Sustained Low-Efficiency Dialysis (SLED) — The Hybrid Modality

Overview & definition

The sustained low-efficiency dialysis (SLED) — also called the extended daily dialysis (EDD) — is a hybrid renal replacement therapy that combines the features of the intermittent haemodialysis (IHD) and the continuous renal replacement therapy (CRRT). A standard haemodialysis machine is run slowly: the low blood and the dialysate flow rates over the extended duration, the daily. The result is the gentle, the efficient clearance with the better haemodynamic tolerance than the IHD, but the less intensive and the cheaper than the CRRT.[1][4]

The prescription

[1]

• The blood flow — the 150 to 200 mL/min (the lower than the standard IHD).[1]
• The dialysate flow — the 100 to 300 mL/min (the lower than the standard IHD).[1]
• The duration — the 6 to 8 hours (up to 12 hours), the daily (or the alternate days).[1]
• The mechanism — the primarily diffusion-based (like the IHD), but the slow and the extended → the gentle clearance.[1]
• The anticoagulation — the less than the CRRT (the shorter session); the heparin, the regional citrate, or none.[1]
• The fluid removal — the slow, the titrated to the haemodynamic tolerance.[1]

The advantages

Vs the IHD

• The better haemodynamic tolerance — the slower, the extended fluid and solute removal causes the less hypotension (the risk for the haemodynamically marginal patient).[1][2]
• The less dialysis disequilibrium — the slower solute shifts reduce the cerebral-oedema risk.[1]
• The better solute clearance per session — the longer session improves the clearance over the standard IHD.[1]

Vs the CRRT

• The uses the standard IHD machine — the cheaper, the more available (the CRRT machines are the scarce in the resource-limited settings).[1][3]
• The less anticoagulation — the shorter session reduces the anticoagulation exposure and the bleeding risk.[1]
• The allows the down-time — the intermittent sessions let the patient mobilise, the attend the procedures, and the imaging (the continuous CRRT limits the mobility).[1]
• The less staff-intensive — the standard machine, the less circuit troubleshooting.[1]
• The less hypothermia and the nutrient loss than the CRRT (the shorter circuit time).[1]

The disadvantages

• The intermittent — not the truly continuous; the solute and the fluid accumulate between the sessions (the gentler than the IHD but the less steady than the CRRT).[1]
• The less middle-molecule clearance than the CVVH (the diffusion-based, not the convective).[1]
• The may need the anticoagulation (the heparin), with the bleeding risk (the less than the CRRT).[1]

The indications

• The AKI in the haemodynamically marginal patient — the better tolerated than the IHD, the less intensive than the CRRT. The ideal middle-ground for the moderately unstable patient.[1]
• The bridge from the CRRT to the IHD — the transitional modality as the patient stabilises and the renal function recovers.[1]
• The resource-limited setting — the CRRT machines scarce; the SLED uses the standard IHD machine.[1][3]
• The patient who needs the down-time — the procedures, the imaging, the mobilisation, the rehabilitation (the intermittent sessions allow this).[1]

The outcomes

The SLED provides the comparable solute control and the haemodynamic stability to the CRRT in the most studies, with the no clear mortality difference. The choice between the SLED, the CRRT, and the IHD depends on the haemodynamic stability, the resources, the anticoagulation needs, and the patient factors (the mobility, the procedures).[1][1][11]

[1]

Red flags

SLED principles — why the hybrid works

The SLED is not merely a dialysis prescription; it is a deliberate physiological compromise engineered to occupy the favourable middle ground between IHD and CRRT. To justify every prescription parameter in a viva, you must understand the physics that links flow rates, duration, and clearance to the clinical effects of haemodynamic stability and adequate solute removal.[4]

The clearance equation — diffusion and the role of Qd/Qb

Diffusive clearance in a haemodialyser obeys the mass-transfer relationship $K = K_0 A \times f(Q_b, Q_d)$, where $K_0 A$ is the membrane mass-transfer-area coefficient (a property of the dialyser — surface area and permeability), and $f(Q_b, Q_d)$ is a function that saturates as the smaller of blood flow and dialysate flow is reached. In standard IHD, $Q_b$ is 300–400 mL/min and $Q_d$ is 500–800 mL/min, so clearance of small solutes (urea) is flow-limited and high — a 4-hour session clears the urea pool efficiently but violently. [1]

In SLED, both flows are deliberately throttled: $Q_b$ 150–200 mL/min, $Q_d$ 100–300 mL/min. Because $Q_d$ is now much lower than $Q_b$, clearance becomes dialysate-limited (the dialysate leaving the filter is nearly fully saturated with urea), and the instantaneous clearance is modest (roughly the dialysate flow rate, 100–300 mL/min). The trick is to run this modest clearance for a long time (6–12 h): total solute removed = clearance × time, so the long session restores the total clearance lost by throttling the flows. The patient gets the same delivered Kt/V as IHD but delivered gently.[3][4]

Why slow = haemodynamically stable

The haemodynamic instability of standard IHD has three roots, all of which SLED mitigates by running slowly:[2]

1. Rate of volume removal (ultrafiltration). To remove 3 L of fluid over a 4-hour IHD session requires ~750 mL/h net ultrafiltration. At this rate, plasma refilling from the interstitium cannot keep up → intravascular volume depletion → hypotension. SLED removes the same 3 L over 8–12 h (250–375 mL/h), well within plasma-refilling capacity. The hypotension rate in the Fliser RCT was significantly lower with extended dialysis than with IHD.
2. Osmotic solute shifts. Rapid urea clearance in IHD lowers plasma osmolality faster than urea can equilibrate out of the brain and tissues → a falling serum osmolality → water shifts into the brain → cerebral oedema and the dialysis disequilibrium syndrome (headache, nausea, seizures, raised ICP). SLED's slow urea clearance preserves a near-constant osmolal gradient → no disequilibrium. This is why SLED (and CRRT) is preferred over IHD in traumatic brain injury, fulminant liver failure, and any patient with raised ICP.
3. Acetate/bicarbonate flux and vasoactive mediator release. The rapid solute and buffer flux of IHD transiently impairs vascular tone and releases vasoactive cytokines; the slower flux of SLED avoids this. [1]

The single-pass batch (Genius) system

A distinctive feature of much of the European SLED literature (including the Schwenger RCT) is the single-pass batch dialysis system (e.g., the Genius, a Fresenius machine).[1][4] A fixed volume of dialysate (typically 75–90 L) is prepared in a single tank, and blood and dialysate are circulated through a high-flux dialyser until the dialysate is spent (which coincides with the 6–8 h session). Because the dialysate is pre-prepared as one batch, there is no continuous dialysate proportioning pump — the machine is mechanically simpler and cheaper to operate. The Genius uses a gravity-driven dialysate circuit (density-based separation of fresh from spent dialysate in a single tank), which further reduces complexity. Most modern ICUs, however, perform SLED on a standard haemodialysis machine (e.g., Fresenius 5008, Nikkiso) in an "extended" programme, which is functionally equivalent.

SLED-f — adding convective clearance

SLED-f (SLED with filtration) is a variant that adds convective clearance (haemofiltration) on top of the diffusive clearance of standard SLED. By pulling extra ultrafiltrate across the membrane and replacing it with substitution fluid, SLED-f gains middle-molecule clearance (β2-microglobulin, vancomycin, some cytokines) that pure diffusive SLED lacks — analogous to the difference between CVVHD and CVVHDF.[3]

[1]

Clinically, SLED-f is used selectively. The Schwenger RCT compared standard SLED (single-pass batch, largely diffusive) against CVVHDF and found equivalent outcomes; the incremental benefit of adding convection to SLED (i.e., SLED-f) over plain SLED has not been demonstrated in a large outcome trial, so it remains a clinician-preference or niche indication.[1][5]

SLED vs CRRT vs IHD — the comprehensive comparison

This is the central exam table. Memorise it: the examiner wants to see that you can defend a modality choice against all three axes — haemodynamics, clearance, and logistics.[1][12][13]

[1]

Choosing a modality by clinical context

The comparison only matters insofar as it drives a choice. The decision tree below is examiner-favoured.[1][13]

[1]

The SLED prescription in detail

A defensible SLED prescription sets four parameters: blood flow, dialysate flow, session duration, and net ultrafiltration (fluid removal) target. The anticoagulation and dialysate composition complete the order.[1][3]

Dose adequacy — Kt/V, EKR, and the "per day" concept

SLED blurs the IHD adequacy metric (Kt/V per session) and the CRRT metric (mL/kg/hr per day). Two frameworks reconcile them:[4][8]

• Per-session Kt/V: target 1.2–1.4 (the standard IHD adequacy). A 70 kg patient with a urea volume ~40 L needs Kt ≈ 48–56 L, achieved by clearance 200 mL/min × 4–5 h or clearance 150 mL/min × 6–7 h.
• Equivalent renal clearance (EKR): averages the intermittently-delivered clearance over 24 h. Daily SLED delivering Kt/V 1.2 typically yields EKR ~20 mL/min — matching the KDIGO CRRT dose (25 mL/kg/hr ≈ 20–25 mL/min EKR for a 70–80 kg patient). This is the physiological justification for saying SLED delivers "CRRT-equivalent" daily clearance. [1]

The RENAL and ATN trials together established that pushing delivered dose above this standard (CRRT 40 mL/kg/hr; IHD/SLED 6×/week) does not improve survival — so the aim is adequacy, not maximalism.[8][9]

Anticoagulation for SLED

SLED needs less anticoagulation than CRRT because the circuit runs for hours rather than days, but it still requires a strategy — a bare dialyser in contact with non-anticoagulated blood will clot within 1–3 h.[1][3]

[1]

Practical pearl: for the routine haemodynamically marginal patient with no active bleed, unfractionated heparin is the workhorse. For the bleeding patient, regional citrate (if the centre has the protocol) or a no-heparin strategy with high Qb is used. Because the SLED session is short, citrate accumulation is much less of a concern than in continuous CRRT — but it still requires the standard calcium monitoring. [1]

Vascular access for SLED

SLED uses the same vascular access as IHD and CRRT: a temporary dual-lumen haemodialysis catheter (12–14 Fr) inserted into a central vein.[1]

[1]

Tip confirmation (right IJV tip at cavo-atrial junction on CXR; femoral tip in IVC) is mandatory before the first session. A malpositioned or kinked catheter produces low blood flow, repeated pressure alarms, access recirculation, and early circuit clotting — the single most fixable cause of a "difficult" SLED run. [1]

Drug dosing on SLED

SLED clears water-soluble drugs by diffusion — but the intermittent, lower-flow nature means the dosing strategy sits between IHD and CRRT. The key principles:[1][10]

1. Small water-soluble drugs ARE cleared (vancomycin, beta-lactams, aminoglycosides, levetiracetam, digoxin, water-soluble vitamins) — sieving coefficient ≈ 1.0.
2. Protein-bound drugs are NOT significantly cleared (ceftriaxone 90% bound, warfarin) — only the free fraction crosses.
3. Clearance occurs mainly during the session (6–12 h/day); off-SLED, the patient has near-zero extracorporeal clearance (only residual renal function). Dosing must account for this on/off pattern.
4. The DALI study confirmed that beta-lactam clearance on extended dialysis is substantial and that underdosing is common — therapeutic drug monitoring (TDM) is recommended for critical antibiotics.[10]

[1]

Pearl: time drug administration relative to the SLED session. For beta-lactams, an extended or continuous infusion during the session maximises time above MIC. Re-dose vancomycin after each SLED session if levels fall. Re-evaluate dosing after every circuit or prescription change. [1]

Complications of SLED

SLED shares the complications of all extracorporeal circuits but at a lower rate than CRRT for the continuous-exposure problems.[1][3]

[1]

FlowSteps — SLED session setup and the daily cycle

[1]

FlowSteps — the CRRT → SLED → IHD transition pathway

SLED's natural place is as the transitional modality as a patient recovers from shock toward stability.[1]

Exam practice — SAQs

[1] [1]

Clinical pearls

Key trials and evidence

Meta-analyses — SLED vs CRRT and modality comparison

Additional red flags

Sample exam question — worked answer

[1]

Worked explanation

Step 1 — Confirm the indication. This patient has an urgent, indisputable indication for RRT: refractory hyperkalaemia (K⁺ 6.1 unresponsive to medical therapy), metabolic acidosis (pH 7.20), and volume overload (4 L positive). The AKIKI and STARRT-AKI trials remind us to wait for an urgent indication before starting RRT — here, the indication is met, so RRT should start now, in whichever modality best matches her physiology.[6][7]

Step 2 — Assess haemodynamic stability and choose the modality. The decision rests on the three-axis comparison: haemodynamics, clearance, and logistics. She is on a single, low-dose vasopressor — she is not in refractory shock on max vasopressors, but she is not fully stable either. This is the textbook haemodynamically marginal patient for whom SLED is the ideal middle ground: better tolerated than standard IHD (Fliser RCT — fewer hypotensive episodes), yet cheaper, less anticoagulation, and with down-time compared to CRRT.[2][12] If she were on escalating high-dose multiple vasopressors with refractory shock, CRRT would be the choice. If she were fully off vasopressors, IHD would suffice. She sits in the SLED sweet-spot.

Step 3 — Defend SLED against CRRT. A common viva challenge: "Why not CRRT, which is the most stable?" The defence is multilayered. First, the evidence shows no mortality or renal-recovery difference between SLED and CRRT (Schwenger RCT 2012; Dalbhi meta-analysis 2021; Zhou network meta-analysis 2021) — CRRT is not superior in outcomes, only marginally smoother haemodynamically.[1][11][13] Second, SLED uses the standard IHD machine and less anticoagulation, so it is cheaper and carries a lower bleeding risk (Berbece cost analysis).[3] Third, the down-time allows the procedures, imaging, and mobilisation that matter for her recovery and prevent ICU-acquired weakness. Fourth, in a centre with only one CRRT machine, choosing SLED reserves the CRRT machine for a patient who truly needs it (max vasopressors). For this marginal patient, the marginal haemodynamic gain of CRRT is not worth the cost and tether.

Step 4 — Defend SLED against IHD. "Why not just haemodialyse her?" Because standard IHD's rapid ultrafiltration and osmotic shifts would precipitate hypotension in a patient already vasopressor-dependent, worsening the AKI and the shock. The Fliser RCT established extended/slow dialysis as haemodynamically superior to IHD.[2] Additionally, her severe acidosis and uraemia make dialysis disequilibrium a concern with high-flux IHD; SLED's slow clearance avoids the osmotic shift.

Step 5 — Set the prescription. Following the prescription protocol:[1][3]

• Blood flow (Qb): 150 mL/min (start low for the vasopressor-dependent patient; titrate to 200 as tolerated).
• Dialysate flow (Qd): 300 mL/min (higher end — she needs aggressive clearance for the K⁺ 6.1 and acidosis; clearance is dialysate-limited, so Qd is the lever).
• Duration: 8 h (long enough for the fluid removal and solute clearance; extend to 10–12 h if fluid removal at a safe UF rate demands it).
• Net ultrafiltration: she is 4 L positive — remove over 8 h at ~400 mL/h (within safe plasma-refill capacity). Do NOT push to 750 mL/h to "finish in 4 h" — that is the IHD mistake.
• Anticoagulation: unfractionated heparin (bolus 30 U/kg then infusion 500–1000 U/h targeting APTT 1.5–2× baseline), unless she has active bleeding — then regional citrate or no-heparin with high Qb.
• Dialysate: bicarbonate-buffered (corrects her acidosis), potassium 2 mmol/L (she is hyperkalaemic but do not use zero-K⁺ — risk of overshoot hypokalaemia), calcium 1.5 mmol/L.
• Dose target: single-pool Kt/V ≈ 1.2–1.4, or EKR ~20 mL/min averaged over 24 h — the adequacy standard set by RENAL and ATN.[8][9]

Step 6 — Monitoring. Hourly BP, MAP, HR, and circuit pressures (arterial, venous, transmembrane) — titrate UF rate down if MAP drops, and extend the session rather than push the UF. Mid-session and end-of-session bloods (Na, K, bicarbonate, glucose, urea, creatinine, phosphate, Mg) — check K⁺ at midpoint because she started hyperkalaemic and you must confirm it is falling without overshoot. Phosphate before the next session (repeated sessions deplete it). Delivered Kt/V at session end (urea reduction ratio). Vancomycin/beta-lactam levels if she is on antibiotics — SLED clears them (DALI), and underdosing causes treatment failure.[10]

Step 7 — Plan the off-period and the transition. Use the 12–18 h down-time for her antibiotic dosing, feeds, physiotherapy, and any imaging. As her shock resolves (vasopressor off, urine output rising, creatinine falling), step down the ladder: CRRT → SLED (current) → IHD → stop. The first IHD session after SLED should use reduced Qb to avoid disequilibrium. Counsel that dialysis dependence is possible (age, sepsis, AKI severity are risk factors; STARRT-AKI showed accelerated-start patients had more residual dependence).[7]

Bottom line for the exam: SLED is the modality of choice for the haemodynamically marginal patient — better tolerated than IHD, cheaper and less anticoagulation than CRRT, with comparable outcomes. Set Qb 150–200, Qd 100–300 (raise Qd for aggressive clearance), duration 6–12 h with the UF rate capped for haemodynamic safety, and standard heparin anticoagulation. Monitor hourly haemodynamics, mid-session electrolytes, delivered Kt/V, and antibiotic levels. The prescription is justified by the Schwenger non-inferiority RCT, the Fliser haemodynamic RCT, and the dose ceiling from RENAL/ATN. [1][2][8][9]

Prognosis and outcomes summary

[1]

The overarching message: SLED is a fully evidence-supported modality, not a compromise. Where the haemodynamic profile, resources, or need for down-time favour it, SLED delivers CRRT-equivalent outcomes at lower cost and complexity. The choice between SLED, CRRT, and IHD should be individualised to the patient's haemodynamic stability, the centre's resources, and the practical need for mobility and procedures — guided by the comparison table and the evidence above.[1][1][11]

References

1. [1]Schwenger V, Weigand MA, Hoffmann O, et al. Sustained low efficiency dialysis using a single-pass batch system in acute kidney injury - a randomized interventional trial: the REnal Replacement Therapy Study in Intensive Care Unit PatiEnts Crit Care, 2012.PMID 22839577
2. [2]Fliser JT, Kielstein JT Efficacy and cardiovascular tolerability of extended dialysis in critically ill patients: a randomized controlled study Am J Kidney Dis, 2004.PMID 14750100
3. [3]Berbece AN, Richardson RMA Sustained low-efficiency dialysis in the ICU: cost, anticoagulation, and solute removal Kidney Int, 2006.PMID 16850023
4. [4]Kielstein JT, Schiffer M, Hafer C Technology Insight: treatment of renal failure in the intensive care unit with extended dialysis Nat Clin Pract Nephrol, 2006.PMID 16932387
5. [5]Schneider AG, Bellomo R, Bagshaw SM, et al. Good-bye CRRT, here comes SLED? ... not so fast! Crit Care, 2012.PMID 23148709
6. [6]Gaudry S, Hajage D, Schortgen F, et al. (AKIKI trial) Initiation Strategies for Renal-Replacement Therapy in the Intensive Care Unit N Engl J Med, 2016.PMID 27181456
7. [7]STARRT-AKI Investigators, Canadian Critical Care Trials Group, ANZICS Clinical Trials Group Timing of Initiation of Renal-Replacement Therapy in Acute Kidney Injury N Engl J Med, 2020.PMID 32668114
8. [8]Bellomo R, Cass A, Cole L, et al. (RENAL Replacement Therapy Study Investigators) Intensity of continuous renal-replacement therapy in critically ill patients N Engl J Med, 2009.PMID 19846848
9. [9]Palevsky PM, Zhang JH, O'Connor TZ, et al. (VA/NIH Acute Renal Failure Trial Network) Intensity of renal support in critically ill patients with acute kidney injury N Engl J Med, 2008.PMID 18492867
10. [10]Roberts JA, Udy AA, Jarrett P, et al. (DALI study) DALI: defining antibiotic levels in intensive care unit patients: are current β-lactam antibiotic doses sufficient for critically ill patients? Clin Infect Dis, 2014.PMID 24429437
11. [11]Dalbhi SA, Alorf R, Alotaibi M, et al. Sustained low efficiency dialysis is non-inferior to continuous renal replacement therapy in critically ill patients with acute kidney injury: A comparative meta-analysis Medicine (Baltimore), 2021.PMID 34941056
12. [12]Russo DS, Eugenio CS, Balestrin IG, et al. Comparison of hemodynamic instability among continuous, intermittent and hybrid renal replacement therapy in acute kidney injury: A systematic review of randomized clinical trials J Crit Care, 2022.PMID 35124346
13. [13]Zhou X, Dong P, Pan J, et al. Renal replacement therapy modality in critically ill patients with acute kidney injury - A network meta-analysis of randomized controlled trials J Crit Care, 2021.PMID 33836397
14. [14]Zhao Y, Chen Y Effect of renal replacement therapy modalities on renal recovery and mortality for acute kidney injury: A PRISMA-compliant systematic review and meta-analysis Semin Dial, 2020.PMID 32149415