Renal, Fluids, Electrolytes & Acid-Base · Topic 3 of 22

Renal Replacement Therapy (HD & CRRT)

Renal · kidney injury & replacement

Renal Replacement Therapy (HD & CRRT)

Dialyze for refractory potassium, fluid, acidosis, uremia or a dialyzable poison; use CRRT when the patient is hemodynamically unstable.

Key numbers

CRRT dose: effluent 20–25 mL/kg/h20–25 mL/kg/h

Citrate accumulation: total/ionized calcium > 2.5tCa/iCa > 2.5

Adequate hemodialysis: urea reduction ≥ 65%URR ≥ 65%

PD peritonitis: cloudy effluent, WBC > 100WBC > 100/µL, > 50% PMN

01 · Defined byEmergent dialysis: refractory K⁺, fluid, acid, uremia

Start RRT emergently for life-threatening hyperkalemia (eg, K⁺ > 6 mmol/L despite treatment), volume overload or metabolic acidosis that medical therapy cannot control, uremic complications (pericarditis, encephalopathy, seizures), or a dialyzable poisoning.

02 · The ruleUnstable patient: choose CRRT over intermittent HD

Use CRRT rather than intermittent hemodialysis for hemodynamically unstable patients and for AKI with acute brain injury, raised intracranial pressure or brain edema; slow removal avoids hypotension and osmotic shifts.

03 · The numberCRRT dose: effluent 20–25 mL/kg/h

Deliver a CRRT effluent volume of 20–25 mL/kg/h in AKI (prescribe higher to allow for downtime); for intermittent or extended RRT in AKI, deliver a weekly Kt/V of 3.9.

What it is

Core idea

CVVH convects, CVVHD diffuses, SCUF removes fluidCVVHD clears small solutes by diffusion (potassium, acidosis); CVVH clears larger molecules by convection and needs replacement fluid; CVVHDF combines both; SCUF removes fluid only.

CRRT avoids the large fluid shifts of HDContinuous therapies remove large fluid volumes slowly, so they suit unstable AKI patients on vasopressors or needing large infused volumes; SLED (8–12-hour sessions) is a hybrid with similar hemodynamic tolerance.

Causes & risk factors

Why it happens

Dialyzable toxins: toxic alcohols, lithium, salicylatesEthylene glycol, methanol, lithium, salicylates and theophylline may need dialysis; large, protein-bound or highly tissue-distributed drugs (digoxin, tricyclics) are not removed.

Diagnosis & monitoring

Work-up

Adequate hemodialysis: urea reduction ≥ 65%A chronic hemodialysis session is adequate with a urea reduction ratio ≥ 65% or Kt/V ≥ 1.2; most patients do well with 3–5-hour sessions 3 times a week.

Do right now

In this order

First moves

Emergent dialysis: refractory K⁺, fluid, acid, uremia

→

Dialysis catheter: right internal jugular first

→

Unstable patient: choose CRRT over intermittent HD

→

Regional citrate anticoagulation for CRRT

Why each step

Emergent dialysis: refractory K⁺, fluid, acid, uremiaStart RRT emergently for life-threatening hyperkalemia (eg, K⁺ > 6 mmol/L despite treatment), volume overload or metabolic acidosis that medical therapy cannot control, uremic complications (pericarditis, encephalopathy, seizures), or a dialyzable poisoning.

Dialysis catheter: right internal jugular firstStart acute RRT through an uncuffed nontunneled catheter placed with ultrasound: right internal jugular first, then femoral, then left jugular, subclavian last; get a chest radiograph before using a jugular or subclavian line.

Unstable patient: choose CRRT over intermittent HDUse CRRT rather than intermittent hemodialysis for hemodynamically unstable patients and for AKI with acute brain injury, raised intracranial pressure or brain edema; slow removal avoids hypotension and osmotic shifts.

Regional citrate anticoagulation for CRRTAnticoagulate CRRT with regional citrate rather than heparin when citrate is not contraindicated; citrate chelates calcium in the circuit, and calcium is reinfused on return. Avoid it in acute liver failure.

Management

Orders

Heparin for intermittent HD; argatroban if HITFor intermittent RRT use unfractionated or low-molecular-weight heparin; with heparin-induced thrombocytopenia use a direct thrombin inhibitor (argatroban preferred) or factor Xa inhibitor; proceed without anticoagulation if bleeding risk is high.

AV fistula: preferred access, needs months to matureAn arteriovenous fistula is the preferred chronic access (fewer infections, more durable); it takes 2–3 months to mature, so create it at least 6 months before dialysis is expected.

Red flags & don'ts

Avoid harm

Citrate accumulation: total/ionized calcium > 2.5 complicationCitrate accumulation causes falling ionized calcium with stable or rising total calcium (total/ionized ratio > 2.5), rising calcium-infusion needs and high anion gap acidosis, often with hyperlactatemia.

Hypotension is the most common HD complication complicationIntradialytic hypotension occurs in 4–17% of treatments, from rapid fluid removal, osmotic shifts, autonomic neuropathy or cardiomyopathy; cramps, nausea and headache are also common.

Disequilibrium: confusion, seizures after rapid HD complicationDialysis disequilibrium — headache, nausea, cramps, agitation, confusion, visual disturbance, seizures, coma — follows too-rapid urea removal that shifts water into the brain; risk is highest at the first dialysis sessions.

PD peritonitis: cloudy effluent, WBC > 100 complicationPD peritonitis = 2 of: pain or cloudy effluent; effluent WBC > 100/µL with > 50% neutrophils; positive culture. Treat at once: vancomycin or first-generation cephalosporin plus ceftazidime or an aminoglycoside, IV or intraperitoneally.

The rules

Know these

No BUN threshold; earlier start does not helpDecide on RRT from the clinical context and trends, not BUN or creatinine alone; in STARRT-AKI, accelerated RRT did not lower 90-day mortality and increased dialysis dependence and adverse events.

Intermittent HD or CRRT?

Commonly confused

Session	3–5 hours, typically 3 times a week
Fluid and solute removal	Fast — large fluid shifts
Best for	Hemodynamically stable patients
Circuit anticoagulation	Unfractionated or low-molecular-weight heparin
Dose	Kt/V ≥ 1.2 per session (chronic)

More detail

Reference

Show 7 reference facts

Stop RRT when kidney function recovers enough for the patient's needs or it no longer fits the goals of care; do not give diuretics to speed recovery or shorten RRT.

Use bicarbonate rather than lactate as the buffer in dialysate and replacement fluid, especially in circulatory shock, liver failure or lactic acidosis.

Hemodialysis lowers serum levels of some drugs (eg, many antibiotics), which then need a post-dialysis supplemental dose; consult a renal dosing reference.

PD catheter removal is needed when peritonitis fails to respond within 5 days, relapses with the same organism, or is fungal.

Watch an AV fistula for infection, thrombosis, aneurysm and pseudoaneurysm: pain, swelling, erythema, absent bruit and pulse, hematoma or prolonged bleeding after needle removal.

KRT does not replace the kidney's endocrine work: erythropoietin and calcitriol deficiency persist and still need treatment.

Peritoneal dialysis is ruled out by loss of peritoneal function, adhesions, recent abdominal wounds, abdominal fistulas or wall defects; adequacy is a weekly Kt/V ≥ 1.7.

Pearl

AEIOU — acidosis, electrolytes (K⁺), intoxication, overload, uremia — when medical therapy fails.

Sources · KDIGO Clinical Practice Guideline for Acute Kidney Injury (section 5: dialysis interventions). Kidney Int Suppl 2012;2:1–138 · Merck Manual Professional — Overview of Kidney Replacement Therapy · Merck Manual Professional — Hemodialysis · Merck Manual Professional — Continuous Hemofiltration and Hemodialysis · Merck Manual Professional — Acute Kidney Injury (dialysis indications) · STARRT-AKI Investigators. Timing of Initiation of Renal-Replacement Therapy in Acute Kidney Injury. N Engl J Med 2020;383:240–251 · Systemic Citrate Toxicity During Regional Citrate Anticoagulation for Continuous Kidney Replacement Therapy. J Clin Med 2026 · Merck Manual Professional — Peritoneal Dialysis · Merck Manual Professional — General Principles of Poisoning (dialysis) · Merck Manual Professional — Chronic Kidney Disease (medication dosing) · Mistry K. Dialysis disequilibrium syndrome prevention and management. Int J Nephrol Renovasc Dis 2019;12:69–77 · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).