Hemodialysis

Hemodialysis is a topic covered in the Washington Manual of Medical Therapeutics.

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General Principles

  • Hemodialysis is by far the most commonly used 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 flows) and using 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 usually necessitate anticoagulation (with systemic heparin or regional citrate) to prevent the filter from clotting. Continuous modalities require specialized nursing and an intensive care setting.
    • The most frequently employed form of CRRT is continuous venovenous hemodiafiltration (CVVHDF).
    • In CVVHDF, blood is slowly pumped counter-currently to a dialysis solution (diffusion), and a replacement fluid (a “cleansed” physiologic solution devoid of uremic toxins) is infused into the circuit to balance most of the ultrafiltrate (convection).
  • Sustained low-efficiency dialysis is a hybrid form of IHD and CRRT used in an intensive care setting. Intermediate blood flows lower the clotting risk if anticoagulation is not used while intermediate treatment lengths (8–10 hours) still allow for adequate clearances. Patients also spend a significant portion of the day off the machine to allow for nonbedside 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 known, although a thrice-weekly schedule is likely adequate.
    • Adequacy is assessed by calculating the clearance of BUN, which serves as a surrogate marker of the “uremic factors.” The urea reduction ratio can be calculated by the following:Descriptive text is not available for this image

      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 thrice-weekly treatments.3
    • Clearance is measured differently in CRRT where dialytic therapy is taking place around the clock, 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 and then 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. One must be vigilant in checking electrolyte levels (particularly calcium and phosphorus) 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.

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General Principles

  • Hemodialysis is by far the most commonly used 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 flows) and using 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 usually necessitate anticoagulation (with systemic heparin or regional citrate) to prevent the filter from clotting. Continuous modalities require specialized nursing and an intensive care setting.
    • The most frequently employed form of CRRT is continuous venovenous hemodiafiltration (CVVHDF).
    • In CVVHDF, blood is slowly pumped counter-currently to a dialysis solution (diffusion), and a replacement fluid (a “cleansed” physiologic solution devoid of uremic toxins) is infused into the circuit to balance most of the ultrafiltrate (convection).
  • Sustained low-efficiency dialysis is a hybrid form of IHD and CRRT used in an intensive care setting. Intermediate blood flows lower the clotting risk if anticoagulation is not used while intermediate treatment lengths (8–10 hours) still allow for adequate clearances. Patients also spend a significant portion of the day off the machine to allow for nonbedside 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 known, although a thrice-weekly schedule is likely adequate.
    • Adequacy is assessed by calculating the clearance of BUN, which serves as a surrogate marker of the “uremic factors.” The urea reduction ratio can be calculated by the following:Descriptive text is not available for this image

      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 thrice-weekly treatments.3
    • Clearance is measured differently in CRRT where dialytic therapy is taking place around the clock, 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 and then 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. One must be vigilant in checking electrolyte levels (particularly calcium and phosphorus) 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.

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