Hemodialysis and Acute Renal Replacement Therapy — Advanced

This patient is haemodynamically unstable on vasopressor support with life-threatening indications for RRT (pulmonary oedema and severe hyperkalaemia with ECG changes). Intermittent haemodialysis may cause rapid fluid and solute shifts and further haemodynamic deterioration. Peritoneal dialysis is slow and inadequate for rapid correction of life-threatening hyperkalaemia and pulmonary oedema in this context. SLED may be better tolerated than conventional intermittent HD but in a patient requiring ongoing vasopressor support CRRT provides the most haemodynamically stable, continuous fluid removal and gradual correction of electrolytes and volume. Conservative measures (insulin/glucose, salbutamol, calcium) are needed immediately for ECG protection, but definitive extra-renal clearance is required. CRRT with appropriate anticoagulation (or no anticoagulation if bleeding risk) is therefore the most appropriate next step.

In CRRT the principal determinant of small-solute clearance is the effluent flow rate (for continuous therapies, clearance approximates the effluent rate): higher effluent flows produce greater small-molecule removal. Anticoagulation affects circuit patency and therefore delivered therapy but does not directly set solute clearance. Dialyser surface area influences clearance but in CRRT the effluent flow is the primary control. Frequent interruptions reduce delivered dose but the question asks for the parameter that most directly determines clearance in the prescription. Patient BMI influences distribution and dosing considerations but is not a direct prescription parameter controlling instantaneous clearance.

In patients with high bleeding risk (recent intracranial haemorrhage) regional citrate anticoagulation is preferred for CRRT because it provides effective circuit anticoagulation while minimising systemic anticoagulant exposure. Systemic unfractionated heparin or LMWH increases bleeding risk and is therefore undesirable. Running circuit without any anticoagulant may be possible but commonly results in frequent circuit clotting and inadequate therapy. Systemic direct thrombin inhibitors avoid heparin but still expose the patient to systemic anticoagulation and bleeding risk; they may be used when citrate is contraindicated but are not preferred in active bleeding. Citrate requires monitoring for ionised calcium (systemic and post-filter), total calcium/ionised calcium ratio for citrate accumulation, and acid–base status.

DDS results from rapid reduction in plasma osmolality (urea) leading to cerebral fluid shifts. The most effective prevention is to minimise the rate of urea removal at the initial session—achieved by using a small surface area dialyser, low blood flow, lower dialysate flow, shorter or more gentle sessions, or use of CRRT/slow low-efficiency dialysis. Increasing dialysate sodium or using cooler dialysate may help haemodynamic stability but are not the primary preventive strategy for DDS. Rapid or high-volume ultrafiltration will worsen cerebral perfusion and is contraindicated. Loop diuretics are ineffective in anuric patients and do not prevent DDS.

A fibrin sheath often causes refractory inability to aspirate blood. While intraluminal tPA may temporarily restore function if thrombus is intraluminal, a mature fibrin sheath often requires radiological intervention: venography-guided fibrin-sheath disruption/stripping or exchange under imaging. Bedside over-the-wire exchange without imaging may reintroduce a catheter into the same sheath and fail. Systemic anticoagulation does not reliably resolve a fibrin sheath. Immediate removal and waiting 2 weeks is not necessary and delays dialysis access; radiological management is the preferred next step.

With severe hyperkalaemia and ECG changes, intravenous calcium is the immediate first step because it stabilises the myocardial membrane. At the same time, give temporising potassium-shifting therapy such as insulin with glucose and nebulised salbutamol, and arrange urgent haemodialysis as definitive potassium removal in a dialysis-dependent patient. Calcium does not lower serum potassium, so definitive therapy and close monitoring must follow without delay.

In suspected or confirmed HIT all heparin products must be stopped. In patients with severe renal failure, anticoagulants that are predominantly renally cleared (e.g., bivalirudin, fondaparinux) accumulate and may be problematic. Argatroban is hepatically metabolised and is therefore the preferred intravenous alternative for anticoagulating extracorporeal circuits in severe renal impairment (dose adjustment for hepatic impairment needed). Fondaparinux is renally cleared and contraindicated in severe renal failure. LMWH and continuing heparin are contraindicated. Bivalirudin is partially renally cleared and requires dose adjustment but is less favourable compared with argatroban in severe renal impairment.

Although several factors influence dialysability (molecular size, protein binding, water solubility), the apparent volume of distribution is the best single predictor: drugs with a small Vd (largely confined to the intravascular space, e.g., Vd < 1 L/kg) are more readily removed by dialysis. Highly protein-bound drugs are less dialysable, and very large molecules are less dialysable, but Vd integrates distribution and is most useful clinically when predicting removal. Hepatic clearance and pKa influence overall drug handling but are not primary predictors of acute dialysability.

Tolerance to ultrafiltration is primarily determined by the capacity of the vascular compartment to be refilled from the interstitial fluid (plasma refill rate). If ultrafiltration rate exceeds plasma refill capacity, intravascular volume falls and hypotension ensues. Dialysate sodium, temperature, residual urine output and haemoglobin all influence haemodynamic stability but the direct determinant of UF tolerance is the balance between UF rate and plasma refilling.

When citrate anticoagulation is contraindicated because of severe hepatic failure and concern about citrate accumulation, bicarbonate-based replacement or dialysate fluid without citrate is generally preferred. The separate anticoagulation decision should be individualised: CRRT may be run without anticoagulation with frequent circuit changes, or a systemic agent may be considered after assessing bleeding risk. Routine citrate dosing should not be used in this setting; if citrate is considered in a selected centre, intensive monitoring and dose adjustment are required.