General Principles

• Hemodialysis is by far the most common form of renal replacement therapy in the United States. Intermittent hemodialysis (IHD) typically runs for 3–4 hours per session and is performed three times weekly. Outpatient, in-center hemodialysis for ESRD generally uses this modality, although variations are available for patients undergoing home treatments.
• Continuous renal replacement therapy (CRRT) can be used in specialized circumstances, particularly when the patient’s hemodynamic status would not tolerate the rapid fluid shifts of IHD. Although less efficient (with slower blood flow) and with slower UF rates, CRRT can achieve equivalent clearances of both solute and fluid compared to IHD due to its continuous, 24-hour nature. The slower blood flows may necessitate anticoagulation (with either systemic heparin or regional citrate) to prevent the filter from clotting. Continuous modalities require specialized nursing and an intensive care setting.
  • There are various modalities of CRRT, including continuous venovenous hemodiafiltration (CVVHDF) and continuous venovenous hemodialysis (CVVHD). None of these modalities have been shown to be superior to the others.
  • In CVVHDF, blood is slowly pumped counter-currently to a dialysis solution (allowing for diffusive clearance), and a replacement fluid (an isotonic physiologic solution devoid of uremic toxins and other waste products) is infused into the circuit to balance most of the ultrafiltrate (convective clearance). CVVHD does not utilize a replacement fluid and thus convective clearance is not performed.
• Sustained low-efficiency dialysis uses intermediate treatment lengths (8–12 hours), allowing for adequate clearances, while patients can spend a significant portion of the day off the machine to allow for non-bedside testing, procedures, and physical therapy.
• Prescription and adequacy
  • IHD typically runs for 3–4 hours and can ultrafilter 3–4 L safely in hemodynamically stable patients. In the chronic setting, IHD is generally performed three times weekly, although the longer interdialytic interval on the weekend has been associated with a heightened mortality risk.¹ In the acute setting, the appropriate interval is not clearly defined, although a three times weekly schedule is likely adequate and remains common practice.
  • Adequacy is assessed by calculating BUN clearance, which serves as a surrogate marker of the “uremic factors.” The urea reduction ratio can be calculated by the following:
  • A reduction rate of ≥65% is considered adequate in the chronic setting.² An adequacy target is less well defined for AKI. Intensive daily hemodialysis was not shown to be superior to standard three times weekly treatments.³
  • Clearance is measured differently in CRRT in which dialytic therapy spans over 24 hours, effectively providing an extracorporeal “GFR.” Drug dosing needs to be adjusted accordingly; an estimate of this clearance can be calculated by the sum of the dialysis fluid, replacement fluid, and net UF rates converted into milliliters per minute. For most circumstances, this approximates a clearance of 20–50 mL/min.
  • With CRRT, the net UF rate can be adjusted as needed, according to the patient’s hemodynamic status. Electrolyte levels (particularly calcium and phosphorus) should be monitored very carefully to ensure they remain within the desired ranges. Ionized calcium levels are especially important to follow when regional citrate anticoagulation is being used.
  • Phosphate, which is predominantly intracellular, is generally poorly removed by IHD; however, in CRRT, there is continuous efflux of this anion, and significant hypophosphatemia can occur.