Chapter 17. Renal Replacement Therapy

• Kt/V & URR: measures of renal replacement therapy (RRT) adequacy
• Dialysate: solution used to facilitate diffusion during RRT
• Dialyzer/filter/dialysis membrane: semipermeable membrane utilized to facilitate solute & fluid removal; surface area & pore size determine size of solutes removed & ultrafiltration of fluid
• Flow rates: QB, blood flow rate; QD, dialysate flow rate; QUF, ultrafiltration flow rate
• Dry weight (kg): body weight, patient is volume neutral
• Vascular access: arteriovenous (AV) in chronic intermittent hemodialysis (IHD); venovenous (VV) in continuous renal replacement therapy (CRRT); AV fistula or graft in chronic RRT; central venous catheters (CVC) in emergent RRT (especially CRRT or hybrid IHD; internal jugular [IJ] preferred, ↑ infection rate with femoral & ↑ risk of stenosis with subclavian)

• Diffusion/dialysis → solute movement across a semipermeable membrane down a concentration gradient
• Convection/ultrafiltration → solute movement in solution (“solvent drag”) driven by hydrostatic pressure resulting in a transmembrane pressure gradient; convection more efficient than diffusion at removing larger molecular weight solutes

Clinical status dictates RRT use:

• A: Acidosis (e.g., metabolic acidosis)
• E: Electrolyte disturbances (e.g., ↑ K+, ↑ Mg++, ↑ Phos, ↑ Ca++)
• I: Intoxications (e.g., lithium, theophylline, valproic acid, ethylene glycol, metformin, carbamazepine, pentoxifylline, methanol)
• O: Fluid Overload (e.g., CHF, pulmonary edema, or uncontrolled HTN)
• U: Uremia (e.g., N/V, loss of appetite, mental status changes, uremic pericarditis, prolonged bleeding time, motor neuropathy)

• Intermittent hemodialysis (IHD): uses both diffusion & ultrafiltration/convection; stage 5 CKD patients often receive adequate RRT with IHD treatments 3×/wk; usual treatment 3–4h with QB ≥300–400mL/min; critically ill patients with AKI may require daily IHD for adequate fluid/solute removal; advantages → rapid removal of volume/solutes & correction of electrolytes; primary disadvantage → hypotension due to large fluid shifts from intravascular space
• Peritoneal dialysis (PD): uses diffusion & ultrafiltration/convection across peritoneal membrane; <10% of stage 5 CKD patients in the United States; dialysate instilled into peritoneal cavity via tunneled indwelling catheter; not commonly used in critically ill patients with AKI; advantages → ↑ patient independence, ↑ hemodynamic stability, ↑ clearance of larger solutes, preservation of residual renal function; disadvantages → ↑ loss of protein & amino acids, ↑ risk of peritonitis & exit site infections, ↑ risk of excess glucose load leading to excess calories & weight gain as well as difficulty with BG control in DM patients
  • Continuous ambulatory PD (CAPD): dialysate exchanges of 1–3L of dialysate instilled & drained 3–4× daily
  • Automated PD (APD): automated cycler instills dialysate several times overnight leaving dialysate in peritoneal cavity for 1–2h before next exchange allowing for longer dialysate free time during day
• Continuous renal replacement therapy (CRRT): often continuous for 24h/d based on clinical status; uses diffusion or ultrafiltration/convection or both; ↓ QB (150–200mL/min) & ↓ QUF (25–50mL/min) vs IHD; ↓ rate of solute removal, but ↑ overall solute removal over 24h; advantage → more consistent fluid & solute removal, ↑ solute & fluid removal overall, useful in hemodynamically unstable patients; disadvantages → ↑ incidence of extracorporeal thrombosis requiring additional anticoagulation, requires specialized equipment, labor intensive, ↑ costs