Introduction

Various renal replacement therapies (RRTs) are available for managing severe acute kidney injury (AKI), including intermittent hemodialysis (IHD), continuous renal replacement therapy (CRRT), and prolonged intermittent RRT. Decisions about technique are dictated by the dialysis indication, clinician preference, outcome data, and, most importantly, hemodynamic status.[1] A 2015 multinational cross-sectional epidemiological study of patients with AKI in intensive care units (ICUs) revealed that CRRT was the preferred treatment modality in 75.2% of sessions, compared to intermittent dialysis in 24.1% of sessions and peritoneal dialysis in 0.7% of sessions.[2]

CRRT comprises techniques that manage solute removal and fluid balance over 24 hours. CRRT filters blood through a semipermeable membrane using various solute transport mechanisms. The specific mechanism defines each CRRT type. The 3 CRRT techniques are continuous venovenous hemofiltration (CVVH), continuous venovenous hemodialysis (CVVHD), and continuous venovenous hemodiafiltration (CVVHDF).

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Anatomy and Physiology

Vascular Access for CRRT

CRRT vascular access employs a dual-lumen catheter placed in either the internal jugular or femoral vein. A nontunneled dual-lumen catheter may be placed if CRRT is only needed in the short term. However, prolonged CRRT use requires either a cuffed or noncuffed tunneled dual-lumen catheter. These vascular access types allow a blood flow rate of 200 to 250 mL/min. Previously established access, such as arteriovenous fistulae or grafts, should not be utilized for CRRT due to possible graft dislodgement or injury.

The right internal jugular vein is the most preferred option for direct access to the superior vena cava as it ensures optimal blood flow.[3] The left internal jugular or femoral veins may be considered in patients with obesity, though these vessels are less favored due to potential blood flow impairment with positional changes. Femoral vein use was traditionally reserved for patients with anatomical internal jugular vein obstruction, eg, stenosis or thrombosis. However, the femoral veins are also currently widely used for CRRT vascular access in patients without obesity. Using the subclavian vein for CRRT vascular access is discouraged because it poses a risk of stenosis, which can lead to difficulties using downstream vessels for future arteriovenous fistulae or grafts.

Patient height and the insertion site dictate the necessary length of the vascular catheter. Right-sided internal jugular catheters typically range from 12 to 15 cm, while left-sided internal jugular catheters are 20 to 24 cm long. Femoral catheters are at least 24 cm in length. Nontunneled catheter placement is typically performed at the bedside under aseptic conditions using ultrasound guidance per the Kidney Disease Improving Global Outcomes (KDIGO) guidelines. Tunneled catheters are placed in the angiography suite with venous cannulation using ultrasound guidance and catheter tip positioning via fluoroscopic guidance. Confirming catheter tip positioning in the superior vena cava via chest radiography is typically required before using nontunneled catheters. The ideal catheter tip location is close to or within the inferior vena cava if a femoral vein is used for vascular access. A higher chance of restricted blood flow, circuit flow interruptions, and filter clotting is anticipated if the catheter is malpositioned.[4]

Principles of CRRT

CRRT aims to mimic the kidney's countercurrent mechanism to achieve effective solute and plasma water transport. Solute transport is achieved either by diffusion, convection, or both. Slow blood flow mechanisms ensure plasma water and effluent equilibration. Diffusion is the primary solute transport method for substances with a molecular weight of less than 1000 Da. Convective transport permits the movement of intermediate molecular weight molecules via high hydrostatic pressure. Convective transport filters molecules with a maximum weight of approximately 15,000 Da.

In contrast to IHD, CRRT dialysate flow rates are far lower than blood flow rates, with an average difference of 15 to 30 mL/min. Small solutes in the plasma water can equilibrate with the effluent at this flow rate, effectively making solute clearance equal to the effluent fluid rate (Qef) or the final output postfiltration. This value comprises the net ultrafiltration (Qnet) plus the replacement fluid rate (QRF) in CVVH. The dialysate flow rate is also considered in CVVHD and CVVHDF.

Effluent

The primary goal of CVVH is to regulate the total ultrafiltrate, which comprises both the replacement and net fluid removed from the patient's bloodstream (see Image. Continuous Venovenous Hemofiltration Circuit). Replacement fluid is administered to maintain proper fluid balance, while net fluid removal represents the volume of fluid extracted during filtration to alleviate fluid overload or remove waste products.

CVVHD involves the utilization of spent dialysate, the fluid that has passed through the dialyzer, along with net fluid removal. This process aids in removing waste substances and excess fluids from the blood (see Image. Continuous Venovenous Hemodialysis Circuit). Conversely, CVVHDF integrates both spent dialysate and total ultrafiltrate. Spent dialysate aids solute removal, while total ultrafiltrate accounts for the combined volume of replacement fluid and net fluid removed, ensuring comprehensive management of fluid and solute balance.

General clearance

The general clearance (K) equation quantitatively measures how efficiently CRRT removes solutes from the blood based on the effluent flow rate and solute concentration gradient between the effluent and the blood. The equation is as follows:

K= QE x CE/CB, where CE/CB is the sieving coefficient, S

where 

"K" represents the clearance or the rate at which solutes are removed from the blood by the CRRT process. K is measured in milliliters per minute and indicates how efficiently the therapy removes solutes.

"QE" refers to the effluent flow rate, which is the rate at which the dialyzed or filtrate fluid is removed from the patient's blood during CRRT. This quantity is typically measured in milliliters per minute and represents the volume of fluid processed by the CRRT machine.

"CE" represents the concentration of solutes in the effluent fluid, which is the fluid that has been filtered or dialyzed and removed from the patient's bloodstream. CE is measured in concentration units (eg, milligrams per deciliter).

"CB" represents the concentration of solutes remaining in the patient's bloodstream after passing through the CRRT circuit. CB is also measured in concentration units (eg, milligrams per deciliter).

A solute with an S of 1 can pass freely through a filter. If S is 0, the solute cannot pass through the filter.[5][6] This factor and the difference in hydrostatic pressure between the dialysate and blood compartments determines the amount of solute that convection will remove. The amount of solute that diffuses through the membrane depends on its concentration gradient, the solute's molecular size, and the membrane's thickness, surface area, and pore size.