Acute Kidney Injury – Dialysis

Dialysis Treatment – CRRT

Dialysis treatment is provided for acute or chronic renal failure with uremia. Acute kidney failure is a common consequence of various life-threatening conditions such as sepsis, acute liver failure, major trauma, acute pancreatitis, etc., and dialysis treatment is often administered in an intensive care unit. Acute kidney failure increases mortality in various severe conditions, and proper dialysis treatment can often improve prognosis and save lives. This chapter describes different types of dialysis methods used in intensive care.

There are many ways to measure kidney function, commonly through the determination of creatinine, urea, and GFR calculation formulas. More precise determinations often use iohexol clearance or chrom-EDTA clearance. For estimating impaired kidney function, formula clearance (eGFR) is fairly adequate. Kidney function decreases significantly with age, while plasma creatinine increases less.

General Indications for Dialysis Treatment

• Urea > 40 mmol/l
• Therapy-resistant hyperkalemia
• Pulmonary edema with FiO₂ > 0.8 despite diuretics and PEEP (up to 10 cm H₂O)
• Hypervolemia in shock, sepsis, or acute liver failure
• Certain poisonings, e.g., methanol, metformin, or ethylene glycol poisoning

There are several types of dialysis used in dedicated dialysis wards or in intensive care units. The most common forms are hemodialysis (HD), continuous veno-venous hemofiltration (CVVHF), plasmapheresis, peritoneal dialysis (PD), or MARS therapy (molecular adsorbent recirculating system). The most common type of dialysis in an ICU today is continuous venovenous dialysis, performed through a single dialysis catheter, typically a variant of a central venous catheter (CVC) with two or three lumens, referred to as a central dialysis catheter (CDK). Peritoneal dialysis is rarely or never performed in ICUs, and MARS treatment is used occasionally at regional centers for treating acute fulminant liver failure, usually as a bridge to liver transplantation.

Dialysis works by perfusing blood through a filter, where the blood is purified through osmosis and filtration of molecules like urea and potassium. Dialysis employs two main principles for purifying the blood: diffusion and convection. In diffusion, the concentration gradient across a semipermeable membrane allows molecules to move through the filter, similar to how tea diffuses from a tea bag into hot water. Diffusion is effective for eliminating small molecules without fluid loss. Clearance represents the amount of a substance excreted via the kidneys, which depends on the urine concentration and volume. Renal clearance represents the relationship between the excretion of a substance per unit of time and its plasma concentration, Cx = Ux x V/Px ml/min. With clearance, one can compare the ability of the kidneys or the dialysis machine to excrete various substances. Clearance depends on the blood flow through the filter and the amount of dialysis fluid per minute. The principle of diffusion is used in hemodialysis, CVVHD, and CVVHDF.

Convection purifies the blood through a pressure gradient, forcing molecules through a membrane along with fluid, much like brewing coffee. Convection can filter larger molecules depending on the membrane size and pore size, comparable to using a large or small coffee filter. Since there are fewer pores for large molecules than for small ones, the filtering capacity saturates faster for larger particles. Convection also results in fluid loss, which can be replaced during dialysis with replacement solutions, allowing fluid balance to be managed. Clearance varies with the volume of fluid removed per unit of time and the volume of replacement fluid. The convection principle is used in SCUF, CVVH, and CVVHDF dialysis methods.

The general indications for dialysis treatment are uremia with urea > 40 mmol/l (relative indication at urea 30-40 mmol/l), therapy-resistant hyperkalemia, pulmonary edema with FiO₂ > 0.8 and simultaneous overhydration and diuretic resistance. In cases of acute life-threatening pulmonary edema with chronic or acute kidney failure, SCUF is the method that can quickly improve the condition, as can treatment with PEEP in a ventilator or CPAP.

Usually, treatment with CVVH, CVVHD, and CVVHDF is managed by an intensivist or anesthesiologist, while treatment with plasmapheresis, MARS, hemodialysis, or PD is managed by a nephrologist. Therefore, it is essential for an anesthesiologist to be well-acquainted with continuous dialysis, which is administered via CDK.

When starting CRRT with continuous dialysis in the ICU, basic settings are usually prescribed in the dialysis machine. These settings vary depending on the type of dialysis machine but usually involve prescribing an outflow dose and fluid distribution. The outflow dose typically starts at 30 ml/kg/hour, which equals the effluent dose. The outflow dose is the sum of the dialysate, replacement fluid pre- and post-filter, and fluid removal. In cases of sepsis, severe liver failure, burns, or other catabolic conditions, higher initial outflows of 35-40-60 ml/kg/hour may be considered.

The balance between replacement fluid and dialysate depends on the size of the particles you primarily want to eliminate. Larger particles require more replacement fluid, while smaller molecules require more dialysate. The settings also depend on the dialysis machine used. Prismaflex requires at least 500 ml post-filter. Increasing the amount of fluid after the filter improves the dialysis efficiency but increases the risk of filter clogging. Predilution reduces the risk of filter clogging but leads to a more significant dilution effect on the blood being dialyzed. The effectiveness of dialysis is monitored through daily measurements of urea and creatinine, as well as weighing the patient and calculating the fluid balance.

Blood flow in the dialysis machine during CRRT usually ranges between 100 and 250 ml/min, with current practice often leaning toward 200-250 ml/min. The advantage of high blood flow is that the volume of dialyzed blood per unit of time increases, making dialysis more efficient and preventing the blood from reaching the same concentration as the dialysate at the end of the filter. Increasing blood flow helps counter this tendency, thus enhancing dialysis efficacy. A high blood flow also reduces the risk of filter clogging. With SCUF, CVVH, and CVVHDF without predilution, there is a risk of hemoconcentration with high hematocrit (EVF), which can reduce filter lifespan. The filtration fraction (FF) is defined as the amount of fluid removed in relation to the amount of blood passing through the filter.

During dialysis treatment, anticoagulation is provided unless the patient is already at significant bleeding risk, to extend the filter’s lifespan. Typically, heparin is used. Anticoagulation is administered to prevent blood from clotting when it contacts plastic tubing and dialysis filters. Coagulation activation is also reduced by diluting the blood through predilution. Typically, heparin is administered in low concentrations, first as a bolus dose, followed by continuous infusion. A bolus dose usually consists of 10 IU/kg, followed by 10 IU/kg/hour. If the dialysis filter clots within 24 hours, the heparin dose may be increased to 15 IU/kg as a bolus, followed by 15 IU/kg/hour. The effect is monitored against PT-INR and APTT. The effectiveness of dialysis is limited by the filter’s lifespan. When the filter’s lifespan is restricted, dialysis settings can be adjusted, or the anticoagulation can be modified. In many cases, the patient’s dialysis catheter may need to be replaced or repositioned to ensure continuous dialysis function.

If filter time is less than 24 hours:

• Calculate “Filtration Fraction” (removal vs. flow)
• If FF < 20-30%: Increase predilution or blood flow

Adjust anticoagulation (heparin infusion)

• Administer Heparin 15 IU/kg bolus + increase infusion by 5 IU/kg
• Measure APTT (should be in the upper normal range)

Electrolyte content in dialysis bags

Citrate Dialysis

As an alternative to heparin in the dialysis machine, citrate can be infused as an anticoagulant. This method is called regional citrate anticoagulation (RCA). Citrate is infused into the blood circuit of the dialysis machine, where it binds calcium, making the blood more fluid and preventing it from coagulating in the filter. Calcium forms a complex with citrate, which is mainly excreted in the ultrafiltrate. Citrate consumes calcium, which must be compensated for during citrate dialysis through continuous intravenous calcium substitution. A certain amount of excess citrate may pass into the patient’s blood and be metabolized into bicarbonate. To prevent the patient from developing hypocalcemia, continuous calcium infusion is administered intravenously, typically via the same catheter used for dialysis. For this purpose, a triple-lumen CDK is often used. Alternatively, calcium can be substituted through another central venous catheter (CVC).

Citrate dialysis is effective even in cases of systemic impact and coagulation disorders, including thrombocytopenia. However, citrate dialysis is not suitable for patients with severe liver failure or shock with impaired microcirculation. Citrate dialysis tolerates significantly lower blood flow in the dialysis machine’s blood circuit compared to heparin infusion, with typical blood flow settings of 100-150 ml/min compared to 200-250 ml/min for heparin-based dialysis. Anticoagulation with citrate is generally very effective and provides longer filter times compared to heparin-based anticoagulation.

In citrate dialysis, there is a small risk of citrate accumulation, which may lead to hypocalcemia. This is managed by adjusting the citrate dose and calcium substitution. Typically, treatment starts with a citrate dose of 3.0 mmol/L in the dialysis circuit and a calcium compensation of 100%, administered into the patient’s blood via the CDK. Calcium compensation is provided with Calcium Chloride APL, 500 µmol/ml (20 mg/ml). Calcium levels are regularly measured in the dialysis circuit (Postfilter-Ca = Pf-Ca²⁺) and in the patient’s blood (S-Ca²⁺), where the Pf-Ca/S-Ca²⁺ ratio can indicate citrate accumulation. For patients showing signs of citrate accumulation after one day of treatment (increased need for intravenous calcium substitution, Pf-Ca/S-Ca²⁺ ratio > 2.4), the citrate dose is reduced in steps of 0.5 mmol/L.

Prescription for CRRT and RCA

1. Dialysis Catheter
   Triple-lumen dialysis catheter is used for CRRT with RCA. The third lumen is used for calcium infusion.
2. Modality
   Start with CVVHDF.
3. Priming
   Normally, CRRT is primed with heparin. In cases of suspected or confirmed heparin-induced thrombocytopenia (HIT), priming is done without heparin.
4. Citrate Concentration
   Initially, choose a citrate concentration of 3 mmol/L (“Citrate Dose” on PrisMAX). The dialysis machine maintains a constant citrate concentration in the extracorporeal circuit by adjusting the pump that delivers the citrate-containing solution (PBP pump).
5. Calcium Compensation
   Initially set to 100%. This means that 100% of calcium losses in the outflow are replaced. Infusion of CaCl is titrated to maintain ionized calcium in the blood at 1.0–1.3 mmol/L. The compensation can be adjusted between 25–200%. The dialysis machine calculates the flow rate and automatically adjusts when other flow rates change to maintain the degree of compensation.
6. Blood Flow and Dialysis Solutions
   Initially follow Table 1 (see below) and prescribe blood, dialysate, and replacement fluid flow post-filter based on the patient’s BMI-adjusted ideal weight. After prescribing the blood flow, the dialysis machine will set the pre-filter replacement fluid flow (PBP pump) with the citrate solution automatically to achieve the prescribed citrate concentration. When the blood flow changes, the PBP pump and calcium infusion rates are automatically adjusted to achieve the set citrate concentration and calcium compensation.

Citrate dialysis (normal settings)

Monitoring Calcium Substitution During Treatment

S-Ca²⁺ and Pf-Ca²⁺ are monitored 1 and 3 hours after the start of treatment, and then every 8 hours if the values remain stable. Pf-Ca²⁺ is measured to ensure optimal anticoagulation, and S-Ca²⁺ is checked to confirm that calcium substitution is sufficient.

Citrate dosing and calcium substitution in regional citrate anticoagulation

If S-Ca²⁺ is below 0.80 mmol/L, administer a bolus dose of Calcium Gluconate (10 mg/ml) 10 ml intravenously.

Metabolic Control

Metabolic corrections are primarily made by adjusting the dialysate flow, which is relatively low in buffer content.

Alkalosis: Increase the dialysate flow (Biphozyl) by 250 ml/hour for each desired change of 3 mmol/L in BE (Base Excess). Consider reducing the citrate dose so that Pf-Ca²⁺ is close to 0.5 mmol/L.

Acidosis: Decrease the dialysate flow by 250-500 ml/hour. Consider additional buffering with intravenous sodium bicarbonate. Also, check for potential citrate accumulation, indicated by a P-Ca/S-Ca²⁺ ratio greater than 2.4, and if confirmed, reduce the citrate dose in steps of 0.5 mmol/L.

CRRT

The goal of all renal replacement therapy (CRRT) is to remove excess waste products and water from the body due to kidney disease. The effectiveness of substance removal is usually expressed as clearance (K) of the substance in question. Impaired kidney function is reflected in a decrease in glomerular filtration rate (GFR). The volume of urine partly depends on the GFR, but also on the kidney’s ability to reabsorb water. If the GFR is zero, the urine volume will also be zero, resulting in anuria. Out of a normal GFR of about 150 ml/min (9 liters/hour), 99% is reabsorbed, and only 1% (about 90 ml/hour) is excreted as urine. Therefore, GFR can be significantly reduced without affecting urine volume if more than 1% of the GFR is excreted as urine.

GFR (the formation of primary urine) cannot be measured directly and must instead be calculated based on the kidney’s ability to excrete a model substance, i.e., the clearance (K) of the model substance. To provide a correct measure of GFR, such a model substance must be freely filtered along with water and should not be reabsorbed or secreted via the tubules.

K is defined in this context as the volume of plasma water completely cleared of a substance within a given time, typically expressed in ml/min. K can be calculated for any substance, but it only corresponds to GFR if it is freely filtered and not reabsorbed or secreted via the tubules. Creatinine is the endogenous substance most commonly used to estimate GFR.

Two healthy kidneys achieve a GFR of approximately 150 ml/min, which can be compared to continuous renal replacement therapy (CRRT), which at best is only about one-third as effective.

CRRT can remove water-soluble substances via two main principles (Figure 1):

1. Convection (hemofiltration) – where water (via ultrafiltration) and dissolved substances are transported across the dialysis filter from the bloodstream. The CRRT machine achieves this by creating a pressure gradient (transmembrane pressure) across the filter. Clearance (K) is determined by the ultrafiltration flow rate across the membrane.
2. Diffusion (hemodialysis) – substances
   
   (but not water) passively diffuse across the dialysis filter from the bloodstream (where the concentration of the substance is high) to the dialysate (where the concentration of the substance is low). Clearance is determined by the dialysate flow rate.

Figure 1. CRRT principles for filtration and dialysis. P_B hydrostatic pressure on the blood side. P_UF hydrostatic pressure on the ultrafiltrate side. P_D hydrostatic pressure on the dialysate side.

The CRRT Circuit

Modern CRRT machines, in addition to the blood pump, have pumps that control the dialysate, replacement fluid (before and after the filter), and ultrafiltrate. The flow in the blood pump, the pre-filter replacement fluid pump (predilution pump), the post-filter replacement fluid pump (postdilution pump), and the dialysate pump can be controlled separately within fairly wide ranges. The flow in the effluent pump (which removes the ultrafiltrate) cannot be manually set but is calculated by the machine to maintain the appropriate pressure difference (TMP) across the filter membrane. The flow in the effluent pump is the sum of the predilution pump, postdilution pump, and the desired fluid removal per hour. All pumps are integrated and connected to alarm functions. If an alarm is triggered in one pump, all pumps (except the blood pump) will stop simultaneously, reducing the risk of problems throughout the circuit.

By controlling the different pumps, continuous renal replacement therapy can be performed as ultrafiltration (CVVH), dialysis (CVVHD), or a combination of both (CVVHDF). Below is a description of the CRRT circuit for the various modalities.

SCUF – Slow Continuous Ultrafiltration

In the simplest form of CRRT, the active flow circuit consists only of the blood pump and the effluent pump (Figure 2). Blood is pumped from the patient using the blood pump into the filter. This creates a positive hydrostatic pressure on the blood side of the filter that slightly exceeds the pressure on the ultrafiltrate side. This pressure difference across the membrane is called transmembrane pressure (TMP). TMP allows a small amount of plasma water (the so-called ultrafiltrate) to be “pressed” from the blood side to the ultrafiltrate side. By using another pump (the effluent pump) that actively “sucks” the ultrafiltrate out of the filter, a negative hydrostatic pressure is created on the ultrafiltrate side. This significantly increases the pressure difference (TMP) across the membrane and increases the convective ultrafiltration. By reducing or increasing TMP, the amount of ultrafiltrate can be precisely regulated.

The set flow in the effluent pump corresponds to the volume of plasma water removed from the patient’s bloodstream per unit of time. This CRRT modality is called SCUF (slow continuous ultrafiltration).

Figure 2. SCUF – Slow Continuous Ultrafiltration

The goal of SCUF treatment is only to remove the patient’s excess water. Keep in mind that in the plasma water transported across the membrane, there are also dissolved molecules, such as creatinine and urea. The clearance of creatinine or urea achieved by the machine in this modality is determined entirely by the flow rate in the effluent pump, i.e., the fluid removal.

If the effluent pump flow rate is 200 ml/h, the convective ultrafiltration flow across the membrane will be 200 ml/h. Clearance (K) for molecules that pass freely through the membrane (such as creatinine and urea) and are thus dissolved in the ultrafiltrate will be: K = Ultrafiltration flow = 200 ml/h, which can also be expressed as 3.3 ml/min. With this setting, we can efficiently remove excess water. However, this is not an effective form of renal replacement therapy.

• K (SCUF) = Ultrafiltration flow

Can we increase fluid removal to increase the clearance of various molecules? Yes, the CRRT machine can handle this, but the patient’s circulation is the limiting factor. Excessive fluid removal would either dehydrate the patient or require large volumes of intravenous fluid to be administered.

How can we remove an adequate amount of water while achieving high clearance? By administering replacement fluid either before (predilution) or after (postdilution) the filter, we can increase the ultrafiltration flow across the membrane without dehydrating the patient.

CVVH – Continuous Veno Venous Hemofiltration

CVVH with Postdilution

In CVVH with postdilution, all replacement fluid is administered after the filter. The postdilution flow is driven by a separate pump (Figure 3).

Figure 3. CVVH with Postdilution

The purpose of the replacement fluid is to maintain an adequate fluid balance while allowing a large volume of ultrafiltrate to be removed from the patient. If the goal is not to dehydrate the patient, the ultrafiltrate flow (the speed of the effluent pump) and the replacement flow (the speed of the postdilution pump) should be equal. If negative fluid balance is desired, the ultrafiltrate flow must be greater than the replacement flow.

• Ultrafiltrate flow = Replacement flow + Fluid removal

For example, if the postdilution pump is set to 3800 ml/h and fluid removal is set to 200 ml/h, the machine will automatically calculate the effluent pump speed to be 3800 ml/h + 200 ml/h = 4000 ml/h. What will the clearance (K) for creatinine and urea be in this example?

Creatinine and urea are small molecules that pass freely through the membrane along with the water. K for these molecules will therefore be equal to the ultrafiltrate flow, which is 4000 ml/h. This can be expressed as 67 ml/min.

• K (CVVHpostdilution) = Ultrafiltrate flow = Replacement flow + Fluid removal

This type of treatment has limitations. The high plasma water flow across the membrane from the blood side to the ultrafiltrate side will significantly increase the hematocrit (Hct) on the blood side, increasing the risk of filter clotting. Figure 4 illustrates how Hct changes on the blood side of the filter during CVVH with postdilution and an ultrafiltrate flow of 4 l/h when the blood flow is set to 200 ml/min (=12,000 ml/h) and the patient’s Hct is 0.3. Hct rises to 0.45 at the end of the filter and returns to 0.31 after the filter (where the replacement fluid is administered).

Figure 4. Hematocrit in the filter during CVVH with Postdilution

One way to assess the risk of filter clotting during CVVH with postdilution is to calculate the filtration fraction (FF). FF indicates the proportion of plasma water removed from the blood during hemofiltration and can be calculated simply as: FF = ultrafiltrate flow/blood flow. The optimal FF for a patient with Hct=0.3 is under 20-25%. At an FF above 30%, the risk of filter clotting increases significantly. Note that the FF calculation does not account for the patient’s Hct. A high FF does not necessarily lead to filter clotting if the patient’s Hct is very low when the blood reaches the filter. Conversely, a low FF can lead to filter clotting if the patient’s Hct is high. However, it is common for ICU patients to have Hct levels between 0.3 and 0.4. In this range, FF provides a good indication of the risk of clotting.

• FF = Ultrafiltrate flow/Blood flow

One way to mitigate the issue of high FF is to administer the replacement fluid before the filter (predilution).

CVVH with Predilution

In CVVH with predilution, all replacement fluid is administered before the filter. The predilution flow is driven by a separate pump (Figure 5).

Figure 5. CVVH with Predilution

The blood entering the filter will be diluted by the replacement fluid. Therefore, the blood entering the filter will have a lower hematocrit compared to the patient’s actual hematocrit.

Figure 6 illustrates how hematocrit changes on the blood side of the filter during CVVH with predilution and an ultrafiltration flow of 4 l/h when the blood flow is set to 200 ml/min (=12,000 ml/h) and the patient’s hematocrit is 0.3. Although hematocrit rises just as much in the filter as it does during CVVH with postdilution, the hematocrit at the end of the filter only reaches 0.31 because the blood entering the filter has been diluted.

Figure 6. Hematocrit in the Filter During CVVH with Predilution

Even CVVH with predilution has its limitations. The waste products present in the blood, which we aim to remove through renal replacement therapy, are diluted by the replacement fluid. The composition of the ultrafiltrate during CVVH with postdilution consists of the patient’s plasma water and the dissolved molecules. In CVVH with predilution, the ultrafiltrate consists of the patient’s plasma water mixed with replacement fluid and dissolved molecules. Since the concentration of dissolved molecules (such as waste products) is lower, the clearance (K) of these molecules will be less in this modality compared to when the replacement fluid is administered after the filter.

To calculate clearance in CVVH with predilution, the dilution factor must be considered. The easiest way to do this is by calculating the dilution factor.

• Dilution Factor = Blood Flow / (Blood Flow + Replacement Flow before the Filter)

In the example illustrated in Figure 6 (Hematocrit in the Filter During CVVH with Predilution), the dilution factor is = 12,000 / (12,000 + 3,800) = 0.76. Clearance (K) is then Ultrafiltrate Flow × Dilution Factor, i.e., 4,000 ml/h × 0.76 ≈ 3,000 ml/h.

• K (CVVH predilution) = Ultrafiltrate Flow × Dilution Factor = (Replacement Flow + Fluid Removal) × Dilution Factor

Compare Figure 4 and Figure 6. Although the same flows were used in both examples, clearance is 1,000 ml/h lower simply by administering all replacement fluid before the filter rather than after the filter.

CVVHD – Continuous Veno Venous Hemodialysis

During pure CVVHD, molecules are removed from the blood side via diffusion, meaning molecules but (in principle) no water pass through the filter from the blood side to the dialysate side. The dialysate flow is driven by a separate dialysate pump, and the dialysate flow rate determines clearance (K), i.e., how efficiently molecules are removed from the blood. Water can also be removed (through ultrafiltration) in this modality, but in that case, the effluent pump must operate at a higher speed than the dialysate pump. This is achieved by setting the desired fluid removal in the machine (Figure 7).

Figure 7. CVVHD

• 
  

  K (CVVHD) = Dialysate Flow

  
  

CVVHDF – Continuous Veno Venous Hemodiafiltration

During CVVHDF, both dialysis and filtration are combined (Figure 8). The total clearance (K) is easiest to calculate by first determining K for the individual modalities and then adding them together.

Figure 8. CVVHDF

General Indications for CRRT

• Urea > 40 mmol/l
• Therapy-resistant hyperkalemia
• Pulmonary edema with FiO₂ > 0.8 despite diuretics and PEEP (up to 10 cm H₂O)

If Filter Time < 24 Hours

• Calculate “Filtration Fraction” (removal vs. flow)
• If FF < 20-30%: Increase predilution or blood flow

Adjust Anticoagulation (Heparin Infusion) if Necessary

• Administer Heparin 15 IU/kg bolus + increase infusion by 5 IU/kg
• Measure APTT (which should be in the upper normal range)

Let’s illustrate this with three examples. In the first example (1), we calculate K during combined CVVHD and CVVH with postdilution. In the second example (2), we calculate K during combined CVVHD and CVVH with predilution. Finally (3), we calculate K during combined CVVHD and CVVH with both pre- and postdilution.

In all examples, we assume a blood flow of 200 ml/min (=12,000 ml/h) and a fluid removal of 100 ml/h.

(1) CVVHDF with Dialysate Flow = 1,000 ml/h and Postdilution Flow = 1,000 ml/h

K (CVVHD) = Dialysate Flow = 1,000 ml/h
K (CVVH postdilution) = Ultrafiltrate Flow = Replacement Flow + Fluid Removal = 1,000 + 100 = 1,100 ml/h

Now we simply sum the contribution from the different modalities:
K (CVVHDF) = K (CVVHD) + K (CVVH postdilution) = 1,000 + 1,100 = 2,100 ml/h, which equals 35 ml/min.

(2) CVVHDF with Dialysate Flow = 1,000 ml/h and Predilution Flow = 1,000 ml/h

K (CVVHD) = Dialysate Flow = 1,000 ml/h
K (CVVH predilution) = Ultrafiltrate Flow = Replacement Flow + Fluid Removal = 1,000 + 100 = 1,100 ml/h

We sum the contribution from the different modalities and get:
K (CVVHDF) = K (CVVHD) + K (CVVH predilution) + K (Fluid Removal) = 1,000 + 1,000 + 100 = 2,100 ml/h

In this example, we must account for the dilution factor. Since the replacement fluid is administered before the filter, all substances present in the blood (including those we wish to remove) will be diluted. This reduces the clearance (K) achieved from both dialysis and filtration.

Dilution factor = Blood Flow / (Blood Flow + Replacement Flow before the filter) = 12,000 / (12,000 + 1,000) = 0.92

The dilution factor tells us that by administering replacement fluid before the filter, we will only achieve 92% of the K that we would otherwise achieve if all replacement fluid was given after the filter.

K (CVVHDF) = 2,100 × 0.92 = 1,932 ml/h or 32 ml/min

(3) CVVHDF with Dialysate Flow = 1,000 ml/h, Predilution Flow = 1,000 ml/h, and Postdilution Flow = 1,000 ml/h

K (CVVHD) = Dialysate Flow = 1,000 ml/h
K (CVVH predilution) = Ultrafiltrate Flow = Replacement Flow before the filter = 1,000 ml/h
K (CVVH postdilution) = Ultrafiltrate Flow = Replacement Flow after the filter + Fluid Removal = 1,000 + 100 = 1,100 ml/h

Note that fluid removal is only counted once. In this example, we chose to add it to the replacement flow after the filter, but we could just as easily have added it to the replacement flow before the filter—the result is the same. Let’s now add the contribution from the different modalities:

K (CVVHDF) = K (CVVHD) + K (CVVH predilution) + K (CVVH postdilution) = 1,000 + 1,000 + 1,100 = 3,100 ml/h

Here, we must again account for the dilution factor. It is still only the replacement fluid administered before the filter (predilution) that dilutes the blood, so the dilution factor remains the same as in example (2), namely 0.92.

K (CVVHDF) = 3,100 × 0.92 = 2,852 ml/h or 48 ml/min

• K (CVVHDF) = (K (CVVHD) + K (CVVH predilution) + K (CVVH postdilution)) × dilution factor

Written by Johan Mårtensson and Claes-Roland Martling, Karolinska University Hospital.

CRRT Compendium

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Continuous Veno-Venous Dialysis (CVVHF)

Continuous veno-venous hemofiltration (CVVHF) can be divided into CVVHD or CVVHD-F, commonly referred to as Prismadialysis. Prismadialysis provides continuous ultrafiltration of the blood with or without dialysis. This method removes waste products and fluid from the bloodstream. Water molecules are pushed through a semipermeable membrane by hydrostatic force, with lower pressure on the dialysate side.

Indications: Acute or chronic kidney failure with uremia. Fluid overload in sepsis and other major surgeries. Poisoning by lithium and certain other toxins.

Complications: Hypotension, hypovolemia, muscle cramps, nausea, vomiting, bleeding, headache, infection, hypersensitivity, hypoxemia, amyloidosis.

Technique: A central dialysis catheter (CDK) with double lumens is typically placed as a central venous catheter (CVC) in a central vein, normally in the right internal jugular vein or the subclavian vein. An X-ray is essential to confirm catheter placement before use. Treatment is continuous, around the clock, for 2-3 weeks. Typical fluid removal is 0.5-2 liters per day, depending on the patient’s fluid balance status. Typical blood flow is 150-250 ml/min, and dialysate flow is 500-1,500 ml/h. Regular weight checks and monitoring of uremia and potassium levels are crucial. Ultrafiltration is possible with Prismadialysis, providing quick relief from pulmonary edema.

Hemodialysis (HD)

Hemodialysis involves blood purification through osmosis and the removal of molecules (such as urea and potassium) and fluid across a semipermeable membrane extracorporeally (outside the body, unlike peritoneal dialysis).

Indications

Acute or chronic kidney failure with uremia. Fluid overload. Poisoning by methanol, ethylene glycol, lithium, and certain other toxins.

Procedure

Hemodialysis is usually performed by filtering blood through a dialysis filter with a surface area of around 2 m² extracorporeally, where blood is drawn out of the body, purified, and returned. Blood is typically removed via a large vascular catheter, fistula, or shunt. A typical blood flow (Qb) in the dialysis filter is around 150 ml/min, and a normal dialysate flow (Qd) is around 500 ml/min. Normal fluid removal is 2-4 liters per dialysis treatment, depending on the patient’s hemodynamic stability, uremic status, and fluid balance. The treatment is usually performed by inserting a needle into a surgically created arteriovenous (AV) fistula on the upper or lower arm. An AV fistula takes 4-8 weeks to mature. A graft may be used if the patient has thin or fragile vessels (though this increases the risk of thrombosis and infection). A central dialysis catheter (CDK) with double lumens may be used acutely if vascular access is lacking, or two catheters can be used—one arterial catheter and one venous catheter, usually in the femoral artery and femoral vein, or two venous catheters. The treatment is performed intermittently 3-4 days per week for 2-4 hours per session. Regular weight checks and monitoring of urea and potassium are important. Anticoagulation is typically administered during treatment.

Ultrafiltration (UF) is possible. UF removes only fluid from the bloodstream and provides the fastest relief during the treatment of pulmonary edema.

Complications of hemodialysis include hypotension, cramps, nausea, vomiting, access problems, amyloidosis, and bleeding complications.

Peritoneal Dialysis (PD)

Peritoneal dialysis (PD) is a dialysis treatment typically performed via a Tenckhoff catheter through the abdominal wall (PD catheter). PD works by removing waste products and fluid through the peritoneum (the lining of the abdominal cavity), which is flushed out with dialysis fluid. The peritoneum acts as the dialysis filter. PD is often used initially in the treatment of chronic kidney failure. The treatment duration is typically up to four years. It is a good option for patients with heart failure (as it provides a more stable blood volume) and for diabetics (as it allows for continuous insulin administration). Inflammatory bowel disease is a contraindication. PD can be performed at home or at outpatient clinics. Known complications of PD include peritonitis and amyloidosis (beta-2 microglobulin). The treatment is usually performed intermittently 3-4 days per week, 2-4 hours per session. Typical fluid removal is 2-4 liters depending on the patient’s hemodynamic stability and current fluid balance. Regular weight checks and monitoring of uremia and potassium levels are important. The primary complications of peritoneal dialysis are related to infection, especially peritonitis, with or without systemic involvement.

Plasmapheresis

Plasmapheresis is a treatment that removes venous whole blood and returns everything except plasma. Normally, it does not remove fluid. Plasma is replaced with donor plasma or albumin solution, typically albumin solution. Antibodies, immune complexes, and pathological proteins are removed. Plasmapheresis is used in conditions such as thrombotic thrombocytopenic purpura (TTP), hemolytic uremic syndrome (HUS), systemic lupus erythematosus (SLE), Wegener’s granulomatosis, Goodpasture’s syndrome, Guillain-Barré syndrome, and myasthenia gravis. A central dialysis catheter (CDK) with double lumens may be used acutely if vascular access is lacking, or an arterial catheter, usually in the femoral artery, and a venous catheter in the femoral vein. The treatment is performed intermittently, three days per week, for 2-4 hours per session. Regular weight checks and monitoring of uremia and potassium levels are important.

MARS (Molecular Adsorbent Recirculating System)

MARS is a dialysis method that removes whole blood, purifies the blood, and returns everything except plasma. It does not usually remove fluid. Plasma is replaced with an albumin solution. The blood is filtered through a carbon filter and ion exchanger, removing antibodies, immune complexes, and pathological proteins. MARS can be used in cases of acute liver failure with encephalopathy and in acute acetaminophen poisoning with liver failure. The system requires a central dialysis catheter (CDK) with double lumens, which can be used acutely if vascular access is lacking, or an arterial catheter, usually in the femoral artery, and a venous catheter in the femoral vein. Treatment is carried out over 2-3 consecutive days, 4-8 hours per session. Regular monitoring of uremia and liver function is necessary. There is a risk of bleeding complications. The system is resource-intensive and is typically used in intensive care units.