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

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Presentation on theme: "Sodium flux during dialysis Sushrut S. Waikar, MD, MPH Renal Division Brigham and Women’s Hospital, Harvard Medical School."— Presentation transcript:

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 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 20% of heart’s output is directed to the kidneys, which filter ~180L plasma per day Kidneys also regulate water and salt balance Kidneys also regulate water and salt balance

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

4 Hemodialysis Kidney function can be partially replaced by hemodialysis Kidney function can be partially replaced by hemodialysis First performed by Wilhelm Kolff in early 1940s First performed by Wilhelm Kolff in early 1940s Procedure is typically performed for 4 hours thrice weekly (e.g., Monday, Wednesday, Friday) 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 Blood flows into tubing that divides into thousands of parallel hollow fibers Each fiber is a semipermeable membrane Each fiber is a semipermeable membrane Outside of the fibers runs the “dialysate” solution Outside of the fibers runs the “dialysate” solution

6 Clearance during dialysis Convection Convection Negative pressure applied, water and dissolved small solutes (< 40 kDa) pass across membrane into the dialysate fluid which is then discarded Negative pressure applied, water and dissolved small solutes (< 40 kDa) pass across membrane into the dialysate fluid which is then discarded Diffusion Diffusion Solutes travel across membrane down concentration gradient Solutes travel across membrane down concentration gradient Blood and dialysate flow in opposite directions, maximizing 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 Get rid of the water that was ingested and produced (during metabolism) since the last procedure – usually 3 liters Done by convection Done by convection Get rid of salt (sodium chloride, potassium chloride) Get rid of salt (sodium chloride, potassium chloride) Maintain acid-base balance Maintain acid-base balance Get rid of nitrogenous waste products (urea) Get rid of nitrogenous waste products (urea)

8 Plasma versus dialysate PlasmaDialysate Sodium (meq/L) 140140 Potassium (meq/L) 5.03.0 Chloride (meq/L) 114110 Bicarbonate (meq/L) 2035 Calcium (meq/L) 2.5 Urea nitrogen (mg/dL) 400

9

10 Urea 70 mg/dL Urea 30 mg/dL

11 Sodium 142 meq/L Sodium ? meq/L

12 Sodium 142 meq/L Sodium ? meq/L Sodium in dialysate: 140 meq/L

13 Sodium 142 meq/L Sodium ? meq/L Sodium in dialysate: 140 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)

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

15 Sodium 142 meq/L Sodium ? meq/L

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

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

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

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


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