Transcript 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
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20% of heart’s output is
directed to the kidneys,
which filter ~180L plasma
per day
Kidneys also regulate
water and salt balance
Background
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Loss of kidney function
leads to death within
days
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Excessive buildup of
nitrogenous waste, acid,
potassium, sodium, water
Kidney function can be
partially replaced by
hemodialysis (~300,000
patients in United States)
Hemodialysis
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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)
Hemodialysis procedure
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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
Clearance during dialysis
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Convection
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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
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Goals of a dialysis procedure
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Get rid of the water that was ingested and
produced (during metabolism) since the last
procedure – usually 3 liters
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Done by convection
Get rid of salt (sodium chloride, potassium
chloride)
Maintain acid-base balance
Get rid of nitrogenous waste products (urea)
Plasma versus dialysate
Plasma
Dialysate
Sodium (meq/L)
140
140
Potassium (meq/L)
5.0
3.0
Chloride (meq/L)
114
110
Bicarbonate (meq/L)
20
35
Calcium (meq/L)
Urea nitrogen (mg/dL)
2.5
40
0
Urea 70
mg/dL
Urea 30
mg/dL
Sodium 142
meq/L
Sodium ?
meq/L
Sodium in dialysate: 140 meq/L
Sodium 142
meq/L
Sodium ?
meq/L
Sodium in dialysate: 140 meq/L
Sodium 142
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)
Sodium ?
meq/L
Factors that influence sodium flux
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Dialysate sodium concentration = 140 meq/L
Plasma sodium concentration = 142 meq/L, changes
during procedure
Convection and diffusion occur simultaneously
Gibbs Donnan effect
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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 142
meq/L
Sodium ?
meq/L
Factors that influence Na in plasma
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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
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Distrbution of salt + water in body
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Water in a 70 kg man
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60% water = 42 liters
Intracellular: 28 liters
 Extracellular: 14 liters
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Interstitial = 10 liters
PLASMA VOLUME = 4 liters
Sodium is the primary extracellular cation, 140
meq / liter
Intracellular sodium
Dialysis membranes
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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
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Old: Cellobiose (saccharide)
New: synthetic membranes (e.g., polysulfone,
polyamide, polymethylmethacrylate)
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Manufactured polymers classified as thermoplastics
Dialysis procedure
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Blood flow ~ 400 ml/min
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40% red blood cells, 60% plasma
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Plasma: 93% water, 7% protein and lipids
Dialysate flow ~ 800 ml/min
Typically 2-4 liters of “ultrafiltration” (volume
removed during 4h procedure)