Sodium-Disorders

Sodium-Disorders - Sodium Disorders: Hyponatremia William...

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Sodium Disorders: Hyponatremia William Harper, MD, FRCPC Assistant Professor of Medicine McMaster University
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P Na (mEq/L) 130 135 140 145 0 5 ADH (pM) Thirst ↓ ECFv Normal Serum [Na] (135-145 mEq/L) Closely Guarded
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1) Complete DI 2) Defective osmoreceptor, normal AVP release to ECFv contraction 3) High-set osmoreceptor: AVP release is sluggish/delayed 4) AVP release at normal Posm but subnormal in amount What is Appropriate Urine Concentration?
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Osmolality Plasma Osmolality: Posm = 2 (Na) + glucose + urea Normal = 2 (140) + 5 + 5 = 290 (275-290 mM) Urine Osmolality: Normal: 400-500 mM » Maximal dilution 50-100 mM (U SG 1.002-1.003) » Maximal concentration 900-1200 mM ( U SG 1.030-1.040) Concentrated Urine: > 500 mM (at least!), U SG > 1.017 i.e. U OSM > P OSM is not enough to R/O Diabetes Insipidus
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Urine Specific Gravity U SG Estimates solute concentration of urine on basis of weight as compared with an equal volume of distilled water Normal Posm is 0.8-1.0% heavier than water so P SG = 1.008-1.010 Each ↑ in U OSM 30-35 mM U SG by 0.1% (0.001) Therefore, U SG of 1.010 ~ U OSM 300-350 mM Larger MW urinary OSM (glucose, radiocontrast, carbenicillin) if present will falsely elevate U SG Nothing falsely lowers U SG
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Hyponatremia Serum OSM Low Normal High Hypotonic Hyponatremia ECFv * Low Normal High Hyperglycemia Mannitol Marked hyperlipidemia (lipemia, TG >35mM) Hyperproteinemia (Multiple myeloma) CHF Cirrhosis Nephrosis Hypothyroidism AI SIADH Reset Osmostat Water Intoxication 1° Polydipsia TURP post-op Renal loss (U Na > 20) Diuretics Thiazide K-sparing ACE-I, ARB IV RTA, Hypoaldo Cerebral salt wasting Extra-renal loss (U Na <10) Bleeding Burns GI (N/V, diarrhea) Pancreatitis * Note: all have ↑ADH SIADH: inappropriate Rest: appropriate
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Rx Hyponatremia Na deficit = 0.6 x wt(kg) x (desired [Na] - actual [Na]) (mmol) When do you need to Rx quickly? Acute (<24h) severe (< 120 mEq/L) Hyponatremia Prevent brain swelling or Rx brain swelling Symptomatic Hyponatremia (Seizures, coma, etc.) Alleviate symptoms “Quickly”: 3% NS, 1-2 mEq/L/h until: Symptoms stop 3-4h elapsed and/or Serum Na has reached 120 mEq/L Then SLOW down correction to 0.5 mEq/L/h with 0.9% NS or simply fluid restriction. Aim for overall 24h correction to be < 10-12 mEq/L/d to prevent myelinolysis
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Rx Hyponatremia (Example) Na deficit (mmol) = 0.6 x wt(kg) x (desired [Na] - actual [Na]) 60 kg women, serum Na 107, seizure recalcitrant to benzodiazepines. Na defecit = 0.6 x (60) x (120 – 107) = 468 mEq Want to correct at rate 1.5 mEq/L/h: 13/1.5 = 8.7h 468 mEq / 8.7h = 54 mEq/h 3% NaCl has 513 mEq/L of Na 54 mEq/h = x 513 mEq 1L x = rate of 3% NaCl = 105 cc/h over 8.7h to correct serum Na to 120 mEq/h Note: Calculations are always at best estimates, and anyone getting
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Sodium-Disorders - Sodium Disorders: Hyponatremia William...

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