Approach to Dialysis

Approach to Dialysis is a topic covered in the Washington Manual of Medical Therapeutics.

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Treatment

  • Modalities
    • Renal replacement therapy is indicated when conservative medical management is unable to control the metabolic derangements of kidney disease. This applies to the acute and chronic settings. Common acute indications include hyperkalemia, metabolic acidosis, and volume overload that are refractory to medical management. Uremic encephalopathy or pericarditis, as well as certain intoxications (methanol, ethylene glycol, or salicylates), can all be indications to initiate dialytic therapy acutely. In the chronic setting, renal replacement therapy is typically begun before the worsening of the metabolic or nutritional status of the patient.
    • Dialysis works by solute diffusion and water transport across a selectively permeable membrane. In hemodialysis, blood is pumped counter-currently to a dialysis solution within an extracorporeal membrane. This can be performed intermittently (3–4 hours during the day) or in a continuous 24-hour fashion depending on hemodynamic stability or goals of therapy. PD uses the patient’s peritoneal membrane as the selective filter, and dialysis fluid is instilled into the peritoneal cavity.
    • Transplantation offers the best long-term survival and most completely replaces the filtrative and endocrine functions of the native kidney. However, it carries the risks that accompany long-term immunosuppression.
  • Diffusion
    • The selectively permeable membrane contains pores that allow electrolytes and other small molecules to pass by diffusion while holding back larger molecules and cellular components of the blood. Movement relies on the molecular size and the concentration gradient. Potassium, urea, Cr, and other waste products of metabolism pass into the dialysis solution while alkaline buffers (bicarbonate or lactate) enter the blood from the dialysis solution.
  • Ultrafiltration/convection
    • Removal of volume is termed ultrafiltration. It can be achieved in hemodialysis via a transmembrane hydrostatic pressure that removes excess fluid from the blood compartment. In PD, water follows its osmotic gradient into the relatively hyperosmolar dialysis solution (usually with dextrose providing the osmotic driving force).
    • As water is removed from the vascular compartment, it drags along solute in proportion to its concentration in the blood. This usually accounts for only a small fraction of the total clearance but can be significantly increased if a physiologic “replacement fluid” is infused into the patient concurrently to prevent hypovolemia, a process termed convective clearance. This strategy is frequently employed by continuous hemodialysis modalities.

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Treatment

  • Modalities
    • Renal replacement therapy is indicated when conservative medical management is unable to control the metabolic derangements of kidney disease. This applies to the acute and chronic settings. Common acute indications include hyperkalemia, metabolic acidosis, and volume overload that are refractory to medical management. Uremic encephalopathy or pericarditis, as well as certain intoxications (methanol, ethylene glycol, or salicylates), can all be indications to initiate dialytic therapy acutely. In the chronic setting, renal replacement therapy is typically begun before the worsening of the metabolic or nutritional status of the patient.
    • Dialysis works by solute diffusion and water transport across a selectively permeable membrane. In hemodialysis, blood is pumped counter-currently to a dialysis solution within an extracorporeal membrane. This can be performed intermittently (3–4 hours during the day) or in a continuous 24-hour fashion depending on hemodynamic stability or goals of therapy. PD uses the patient’s peritoneal membrane as the selective filter, and dialysis fluid is instilled into the peritoneal cavity.
    • Transplantation offers the best long-term survival and most completely replaces the filtrative and endocrine functions of the native kidney. However, it carries the risks that accompany long-term immunosuppression.
  • Diffusion
    • The selectively permeable membrane contains pores that allow electrolytes and other small molecules to pass by diffusion while holding back larger molecules and cellular components of the blood. Movement relies on the molecular size and the concentration gradient. Potassium, urea, Cr, and other waste products of metabolism pass into the dialysis solution while alkaline buffers (bicarbonate or lactate) enter the blood from the dialysis solution.
  • Ultrafiltration/convection
    • Removal of volume is termed ultrafiltration. It can be achieved in hemodialysis via a transmembrane hydrostatic pressure that removes excess fluid from the blood compartment. In PD, water follows its osmotic gradient into the relatively hyperosmolar dialysis solution (usually with dextrose providing the osmotic driving force).
    • As water is removed from the vascular compartment, it drags along solute in proportion to its concentration in the blood. This usually accounts for only a small fraction of the total clearance but can be significantly increased if a physiologic “replacement fluid” is infused into the patient concurrently to prevent hypovolemia, a process termed convective clearance. This strategy is frequently employed by continuous hemodialysis modalities.

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