Indications for Dialysis
Dialysis has two main functions: solute clearance (waste removal) and ultrafiltration (volume removal). The mnemonic “AEIOU” includes the major indications for dialysis:
- Acidosis: if severe (pH <7.2) and refractory to HCO3 OR unable to give HCO3 due to volume overload.
- Electrolytes: usually hyperkalemia >6.5mmol/L OR lower with ECG changes. Temporize with calcium gluconate IV, D50+10units IV insulin, beta-agonist nebulizers, loop diuretic (if not ESRD and hypervolemic or euvolemic) and HCO3 (if pH <7.2).
- Ingestions: lithium or ethylene glycol.
- Overload: pulmonary edema that cannot be temporized with nitrates and large doses of loop diuretics (e.g., furosemide 160–200mg IV).
- Uremia: symptomatic with encephalopathy, pericarditis, neuropathy, seizures, platelet dysfunction with severe bleeding, and intractable nausea/vomiting.
While it is axiomatic that one should not delay renal support until the development of these conditions, there is little trial evidence that early intervention improves clinical outcomes (patient survival and kidney recovery) in AKI.
Modes of Renal Replacement Therapy (RRT)
- Intermittent hemodialysis (IHD):
- Mechanism: solute and volume removal over short period (~2-4 hours). Dialysate runs countercurrent to blood and solute removal occurs through diffusion. Volume is removed via negative pressure created from the HD machine.
- Advantages: more rapid solute removal (especially important in hyperkalemia and toxic ingestions), lower risk of bleeding (use for uremic bleeding), greater availability.
- Complications: intradialytic hypotension (15-30%), wrong dialysate, allergic reaction to dialyzing membrane, air embolus, dialysis disequilibrium syndrome (osmotic shifts cause cerebral edema; s/s include headache, nausea, disorientation, seizure).
- Pure ultrafiltration (PUF):
- Mechanism: volume removal over short period. Similar to IHD, volume is removed via negative pressure created from the HD machine. But NO dialysate runs countercurrent (thus no need to re-dose medications post PUF).
- Advantages: less hypotension than IHD because less solute shifts. Can draw off volume faster.
- Complications: hypotension from fluid shifts, allergic reaction to dialyzing membrane, air embolus.
- Continuous renal replacement therapy (CRRT):
- Mechanism: runs nearly continuously (23hrs/day). Can be either by dialysis, hemofiltration, or a combination. Dialysis involves diffusion of solutes across a semipermeable membrane according to concentration gradients. Hemofiltration removes solute via convection: hydrostatic pressure pushes water and solutes through a filter membrane and replaces that volume with a replacement fluid.
- CVVHD (continuous venovenous hemodialysis): pre-mixed dialysate runs countercurrent is used to remove electrolytes by diffusion.
- CVVH (continuous venovenous hemofiltration): solutes are removed by convection (solvent removed and dissolved solutes are removed with solvent).
- CVVHDF (hemodiafiltration, a combination of the two).
- Advantages: preferred modality for hemodynamically unstable patients. Greater precision in removal of solutes, can slowly and safely remove large volumes of fluid (50-400mL/hr over 24 hrs rather than 0-2000mL over 3-4 hours with IHD), lower incidence of cardiac arrhythmias, less effect on intracranial pressure than IHD.
- Disadvantages: slower for acute solute removal (K, toxins), may need continuous anticoagulation with heparin or citrate which increases bleeding risk, requires ICU level care, less availability outside of tertiary medical centers.
- Access: can't use AV fistula or AV graft because of risk of access damage. Use temporary or tunneled catheter.
- Complications: worsening hypotension, clotting the filter, air embolus, allergic reaction, need to re-dose antibiotics, limited patient mobility.
- Mechanism: runs nearly continuously (23hrs/day). Can be either by dialysis, hemofiltration, or a combination. Dialysis involves diffusion of solutes across a semipermeable membrane according to concentration gradients. Hemofiltration removes solute via convection: hydrostatic pressure pushes water and solutes through a filter membrane and replaces that volume with a replacement fluid.
- Peritoneal dialysis: see section Peritoneal Dialysis.
Choosing a Modality
- CRRT vs IHD: there has been no difference in ICU or overall mortality between CRRT and IHD in hemodynamically stable ICU patients with AKI, though it does achieve better hemodynamic parameters. CRRT remains the preferred modality for hemodynamically unstable patients.
- CVVH vs CVVHD: equivalent outcomes. CVVH offers less hypotension, more flexibility. CVVHD is cheaper with less room for pharmacy error.
Access
- Temporary: non-tunneled dialysis catheter (e.g. Quinton, Niagara, or Trialysis catheter) for AKI requiring dialysis for less than 3 weeks.
- Placed in IJ or femoral veins. In general avoid subclavian due to risk of subclavian stenosis.
- Risk of infection: remove femoral lines <3-5 days, IJ lines <14 days.
- Permanent: AV fistula (best) > AV graft > tunneled catheter (e.g. Tesio, “PermCath” or Palindrome catheter).
- Should only be accessed by HD or ICU RN, but can be used in code situations.
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