SLED Dialysis: Key Insights and Practical Tips

Sustained low-efficiency dialysis (SLED) is a hybrid renal replacement therapy used primarily in critically ill patients with acute kidney injury. Combining elements of intermittent hemodialysis and continuous renal replacement therapy, it provides an effective yet flexible approach to fluid and solute management in unstable patients.

Its increasing use in intensive care units underscores the need for a thorough understanding of its mechanics, terminology, and practical considerations.

Basics Of Therapy Mechanics

SLED operates with prolonged treatment durations and lower blood and dialysate flow rates, striking a balance between intermittent hemodialysis (IHD) and continuous renal replacement therapy (CRRT). This method enables gradual solute clearance and fluid removal, reducing hemodynamic instability in critically ill patients. Unlike IHD, which lasts three to four hours with high-intensity clearance, SLED extends to 6–12 hours, minimizing abrupt osmotic shifts that could compromise cardiovascular function.

Solute transport in SLED relies on diffusion and convection. Diffusion, driven by concentration gradients, removes small molecules like urea and creatinine, while convection, enhanced by ultrafiltration, aids in clearing middle-sized molecules. Dialysate flow rates and membrane permeability influence the balance between these processes, allowing adjustments to optimize clearance while maintaining stability. Studies indicate that SLED can achieve urea clearance rates comparable to CRRT when configured properly, making it a viable alternative in settings where continuous therapy is impractical.

Temperature control is critical, as prolonged sessions can lead to heat loss and potential hypothermia. Modern dialysis machines incorporate temperature regulation, but clinical monitoring remains necessary, particularly in septic patients or those prone to thermal dysregulation. Anticoagulation strategies must also be carefully managed to prevent clotting while minimizing bleeding risks. Regional citrate anticoagulation is increasingly favored due to its ability to limit systemic anticoagulant exposure, though it requires close monitoring of ionized calcium levels to prevent metabolic complications.

Key Terminology

A clear understanding of SLED terminology is essential for optimizing treatment and ensuring patient safety. Key parameters influencing efficacy include blood flow rate, dialysate composition, and ultrafiltration rate.

Blood Flow Rate

Blood flow rate (Qb) refers to the volume of blood passing through the dialysis circuit per unit of time, typically measured in milliliters per minute (mL/min). In SLED, Qb is generally set between 150 and 250 mL/min, lower than the 300–500 mL/min used in IHD, to reduce hemodynamic fluctuations in critically ill patients.

Higher Qb enhances diffusion by increasing the concentration gradient across the dialysis membrane, but excessively high rates can cause vascular access complications like recirculation or dysfunction, particularly in patients with central venous catheters. A Nephrology Dialysis Transplantation (2021) study found that maintaining Qb within the recommended range optimizes urea clearance while minimizing intradialytic hypotension risk. Patient-specific factors, such as vascular access type and cardiac output, must also be considered when determining Qb.

Dialysate Composition

Dialysate composition is crucial for maintaining electrolyte balance and acid-base homeostasis during SLED. It typically includes sodium, potassium, calcium, magnesium, chloride, bicarbonate, and glucose, with concentrations tailored to the patient’s metabolic needs. Unlike CRRT, where dialysate is continuously adjusted, SLED often uses fixed formulations similar to IHD, though modifications may be necessary based on lab values.

Sodium concentrations range from 135 to 145 mmol/L to prevent osmotic shifts that could cause neurological complications. Potassium levels are adjusted based on pre-dialysis serum potassium, with standard dialysate containing 2–4 mmol/L to prevent rapid shifts that may trigger arrhythmias. Bicarbonate, typically 30–35 mmol/L, corrects metabolic acidosis, a common issue in acute kidney injury. A 2022 Kidney International Reports review emphasized the importance of individualized dialysate prescriptions in SLED, particularly for patients with sepsis or multi-organ dysfunction, where electrolyte disturbances are common.

Ultrafiltration Rate

Ultrafiltration rate (UFR) refers to the volume of fluid removed per hour during dialysis, measured in milliliters per hour (mL/h). In SLED, UFR is generally set between 100 and 500 mL/h, depending on the patient’s fluid status and hemodynamic tolerance. This gradual removal reduces the likelihood of intradialytic hypotension compared to the more aggressive rates used in IHD.

Determining UFR requires assessing fluid balance, including cumulative intake, urine output, and extravascular fluid shifts. Excessive ultrafiltration can lead to hypovolemia, causing hypotension, tachycardia, and reduced organ perfusion. Conversely, inadequate removal may result in volume overload, exacerbating pulmonary edema and hypertension. A Critical Care Medicine (2023) study found that individualized UFR adjustments based on dynamic hemodynamic monitoring improved patient outcomes in SLED, highlighting the need for continuous reassessment.

Duration And Frequency

SLED scheduling balances metabolic demands with healthcare logistics. Unlike IHD, which follows a fixed regimen of three to four sessions per week, SLED allows greater flexibility in duration and frequency. Sessions typically last 6–12 hours, enabling gradual solute and fluid removal, which benefits critically ill patients who cannot tolerate rapid volume shifts.

Frequency varies based on clinical needs, with some patients requiring daily treatments while others benefit from an every-other-day approach. Factors such as fluid overload, electrolyte imbalances, and metabolic waste accumulation dictate treatment frequency. A Critical Care (2022) study found that patients with sepsis-induced acute kidney injury had better fluid management and metabolic control when SLED was performed at least five times per week.

Session length influences toxin clearance and acid-base regulation. Extending SLED beyond eight hours enhances middle-sized molecule removal, which can mitigate systemic inflammation in acute kidney injury. However, prolonged sessions may increase the risk of catheter-related infections and patient discomfort. Clinicians must weigh the benefits of extended treatment against potential complications, adjusting session length based on patient response.

Equipment Setup

Proper equipment configuration is essential for effective SLED. The dialysis machine must support prolonged treatments while maintaining stable flow rates and precise ultrafiltration control. Machines designed for IHD, such as the Fresenius 2008K or Baxter Prismaflex, can be adapted for SLED by adjusting flow settings and treatment parameters. Ensuring compatibility with SLED protocols is crucial, as not all machines are designed for extended therapy sessions.

Dialyzer membrane selection significantly impacts solute clearance and hemodynamic tolerance. High-flux membranes, typically made from synthetic materials like polyethersulfone or polysulfone, are preferred for their superior permeability and biocompatibility. These membranes facilitate middle-sized molecule removal while minimizing inflammatory responses. The dialyzer’s surface area must also be considered, as larger surface areas enhance clearance but may increase protein loss.

Vascular access is critical, with central venous catheters (CVCs) being the most common due to their ease of placement and suitability for critically ill patients. Optimal catheter positioning—typically in the internal jugular or femoral vein—ensures adequate blood flow while minimizing complications such as recirculation or thrombosis. Catheter patency must be maintained with appropriate anticoagulation strategies, as clot formation can disrupt treatment and increase dysfunction risk. Regular assessment of access function, including flow rate monitoring and pressure readings, is essential to prevent complications.

Pharmacokinetic Factors

Medication management in SLED requires understanding how therapy affects drug absorption, distribution, metabolism, and excretion. Since SLED operates with lower flow rates and extended durations compared to IHD, drug clearance varies significantly.

Drug removal depends on molecular weight, protein binding, and volume of distribution. Low molecular weight compounds, such as aminoglycosides, are efficiently cleared, whereas larger molecules like vancomycin exhibit variable removal depending on dialyzer characteristics. Highly protein-bound drugs, such as phenytoin, are less affected by dialysis, as only the unbound fraction is subject to clearance. The volume of distribution also plays a significant role—medications with a large volume of distribution, such as digoxin or lipophilic sedatives, are minimally removed, whereas hydrophilic drugs with a lower volume of distribution are more susceptible to dialysis-related clearance.