Renal Replacement Therapy in Critical Care

1. An idiots� guide Renal Replacement Therapy in Critical Care

2. �There�s nothing more dangerous than a resourceful idiot� Scot Adams, American Cartoonist

3. Removing badness from the blood First used in the Korean War Dialysis, cf Greek �to pass through�

4. Indications Oliguria (urine output <200 mL/12 h) Anuria/extreme oliguria (urine output <50 mL/12 h) Hyperkalaemia ([K] >6.5 mEq/L) Severe acidaemia (pH <7.1) Azotemia ([urea] >30 mg/dL) Pulmonary oedema Uraemic encephalopathy Uraemic pericarditis Uraemic neuropathy/myopathy Severe dysnatraemia ([Na] <115 or >160 mEq/L) Hyperthermia Drug overdose with dialyzable toxin �to clear poison or fluid�

5. Dialysis (....diffusion) Solutes flow down an electrochemical gradient, across a membrane. Solute removal is proportional to dialysate flow rate Dialysate flows counter-current to blood

6. Ultrafiltration (..... convection) Water moves along a hydrostatic gradient across a filter Solute moves by solvent drag

7. �There are no stupid questions, but there are a lot of inquisitive idiots� Larry Kersten, American Sociologist

8. How can I dialyse a patient? Intermittent haemodialysis Peritonal Dialysis Rarely used in UK ICUs as labour intensive and risks infection, but nb CAPD Continuous Haemodiafiltration Arterio-venous (pts own BP drives blood through the filter) Veno-venous (blood is pumped � doesn�t rely on BP) Plasma exchange, esp in immune disorders, eg GBS Plasma is removed / exchanged by filtration or centrifugation

9. Intermittent haemodialysis Gold standard � though patient must be haemodynamically stable Dialysate is typically deionised water Blood flow typically 200-400mL/min, dialysate flow 500mL/min, filtration rate of 300-2000mL/hr and urea clearance of 150/250mL/min Complications mostly due to fluid and osmitic shifts

10. Problems with IHD Removal of intravascular volume quicker than it can be replaced from the extravascular space can cause cardiovascular collapse � particularly if intravascularly deplete. Hypotension can cause ischaemic injury, particulary in AKI or head injury. Intermittent by nature, so ICU patients may develop overload in-between sessions (nb reduced venous capacitance)

11. Dialysis Disequilibrium syndrome Self-limiting syndrome typically after first dialysis of very uraemic patients. Characterised by nausea, vomiting, headache, seizures and coma. Syndrome is triggered by rapid reduction in plasma osmolality causing cerebral (cellular) oedema. Treatment Supportive Hypertonic saline / manitol

12. �Watson, you idiot. Somebody stole our tent.....� Sherlock Holmes, Sleuth

13. Continuous veno-venous heamofiltration (CVVH) Convective dialysis Filtration rate is high Electrolyte replacement solution is required Removes a lot of middle molecules, e.g. cytokines Slow continuous ultra-filtration (SCUF) is �slower� and doesn�t use a replacement fluid (i.e. removes volume only)

14. Continuous veno-venous haemodialysis (CVVHD) Continuous diffusive dialysis Mostly small molecules are removed

15. Continuous veno-venous haematodiafiltration (CVVHDF) Diffusive and convective dialysis Small and middle molecules removed Requires dialysate and replacement fluid Most popular mode

16. �He is a dreamer, a thinker, a speculative philosopher... or, as I like to put it, an idiot� Christina Hallsworth, my wife

17. Advantages of CRRT Suitable for use in haemodynamically unstable patients. Precise volume control, which is immediately adaptable to changing circumstances. Very effective control of uraemia, hypophosphataemia and hyperkalaemia. Rapid control of metabolic acidosis Available 24 hours a day with minimal training. Safer for patients with brain injuries and cardiovascular disorders (particularly diuretic resistant CCF). May have an effect as an adjuvant therapy in sepsis. Probable advantage in terms of renal recovery Makes space for TPN in anuria

18. Disadvantages of CRRT Expense � probably the same as IHD. Anticoagulation � to prevent extracorporeal circuit from clotting. Complications of line insertion and sepsis. Risk of line disconnection. Hypothermia. Theraputic drugs doses need adjusting � nb vasoactive drugs Severe depletion of electrolytes � particularly K+ and PO4, where care is not taken.

19. Using CVVHDF CVVHDF is similar to IHD � but in slow motion Requires a 12F double lumen catheter (VasCath) in a big vein Typically Blood flow 100-200mL/min Filtration rate 10-20mL/min Urea clearance 10-20mL/min To increase the urea clearance, you can increase the blood flow rate, dialysate flow rate, or both. Membrane is usually a hollow fibre polyacrilonitrile, polyamide or polysulphone with a surface area of 0.6-1m2

20. Using CVVHDF Anticoagulation Classically heparin, but : Risks bleeding Requires antithrombin 3 Causes HIT PGI2 (prostacyclin � short t� ), Citrate (binds Ca+, metabolised to bicarbonate in liver) LMWH Aprotonin

21. Using CVVHDF Typically the dialysate and replacement fluid are similar to �what you want the blood to be�, i.e. Hartmanns Watch potassium, calcium and phosphate levels closely There is often no bicarbonate in the dialysate, and bicarb in the blood is replaced with lactate from the dialysate. This can be a problem in liver failure � best to use a lactate free dialysate

22. �When you left home you deprived the village of it�s idiot� Chris Hallsworth, my father