasedelayed with respect thereto by an angle /2. This is due to the inductance of the circuit which, like the mechanical system inertia counteracts on the change in instantaneous current changecorrespondingly on the change in instantaneous values of voltage.

Combining the maximum current I = U / L with the formula of Ohm's law, we see that in the circuit with the inductance value of the resistance has a value of L, which is denoted by , is called the inductive reactance of the circuit and is expressed in ohms when substituting L in the Henry and Hertz. In the circuit comprising active R and inductive impedance , retard angle on the phase is determined from the current tg = , ie can have values ​​from 0 to .

Voltage graphs u and current i in the circuit with the inductance when the steady state the circuit. They can also be directly observed on the screen of the oscilloscope.

Consider the chain, containing a capacitor with capacity C, to which applied an alternating voltage U = sin( t). The instantaneous values ​​of the current i in the circuit is determined by the conditions of mutual balancing of the instantaneous applied voltage U and with opposite acting potential difference between the plates of the capacitor:

U sin ,

where q=CU sin().

The current i = d q/d t= CU cos ( t)=I sin ( t + ),

where I = CU .

Fig. 3.

sinusoidal character, and leads the voltage in phase by . (fig.3.)

Combining the maximum current I C U by the formula of Ohm's law, we see that in a circuit with a capacity resistance value has the value that is denoted Х is called the capacitance of the circuit and is expressed in Om, by substituting C in farads, and in Hertz. In the circuit comprising a capacitive Х and active resistance R the phase shift angle determined from the condition tg =X \ R, has a value from zero to .

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