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Electrolyte Free Water Clearance

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this is an 18 slide subset of the way too long lecture on sodium called Hyponatremia Dreadnaught

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Electrolyte Free Water Clearance

  1. 1. True hyponatremia <ul><li>Hyponatremia occurs when water intake exceeds water excretion. </li></ul>
  2. 2. True hyponatremia <ul><li>Hyponatremia does not occur when sodium excretion exceeds sodium intake. </li></ul>Negative salt balance causes hypovolemia
  3. 3. <ul><li>If a person drinks more water than the kidney is capable of clearing the excess water will dilute the plasma. </li></ul>Causes of hyponatremia: Increased intake <ul><li>To exceed the maximal renal clearance of water an adult needs to drink about 18 liters a day. </li></ul>clearing
  4. 4. Water clearance <ul><li>Clearance as a general concept: </li></ul><ul><ul><li>The clearance of any substance is the volume of blood cleared of that substance in a set unit of time. </li></ul></ul><ul><li>Water clearance </li></ul><ul><ul><li>Total water clearance is equal to urine output. Not a useful concept. </li></ul></ul>
  5. 5. Free water clearance and soup <ul><li>Imagine urine divided into two components </li></ul><ul><ul><li>A solute component containing all of the solute at the same osmolality as plasma. </li></ul></ul><ul><ul><ul><li>Loss of this component does not change plasma osmolality </li></ul></ul></ul><ul><ul><ul><li>Ladle of soup </li></ul></ul></ul><ul><ul><li>A free water component providing the balance of the volume. </li></ul></ul><ul><ul><ul><li>Loss of this solute free water will change serum osmolality. </li></ul></ul></ul><ul><ul><ul><li>Boiling off water from soup </li></ul></ul></ul>In regards to sodium all that matters is the free water component
  6. 6. Free water clearance 0.5 liter free water Solute component (plasma osmolality 284 mOsm/kg) Use clearance to calculate the osmolar clearance 1 liter 142 mOsm/Kg 0.5 liter 284 mOsm/Kg
  7. 7. Free water clearance Solute component (Solute Clearance) ? 0.5 liter Zero mOsm/Kg Free water component (Free water Clearance) The free water component equals urine volume minus the solute component 1 liter 142 mOsm/Kg 0.5 liter 284 mOsm/Kg
  8. 8. Free water clearance 0.5 liter 568 mOsm/Kg 1 liter 284 mOsm/Kg -0.5 liter 568 mOsm/Kg
  9. 9. Free water clearance Solute component (plasma osmolality 284 mOsm/kg) Use clearance to calculate the osmolar clearance 1 liter 284 mOsm/Kg 0.5 liter 568 mOsm/Kg
  10. 10. Free water clearance Solute component (Solute Clearance) ? Free water component (Free water Clearance) 0.5 liter 568 mOsm/Kg – 0.5 liter Zero mOsm/Kg 1 liter 284 mOsm/Kg
  11. 11. Free water clearance: Implications Dilute urine increases serum osmolality Concentrated urine de-creases serum osmolality Dilute urine Solute free water Concentrated urine Negative free water Na + Na +
  12. 12. Free water clearance: The math
  13. 13. Free water clearance: Math Examples
  14. 14. Electrolyte free water clearance <ul><li>Osmolality doesn’t cause problems, rather tonicity causes changes in cell volume which cause clinical syndromes. </li></ul><ul><li>So free water clearance must be refined to measure clinically significant changes in tonicity. </li></ul>
  15. 15. Electrolyte free water clearance <ul><li>Osmotically active particles (those that contribute to tonicity): </li></ul><ul><ul><li>Sodium </li></ul></ul><ul><ul><li>Potassium </li></ul></ul><ul><ul><li>Albumin, calcium and others </li></ul></ul><ul><li>Sodium is the dominant osmotically active solute of serum to the point that others can be ignored. </li></ul><ul><li>Urine has a significant potassium content so in urine sodium and potassium are equal partners in determining urinary tonicity. </li></ul>
  16. 16. Electrolyte free water clearance <ul><li>Convert the free water clearance calculation to electrolyte free water clearance </li></ul><ul><ul><li>Substitute urine osmolality with the sum urine Na + K </li></ul></ul><ul><ul><li>Substitute serum osmolality with serum sodium </li></ul></ul>Free water clearance Electrolyte free water clearance
  17. 17. Electrolyte free water clearance: CHF vs. SIADH <ul><li>Heart Failure </li></ul><ul><ul><li>Urine Osmolality: 800 </li></ul></ul><ul><ul><li>Serum Osmolality: 270 </li></ul></ul><ul><ul><li>Urine Volume: 800 </li></ul></ul><ul><ul><li>Urine Na: 5 </li></ul></ul><ul><ul><li>Urine K: 40 </li></ul></ul><ul><ul><li>Serum Na: 125 </li></ul></ul><ul><li>SIADH </li></ul><ul><ul><li>Urine Osmolality: 800 </li></ul></ul><ul><ul><li>Serum Osmolality: 270 </li></ul></ul><ul><ul><li>Urine Volume: 800 </li></ul></ul><ul><ul><li>Urine Na: 125 </li></ul></ul><ul><ul><li>Urine K: 40 </li></ul></ul><ul><ul><li>Serum Na: 125 </li></ul></ul>
  18. 18. Etiology of hyponatremia <ul><li>Hyponatremia occurs when water intake exceeds water excretion. </li></ul><ul><li>Hyponatremia occurs when water intake exceeds electrolyte free water clearance. </li></ul>Ingestion > EFW clearance

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