Cardiac surgery in children. Cardiac surgery in children This type of treatment cannot be performed when

Author of the article: Semenisty Maxim Nikolaevich

What is ultrafiltration?

Ultrafiltration is one of the methods for normalizing water homeostasis with excess fluid in the body. This method is based on the removal of protein-free fluid from the blood, passing through artificial or natural membranes, which play the role of a filter (ultrafilter). The most commonly used dialysis membrane, hemofiltration membrane or peritoneum (natural membrane). The main source of ultrafiltrate formation is extracellular fluid, which enters the bloodstream under the pressure of plasma proteins (proteins). The main difference from diuretic methods is that ultrafiltration is capable of dosed dehydration and thereby preserves the electrolyte composition of the blood. The acid-alkaline state of the blood is also preserved. In the case of a sharp simultaneous removal of too much fluid, hyperkalemia may develop, accompanied by metabolic acidosis, an increase in hematocrit and an increase in blood viscosity is also possible.

The process of ultrafiltration of liquid in the body is due to the presence of filtration pressure between the filtration membrane. There are only two pressures: osmotic pressure (the tendency of a liquid to move from an area of ​​a low concentration gradient to an area of ​​a high concentration gradient) and a hydrostatic gradient. Based on this, two types of ultrafiltration are distinguished: osmotic and hydrostatic ultrafiltration.

Ultrafiltration: osmotic.

Osmotic ultrafiltration is usually performed during peritoneal dialysis. To achieve the desired effect, you need to get the osmotic pressure higher than the osmotic pressure in the blood. Only then is ultrafiltration possible. The most commonly used is glucose. For example: a glucose solution is injected into the abdominal cavity with the content of glucose itself slightly higher than in the blood. Then, the fluid tends from the blood into the abdominal cavity, where it is then pumped out. In this way, the water content in the patient's body can be regulated.

Ultrafiltration: hydrostatic.

This type of ultrafiltration is usually carried out using a special device - a dialyzer. The dialyzer creates a negative pressure between the hydrostatic pressure of the dialysate and the hydrostatic pressure of the blood. Depending on this pressure, the ultrafiltration rate can be adjusted. The permeability coefficient is calculated as the amount of ultrafiltrate that passes through the membrane in 1 hour. By the value of this coefficient, all dialyzers can be classified. They come in low, medium and high permeability. The device of each device allows you to enter the required speed and mode of ultrafiltration, which is very convenient. There are a number of devices that allow you to independently conduct ultrafiltration, while adjusting the speed of the process and the dialysis coefficient using electromagnetic flowmetry.

Ultrafiltration with an active speed of 5 to 35 ml/min eliminates high fluid retention in the body in just a few hours. But even with the help of spontaneous continuous ultrafiltration within just one day, about 20 liters of fluid can be removed from the body.

Applied to patients with heart failure, ultrafiltration can reduce venous blood pressure and central blood volume to restore the efficiency of the heart muscle. In patients with uremia, ultrafiltration in combination with hemodialysis can significantly increase the quality of blood purification, and one should not forget about the replacement infusion of fluid into the body. Edema of any organ or tissue is also an urgent indication for ultrafiltration. Ultrafiltration is also applicable to patients with renal insufficiency. In such patients, fluid retention is due to oliguria. However, the consistent use of ultrafiltration is associated with high risks of Collapse.

As for contraindications, then with caution they are used or refused in patients with hypovolemia, hypotension (arterial), intoussication with glycosides, etc. pathologies.

During hemodialysis, water moves from the blood into the dialysate under the influence of a hydrostatic pressure gradient inside the dialyzer. The ultrafiltration rate depends on the dialyzer membrane pressure (transmembrane pressure), calculated as the blood side pressure minus the dialysate side pressure. The blood pressure in the dialyzer depends on the speed of the blood pump and changes within a small range during the procedure. Thus, ultrafiltration can be controlled to a greater extent by changing the pressure of the dialysis fluid.

Fundamentally, there are two ultrafiltration control systems, conventionally called pressure and volume. The following is a brief description of how each system works:

In a single pump UV pressure control system, a throttle restricts the flow of dialysis fluid. As the vacuum pump speed increases, the pressure in the dialyzer will decrease.

In a UV pressure control system with two pumps, a vacuum in the dialyzer is created due to the increased speed of the vacuum pump relative to the boost pump.

In the volumetric UV control system, the main element is the duplex pump. Its peculiarity is that it simultaneously supplies and withdraws the same amount of fluid from the dialyzer. In this case, the vacuum is set by the ultrafiltration pump.

The main disadvantage of systems with pressure controlled ultrafiltration is the limitation on the ultrafiltration coefficient (KUF) of the used dialyzers, which is explained by the error in measuring the transmembrane pressure.

The ultrafiltration coefficient is the amount of fluid passing through the membrane in 1 hour per 1 mm Hg. transmembrane pressure gradient

For example, when using a dialyzer with KUF 60 ml/h/mmHg. and TMR measurement accuracy +/-3 mm Hg. the error of the ultrafiltration system will be +/- 180 ml/h. The maximum KUF value depends on the particular design of the hydraulic system.

For example, HD-secura, which uses two pressure sensors before and after the dialyzer and has a special mode of operation with high permeability membranes, can work with dialyzers with KUF up to 60 ml/h/mmHg. inclusive.

Disadvantages of the ultrafiltration control system by volume are: firstly, intermittent flow of dialysis fluid and, consequently, a decrease in the effectiveness of the procedure, and secondly, sensitivity to air penetration into the closed circuit, which requires a special deaeration system.

Ultrafiltration control system

In the early models of hemodialysis machines, the measurement of fluid removed from the patient during dialysis was not performed; control was carried out only according to the established TMP, the rate of liquid removal was determined approximately as the product of TMP on KUF. A significant error in this kind of calculations was caused by: 1. discrepancy between the KUF value determined in vitro and the real one; 2. decrease in KUF during dialysis; 3. inaccuracy in determining TMR.

Modern hemodialysis equipment automatically determines the rate of fluid removal and displays the relevant information on the display, which allows for hemodialysis with a programmed change in ultrafiltration.

The possibility of dialysis with a variable UV rate during therapy in the case of sodium profiling has already been discussed. The second therapy option is to vary the UV rate at a stable (140 - 142 mmol / l) sodium concentration in the dialysis solution. The most popular is the technique in which the UV rate in the first hour of dialysis is gradually increased to a maximum, which is maintained during the first half of dialysis therapy, and then gradually decreases (to zero) at the end of the procedure. Naturally, the proposed treatment regimens are to some extent conditional and are subject to individual correction in each specific case.

In the case of the UV control system by volume, the way to control the ultrafiltrate is suggested by the very design of the hydraulic part: the calculation of the speed of the UV pump.

In the case of an ultrafiltration pressure control system, there are at least two options for controlling the removed liquid. The first, when, based on the measurement of the inlet and outlet flows of dialysate, a conclusion is made about the amount of ultrafiltrate received from the blood, and the second option, when the rate of fluid removal is judged by the filling of a special electrode chamber.

The error of the UV measurement system should be no worse than 50 - 60 ml/h. At a lower value, it becomes indistinguishable against the background of inaccuracy in determining the "dry" weight of the patient, food during dialysis and the injected saline.

As an additional improvement to the UV control system, mention should be made of the possibility of preventing back filtration.

It is believed that the dialysis fluid does not have to be completely sterile, because the dialyzer membrane is a fairly effective barrier to bacteria and their endotoxins. However, under certain conditions, the presence of bacterial products can play a negative role.

If dialysis is carried out at a low ultrafiltration rate, then a change in the direction of pressure can be observed in a certain area of ​​the dialyzer, and hence reverse filtration, the penetration of dialysis fluid into the blood.

The part of the dialyzer most susceptible to reverse filtration is where dialysis fluid enters and blood leaves the dialyzer. If reverse filtration occurs, it is primarily in this place. Since blood outlet pressure is measured on all machines, a reasonable way to control back filtration is to install a dialysate inlet pressure sensor. Such sensors are, for example, installed in HD-secura and DW1000. The machine gives an alarm when the dialysate inlet pressure approaches the blood outlet pressure, thereby warning of back filtration.

If when using a standard membrane under reverse filtration conditions, the probability of penetration of bacteria and endotoxins is small (although such cases are noted), then when working under the same conditions with a high-permeability membrane, the pore sizes of which are relatively large, the probability of penetration of bacterial products into the blood increases, which can lead to unwanted side effects.

Another way to prevent the possible consequences of the penetration of bacterial products into the blood is the installation of special filters of the dialysis fluid to remove bacteria and endotoxins, as well as hemodialysis using a sterile dialysis solution.

Ultrafiltration I Ultrafiltration

a method for correcting water homeostasis with excess water in the body by removing protein-free fluid from the blood through natural or artificial membranes that play the role of an ultrafilter. Most often, the peritoneum, artificial dialysis and hemofiltration membranes are used as an ultrafilter. The source of ultrafiltrate formation is mainly extracellular fluid entering the bloodstream under the action of oncotic pressure of plasma proteins. Unlike diuretics, ultrafiltration allows for dosed dehydration with little effect on the electrolyte composition and acid-base state of the blood. With the simultaneous removal of a large amount of fluid (several liters), a tendency to hyperkalemia, metabolic acidosis, an increase in hematocrit and blood viscosity, and an accelerated increase in azotemia develops.

Ultrafiltration of fluid in the blood is achieved by creating a pressure difference on both sides of the filtration membrane: osmotic or hydrostatic. Accordingly, osmotic and hydrostatic W are distinguished.

Osmotic U. is usually carried out during peritoneal dialysis. To obtain the effects, it is necessary that the dialysate solution be higher than the osmotic pressure of the blood. Glucose is mainly used as an osmotically active substance, adding it to 1 l isotonic salt solution in the amount of 15, 25 or 42.5 g/l, that, when the solution is injected into the abdominal cavity, it makes it possible to obtain, respectively, 200, 400 or 800 ml ultrafiltrate. After 4-6 h when the difference between the osmotic pressure of the blood and the solution disappears, all fluid from the abdominal cavity is removed. Selecting for dialysis with a certain concentration of glucose, regulate the water content in the patient's body.

Hydrostatic U. is usually carried out with the help of a dialyzer, on the membrane of which a positive difference is created between the blood pressure and the hydrostatic pressure of the dialysis solution. the value of this difference, called the transmembrane pressure, as well as the permeability of the membrane for the ultrafiltrate depends on the rate of ultrafiltration. The permeability coefficient is expressed by the amount of ultrafiltrate (in ml) passing through the membrane in 1 h for each mmHg st. transmembrane pressure. According to the value of this coefficient, all manufactured dialyzers are small (2-3 ml/mmHg st. in 1 h), medium (4-6 ml/mmHg st. in 1 h) and large (8-12 ml/mmHg st. in 1 h) permeability. The design of the devices allows you to set the required U. mode according to the selected transmembrane pressure. By subtracting from the latter the blood pressure measured by the direct method in the venous bubble chamber, the pressure of the solution on the outside of the membrane is determined, which is necessary to obtain the required ultrafiltration rate. The pressure of the solution in the apparatus is controlled manually or automatically according to the set transmembrane pressure. There are devices in which the control of U. is carried out on the principle of volumemetry or electromagnetic flowmetry. The limiting value of the transmembrane pressure should not reach the bursting pressure (approximately 600 mmHg st.).

Ultrafiltration at speeds from 5 to 35 ml/min eliminates a fairly significant fluid retention for several hours. With some variants of the method, for example, with the help of constant spontaneous (due to blood pressure) arteriovenous U., for 1 day. can be removed from the body if necessary 15-20 l liquids, completely eliminating edema.

In patients with heart failure, U. effectively reduces the central volume and central blood, restoring the heart and eliminating ventilation and gas exchange disorders. In patients with uremia, the combination of hemodialysis with large U., which is usually combined with fluid replacement infusion, improves the quality of blood purification (primarily from substances of medium molecular weight) and accelerates the regression of many of the dangerous symptoms of uremia.

Indications for urgent use of U. are pulmonary edema of any etiology, as well as cerebral edema that develops in connection with acute water stress. Along with other methods, U. is used in the complex treatment of patients with anasarca, with edema due to congestive heart failure (especially in the presence of resistance to diuretics and glycosides) or nephrotic syndrome without renal failure, with fluid retention in the body after surgery with cardiopulmonary bypass and hemodilution. In addition, U. is an integral part of the program of hemodialysis treatment of patients with renal insufficiency, in which fluid is retained due to oliguria. Sequential use of U. and hemodialysis in such patients is advisable only in cases where their joint implementation creates a threat of development .

Ultrafiltration is carried out only in a hospital. The procedure is performed in the position of the patient on a functional bed. Before the start of the procedure, the patient is administered at a dose of 15-30 per 1 kg body weight to prevent blood clotting at the time of filling the dialyzer; in the process of ultrafiltration, a constant infusion of heparin is carried out at a rate of 10-15 units per 1 kg body weight per hour. Throughout the procedure, the ultrafiltration mode is controlled; if necessary, with the help of special devices, its speed is regulated and the patient's fluid balance is maintained. The effectiveness of the procedure is assessed by the amount of fluid removed, the decrease in the patient's body weight, and the regression of symptoms of overhydration. Particular attention is paid to the dynamics of the filling of the jugular veins, the frequency of pulse and respiration, peripheral edema, ascites, hydrothorax, hydropericardium, liver size, wet rales in the lungs, discoloration of the blood in the extracorporeal system. For an objective characterization of the effectiveness of treatment, in some cases, repeated chest radiography is performed, the dynamics of central venous pressure, volumes of circulating plasma and extracellular fluid are noted. After W. is almost always observed.

Complications in the process of U. can be hypovolemia, in the muscles of the legs and arms, spastic pain in the abdomen and chest, hoarseness,. In the case of severe hypovolemia, it can develop with loss of consciousness, generalized convulsions and respiratory arrest. It should be borne in mind that severe collapse is rarely the result of an error during U., rather, it can be a manifestation of sudden onset of internal bleeding, cardiac tamponade, myocardial infarction, bacterial shock, adrenal insufficiency. The threat of collapse increases during U. in patients receiving β-blockers and. Treatment of emerging complications is carried out immediately. Muscle cramps that occurred before reaching the desired result U. are stopped without interrupting the procedure with infusions of 60-80 ml 40% glucose solution, 20 ml 10% calcium gluconate solution, 20-40 ml 10% sodium chloride solution. in case of arterial hypotension, it consists in timely lowering the head end of the bed below the horizontal level, reducing the speed or stopping ultrafiltration, and slowing down arteriovenous blood perfusion. Then, based on the situation, an infusion of 500 ml 5% glucose solution, prepared on a polyion basis (easier to perform through the arterial line of the dialysis system using a pump); if necessary, enter 200 ml 20% albumin solution, 30-60 mg prednisolone, returned from the apparatus.