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Sodium flux during dialysis

Sodium flux during dialysis. Sushrut S. Waikar, MD, MPH Renal Division Brigham and Women’s Hospital, Harvard Medical School. Background. Kidneys filter the blood, clearing it of waste products 20% of heart’s output is directed to the kidneys, which filter ~180L plasma per day

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Sodium flux during dialysis

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  1. Sodium flux during dialysis Sushrut S. Waikar, MD, MPH Renal Division Brigham and Women’s Hospital, Harvard Medical School

  2. Background • Kidneys filter the blood, clearing it of waste products • 20% of heart’s output is directed to the kidneys, which filter ~180L plasma per day • Kidneys also regulate water and salt balance

  3. Background • Loss of kidney function leads to death within days • Excessive buildup of nitrogenous waste, acid, potassium, sodium, water • Kidney function can be partially replaced by hemodialysis (~300,000 patients in United States)

  4. Hemodialysis • Kidney function can be partially replaced by hemodialysis • First performed by Wilhelm Kolff in early 1940s • Procedure is typically performed for 4 hours thrice weekly (e.g., Monday, Wednesday, Friday)

  5. Hemodialysis procedure • Blood flows into tubing that divides into thousands of parallel hollow fibers • Each fiber is a semipermeable membrane • Outside of the fibers runs the “dialysate” solution

  6. Clearance during dialysis • Convection • Negative pressure applied, water and dissolved small solutes (< 40 kDa) pass across membrane into the dialysate fluid which is then discarded • Diffusion • Solutes travel across membrane down concentration gradient • Blood and dialysate flow in opposite directions, maximizing concentration gradient

  7. Goals of a dialysis procedure • Get rid of the water that was ingested and produced (during metabolism) since the last procedure – usually 3 liters • Done by convection • Get rid of salt (sodium chloride, potassium chloride) • Maintain acid-base balance • Get rid of nitrogenous waste products (urea)

  8. Plasma versus dialysate

  9. Urea 70 mg/dL Urea 30 mg/dL

  10. Sodium 142 meq/L Sodium ? meq/L

  11. Sodium in dialysate: 140 meq/L Sodium 142 meq/L Sodium ? meq/L

  12. Sodium in dialysate: 140 meq/L Sodium 142 meq/L Sodium ? meq/L CONVECTIVE CLEARANCE: water and small solutes are removed by negative pressure across membrane DIFFUSIVE CLEARANCE: sodium moves down its concentration gradient (in either direction, depending on plasma concentration; dialysate sodium usually fixed)

  13. Factors that influence sodium flux • Dialysate sodium concentration = 140 meq/L • Plasma sodium concentration = 142 meq/L, changes during procedure • Convection and diffusion occur simultaneously • Gibbs Donnan effect • Large negatively charged proteins NOT able to pass through membrane; electroneutrality must be maintained • Sodium and other cations less “permeable” than anions like chloride, bicarbonate

  14. Sodium 142 meq/L Sodium ? meq/L

  15. Factors that influence Na in plasma • Plasma sodium concentration itself has many determinants, not just mass balance of sodium and water • Also affected by potassium mass balance • Water and sodium flux in various body compartments

  16. Distrbution of salt + water in body • Water in a 70 kg man • 60% water = 42 liters • Intracellular: 28 liters • Extracellular: 14 liters • Interstitial = 10 liters • PLASMA VOLUME = 4 liters • Sodium is the primary extracellular cation, 140 meq / liter • Intracellular sodium

  17. Dialysis membranes • Hollow fibers ~12,000 in parallel • 20 – 24cm length, diameter 180 to 220 um, 6 to 15 um thickness • Pores: avg diameter 30 Angstroms, 10^9 in number • Old: Cellobiose (saccharide) • New: synthetic membranes (e.g., polysulfone, polyamide, polymethylmethacrylate) • Manufactured polymers classified as thermoplastics

  18. Dialysis procedure • Blood flow ~ 400 ml/min • 40% red blood cells, 60% plasma • Plasma: 93% water, 7% protein and lipids • Dialysate flow ~ 800 ml/min • Typically 2-4 liters of “ultrafiltration” (volume removed during 4h procedure)

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