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 that 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 maintained. 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 are 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 carried out during peritoneal dialysis. To achieve the desired effect, you need to obtain an 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 with a glucose content slightly higher than in the blood is injected into the abdominal cavity. Then, the fluid rushes 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 solution and the hydrostatic pressure of the blood. Depending on this pressure, the ultrafiltration rate can be adjusted. The permeability coefficient is calculated in the form of the amount of ultrafiltrate that passes through the membrane in 1 hour. All dialyzers can be classified according to the value of this coefficient. They come in low, medium and high permeability. The design of each device allows you to enter the required speed and ultrafiltration mode, which is very convenient. There are a number of devices that allow you to carry out ultrafiltration yourself, while regulating the speed of the process and the dialysis coefficient using electromagnetic flowmetry.

Ultrafiltration with an active speed of 5 to 35 ml/min allows you to eliminate high fluid retention in the body within just a few hours. But even with the help of spontaneous constant ultrafiltration, approximately 20 liters of fluid can be removed from the body over the course of just one day.

Applicable to patients with heart failure, ultrafiltration can reduce venous blood pressure and central blood volume to restore cardiac function. In patients with uremia, ultrafiltration combined with hemodialysis can significantly increase the quality of blood purification, and one should not forget about fluid replacement infusion into the body. Also, an urgent indication for ultrafiltration is swelling of any organ or tissue. Ultrafiltration is also applicable to patients with renal failure. In such patients, fluid retention is due to oliguria. However, the consistent use of ultrafiltration is associated with a high risk of collapse.

As for contraindications, they are used with caution or refused in patients with hypovolemia, hypotension (arterial), intoxication 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 rate of ultrafiltration depends on the pressure across the dialyser membrane (transmembrane pressure), calculated as the blood pressure minus the dialysate pressure. The blood pressure in the dialyzer depends on the speed of the blood pump and changes within small limits during the procedure. Thus, ultrafiltration can be controlled to a greater extent by changing the pressure of the dialysis fluid.

There are fundamentally 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 dialyzer pressure will decrease.

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

In the UV volume control system, the main element is the duplex pump. Its feature is that it simultaneously supplies and withdraws the same amount of liquid 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 dialyzers used, which is explained by the error in measuring transmembrane pressure.

The ultrafiltration coefficient is the amount of liquid passing through the membrane in 1 hour per 1 mmHg. 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 specific hydraulic system design.

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.

The disadvantages of the ultrafiltration control system by volume are: firstly, an 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 first models of hemodialysis machines, the fluid removed from the patient during dialysis was not measured; control was carried out only according to the established TMP; the rate of liquid removal was determined approximately as the product of TMP and 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 of TMP determination.

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

The possibility of performing dialysis with a variable UV rate during therapy in the case of sodium profiling has already been discussed. The second treatment option is to vary the UV rate at a stable (140 - 142 mmol/l) sodium concentration in the dialysate solution. The most popular is a technique in which the UV rate in the first hour of dialysis increases stepwise 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 a volumetric UV control system, the method of ultrafiltrate control is suggested by the design of the hydraulic part itself: calculating the operating speed of the UV pump.

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

The operating error of the UV measurement system should be no worse than 50 - 60 ml/h. With 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 solution.

An additional improvement to the UV control system is the ability to prevent backfiltration.

It is believed that dialysis fluid does not need 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 may be observed in a certain area of ​​the dialyzer, which means reverse filtration of the penetration of dialysis fluid into the blood.

The part of the dialyzer most susceptible to reverse filtration is where the dialysis fluid is supplied and the 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 monitor reverse filtration is to install a dialysate inlet pressure sensor. Such sensors are, for example, installed in HD-secura and DW1000. The machine generates an alarm when the dialysate inlet pressure approaches the blood outlet pressure, thereby warning of the occurrence of reverse filtration.

If, when using a standard membrane under conditions of reverse filtration, the likelihood of the penetration of bacteria and endotoxins is small (although such cases are noted), then when working under the same conditions with a highly permeable membrane, the pore sizes of which are relatively large, the likelihood of the 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 to install special dialysis fluid filters to remove bacteria and endotoxins, as well as hemodialysis using a sterile dialysis solution.

Ultrafiltration I Ultrafiltration

a method of 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 U 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 an amount of 15, 25 or 42.5 g/l, which, when injecting the solution into the abdominal cavity, allows one 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 is removed from the abdominal cavity. By selecting a certain concentration of glucose for dialysis, they regulate the water content in the patient’s body.

Hydrostatic ultrasonography is usually carried out using a dialyzer, on the membrane of which a positive difference is created between the blood pressure and the hydrostatic pressure of the dialysate solution. The magnitude of this difference, called transmembrane pressure, as well as the permeability coefficient of the membrane for ultrafiltrate, determines the ultrafiltration rate. 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 ultrasonic 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 value of the solution pressure on the outside of the membrane necessary to obtain the required ultrafiltration rate is determined. The solution pressure in the apparatus is adjusted manually or automatically according to the specified transmembrane pressure. There are devices in which monitoring of fluid is carried out on the principle of volumetry or electromagnetic flowmetry. The limit value of the transmembrane pressure should not reach the value of the destruction pressure (approximately 600 mmHg st.).

Ultrafiltration at speeds from 5 to 35 ml/min eliminates quite significant fluid retention within a few hours. With some variants of the method, for example, using 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 swelling.

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

Indications for emergency 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 artificial circulation and hemodilution. In addition, U. is an integral part of the hemodialysis treatment program for patients with renal failure in whom fluid is retained due to oliguria. Consecutive use of U. and hemodialysis in such patients is advisable only in cases where their joint implementation creates a threat of development of collapse and .

Ultrafiltration is carried out only in a hospital setting. The procedure is performed with the patient in a functional bed. Before starting the procedure, the patient is administered 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. The ultrafiltration mode is monitored throughout the entire procedure; 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 U. it is almost always observed.

Complications during exercise may include hypovolemia in the muscles of the legs and arms, spastic pain in the abdomen and chest, hoarseness, etc. In 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 in performing ultrasound; rather, it may be a manifestation of sudden onset internal bleeding, cardiac tamponade, myocardial infarction, bacterial shock, or adrenal insufficiency. The threat of collapse increases when U. is carried out in patients receiving β-blockers and. Treatment of emerging complications is carried out immediately. Muscle cramps that occur before the required result is achieved 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 is to promptly lower the head end of the bed below the horizontal level, reduce the speed or stop ultrafiltration, and slow down arteriovenous blood perfusion. Then, based on the situation, an infusion of 500 ml 5% glucose solution prepared on a polyionic basis (this is easier to do through the arterial line of the dialysis system using a pump); if necessary, enter 200 ml 20% albumin solution, 30-60 mg prednisolone is returned from the device.