Water And Sodium

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Dr Chow Yok Wai

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Water And Sodium

  1. 1. Sodium and Water Physiology <ul><li>Consider water and Na separately as regulation is independent </li></ul><ul><li>ECF  Na + </li></ul><ul><ul><li>Na + content  ECF volume </li></ul></ul><ul><ul><li>Na + concentration  ICF volume </li></ul></ul><ul><ul><ul><li>Reflects tonicity of body fluids </li></ul></ul></ul><ul><ul><ul><li>Hyponatremia  swollen cells </li></ul></ul></ul><ul><ul><ul><li>Hypernatremia  shrunken cells </li></ul></ul></ul>
  2. 2. Sodium and Water Physiology <ul><li>Thirst and release of ADH are stimulated by shrunken cells + ECF volume contraction </li></ul><ul><li>ADH is major hormone controlling water excretion </li></ul><ul><li>Water  60% of body mass </li></ul><ul><ul><li>2/3 of body water  ICF </li></ul></ul><ul><ul><li>1/3 of body water  ECF </li></ul></ul>
  3. 3. Sodium and Water Physiology <ul><li>Particles restricted to a compartment determine its volume </li></ul><ul><ul><li>Na + (and Cl, HCO3) determines ECF volume </li></ul></ul><ul><ul><li>K + (held by macromolecular anions) determines ICF volume </li></ul></ul>
  4. 4. Sodium and Water Physiology <ul><li>Water crosses cell membrane rapidly till osmolality is equal on both sides of the membrane </li></ul><ul><li>But some particles do not </li></ul><ul><ul><li>Permeability differences </li></ul></ul><ul><ul><li>Transporters </li></ul></ul><ul><ul><li>Active pumps </li></ul></ul><ul><li>Tonicity (effective osmolality) = total osmolality – urea - alcohol </li></ul>
  5. 6. Take home message <ul><li>Content of Na + determines ECF volume </li></ul><ul><li>Concentration of Na + in the ECF reflects ICF volume </li></ul>
  6. 7. Distribution of Ultrafiltrate across capillary membranes <ul><li>Movement of ultrafiltrate of plasma across capillary membranes do not cause water to shift between ECF – ICF </li></ul><ul><li>Hydrostatic pressure (HP) – Colloidal osmotic pressure (COP)  UF </li></ul><ul><li>Increase HP  venous HPT  CCF, venous obstruction </li></ul><ul><li>Support stockings increase HP </li></ul>
  7. 8. Water Physiology <ul><li>Defense of tonicity involves thirst and excretion or conservation of electrolyte free water (EFW) </li></ul><ul><li>Control of tonicity is sensitive, responding to 1-2% changes </li></ul><ul><li>Change of tonicity is synonymous with [Na + ] in plasma </li></ul><ul><ul><li>Reduction in tonicity  thirst reduction, increase EFW excretion </li></ul></ul>
  8. 9. Mechanism of excretion of EFW <ul><li>Osmolality/tonicity receptors in thirst center and ADH release center  drink more + conserve EFW from kidneys </li></ul><ul><li>Excretion of a dilute urine requires 3 steps </li></ul><ul><ul><li>Delivery of saline to thick ascending limb of loop of Henle </li></ul></ul><ul><ul><li>Separation of salt and water (reabsoprtion of NaCl without water) </li></ul></ul><ul><ul><li>Maintenance of separation (AND secretion must cease) </li></ul></ul>
  9. 10. More to remember…. <ul><li>To assess medullary hyperosmolality, measure urine osmolality after ADH acts </li></ul><ul><li>To assess ADH action, you must know the medullary osmolality </li></ul><ul><li>To assess if urine will lead to rise/fall in plasma Na, to determine [Na + ] and [K + ] in urine. Compare this sum of [e - ] urine with plasma </li></ul>
  10. 11. 0.45 Saline <ul><li> 500mls 0.9% 500mls H20 </li></ul><ul><li> -2/3 ICF, 1/3 ECF </li></ul>
  11. 12. Hyponatremia
  12. 13. Outline of major principles <ul><li>Plasma [Na + ] reflects ICF volume </li></ul><ul><li>Na + content reflects ECF volume </li></ul><ul><li>Acute Hyponatremia- What is the source of EFW? </li></ul><ul><li>Chronic Hyponatremia- Why is ADH present? </li></ul><ul><li>Basis for hyponatremia </li></ul><ul><ul><li>Source of EFW </li></ul></ul><ul><ul><li>ADH secretion to prevent EFW excretion </li></ul></ul>
  13. 14. Figure 7.1
  14. 15. Acute Hyponatremia <ul><li>3 common causes of EFW </li></ul><ul><ul><li>D5% administration as IV </li></ul></ul><ul><ul><li>Clear fluid administration </li></ul></ul><ul><ul><li>Generation of EFW by desalination when isotonic/hypotonic saline is adminstered </li></ul></ul><ul><ul><ul><li>Kidney must excrete urine that’s hypertonic to infusate </li></ul></ul></ul><ul><li>Immediate goal is to shrink expanded ICF volume </li></ul><ul><li>Hypertonic saline </li></ul>
  15. 16. Prevention <ul><li>Do not give solutions that are hypotonic to the urine if polyuria is present </li></ul><ul><li>Do not give solutions that are hypotonic to the body fluids in the oliguric patient </li></ul><ul><li>Give isotonic fluids only to replace losses and to maintain hemodynamics </li></ul><ul><li>Suspicious of good U/O as urine might be hypertonic to the infused solutions and generate EFW </li></ul>
  16. 17. Acute hyponatremia- therapy <ul><li>Correct Na + with hypertonic saline till Na + is 130mmol/l </li></ul><ul><li>Prevention of further fall of sodium </li></ul><ul><ul><li>Input </li></ul></ul><ul><ul><ul><li>If input=output with respect to Na, K and H20, then  no change in sodium concentration </li></ul></ul></ul><ul><ul><ul><li>If hypertonic urine is excreted, the same volume and same composition of hypertonic saline must be administered </li></ul></ul></ul>
  17. 18. Acute hyponatremia- therapy <ul><li>Output </li></ul><ul><ul><li>Aim is to lower [Na + + K + ] in urine so that isotonic fluids can be administered </li></ul></ul><ul><ul><li>Loop/ osmotic diuretic can render urine less hypertonic </li></ul></ul><ul><ul><li>Once ADH release is no longer present/ diminished, can then stop diuretics and plasma [Na + ] will rise </li></ul></ul>
  18. 19. Table 7.3
  19. 20. Chronic hyponatremia <ul><li>Most common electrolyte abnormality in hospitalised patients </li></ul><ul><li>Most pt is asymp as adaptive responses have taken place (brain cells have normalised ICF volume) </li></ul><ul><li>Danger is too rapid rise in plasma [Na + ]  central pontine myelinosis </li></ul><ul><li>To develop hyponatremia, source of EFW + excretion/release of ADH must be present </li></ul>
  20. 21. Chronic hyponatremia <ul><li>ADH is released when ECF volume is low </li></ul><ul><li>Deducing whether ECF volume is contracted </li></ul><ul><ul><li>Loss of Na via renal cause </li></ul></ul><ul><ul><ul><li>Diuretic </li></ul></ul></ul><ul><ul><ul><li>Renal salt wasting </li></ul></ul></ul><ul><ul><ul><li>Osmotic agents (glucose) </li></ul></ul></ul><ul><ul><ul><li>Rate of K + should be examined </li></ul></ul></ul><ul><ul><ul><ul><li>Low urine [K + ] + renal Na + loss + ECF contraction  low aldosterone bioactivity </li></ul></ul></ul></ul><ul><ul><ul><ul><li>High urine [K + ] + renal Na + loss + ECF contraction  abnormal loss occurred in PCT, loop of henle, early DCT </li></ul></ul></ul></ul><ul><ul><li>Loss of Na via non renal cause </li></ul></ul><ul><ul><ul><li>GIT </li></ul></ul></ul><ul><ul><ul><li>Skin </li></ul></ul></ul>
  21. 22. Chronic hyponatremia <ul><li>‘ effective’ ECF volume is decreased (maldistribution) </li></ul><ul><ul><li>Edema states </li></ul></ul><ul><ul><li>Congestive cardiac failure </li></ul></ul>
  22. 24. Hypernatremia
  23. 25. Outline of major principles <ul><li>Hypernatremia is not a disease </li></ul><ul><ul><li>Look for its cause and underlying disease </li></ul></ul><ul><li>Hypernatremia </li></ul><ul><ul><li> ICF volume contraction </li></ul></ul><ul><ul><li>Brain is most susceptible  CNS hemorrhage </li></ul></ul><ul><li>Thirst </li></ul><ul><ul><li>Pt will not permit hypernatremia if thirst mechanism is intact </li></ul></ul>
  24. 26. Outline of major principles <ul><li>Urine Osmolality </li></ul><ul><ul><li>Diabetes Insipidus </li></ul></ul><ul><ul><ul><li>Large urine amount </li></ul></ul></ul><ul><ul><ul><li>Low osmolar urine </li></ul></ul></ul><ul><ul><li>Osmotic/pharmacological diuresis </li></ul></ul><ul><ul><ul><li>Large urine amount </li></ul></ul></ul><ul><ul><ul><li>Slightly hyperosmolar urine </li></ul></ul></ul><ul><ul><li>Non renal water loss without water intake </li></ul></ul><ul><ul><ul><li>Small urine amount </li></ul></ul></ul><ul><ul><ul><li>Maximally hyperosmolar urine </li></ul></ul></ul>
  25. 27. Outline of major principles <ul><li>Hypernatremia </li></ul><ul><ul><li>Na + gain  uncommon </li></ul></ul><ul><ul><li>EFW loss </li></ul></ul>
  26. 28. Etiology of Hypernatremia <ul><li>True [Na + ] plasma  152 mmol/l </li></ul><ul><li>6-7% non aqueous volume (lipids, proteins) </li></ul><ul><li>Hypernatremia is almost always d/t water loss in the present of a thirst defect </li></ul><ul><li>4 questions to ask </li></ul><ul><ul><li>What’s the ECF volume? (Na + gain) </li></ul></ul><ul><ul><li>Body weight change? (H2O gain) </li></ul></ul><ul><ul><li>Normal thirst response? </li></ul></ul><ul><ul><li>Normal renal response? (ADH response) </li></ul></ul>
  27. 29. Approach to pt with hypernatremia
  28. 30. Hypernatremia due to water loss <ul><li>Non renal water loss </li></ul><ul><ul><li>Respiratory tract, skin, fever, hyperventilation, GIT (Hypotonic) </li></ul></ul><ul><li>Renal water loss </li></ul><ul><ul><li>Usually a/w thirst defect </li></ul></ul><ul><ul><li>Usually a/w polyuria </li></ul></ul><ul><ul><li>Usual causes </li></ul></ul><ul><ul><ul><li>Diabetes Insipidus </li></ul></ul></ul><ul><ul><ul><li>Osmotic diuresis </li></ul></ul></ul>
  29. 31. Central DI <ul><li>d/t lack of ADH </li></ul><ul><ul><li>ADH is synthesized from paraventricular and supraoptic nuclei </li></ul></ul><ul><ul><li>ADH then transported via axonal flow to posterior pituitary </li></ul></ul><ul><li>CNS disorder </li></ul><ul><li>Polydypsia, polyuria </li></ul><ul><li>Large urine amount (3-20L depending on GFR) </li></ul><ul><li>Hypo-osmolar urine (< 150 mosm/l) </li></ul><ul><li>ECF normal </li></ul><ul><li>Hypernatremia </li></ul><ul><li>Hypernatremia worsens and polyuria occurs with judicious water administration </li></ul><ul><li>ADH administration raises urine osmolality </li></ul>
  30. 32. Nephrogenic DI <ul><li>ADH fails to act </li></ul><ul><ul><li>Failure to increase water permeability of collecting duct </li></ul></ul><ul><li>Loss of medullary hypertonicity </li></ul><ul><ul><li>Medullary interstitial defect or infirtrate </li></ul></ul>
  31. 33. Treatment of water deficit <ul><li>Stop ongoing Water Loss </li></ul><ul><ul><li>Rectify ADH deficiency </li></ul></ul><ul><ul><li>Stop osmotic agent </li></ul></ul><ul><li>Replacing Water Deficit </li></ul><ul><ul><li>D5%- ideal EFW administration </li></ul></ul><ul><ul><li>½ NS- not appropriate if polyuria is present and [Na + ] in urine < in IVD </li></ul></ul><ul><ul><ul><li>1L 1/2NS </li></ul></ul></ul><ul><ul><ul><ul><li> 500mls EFW available </li></ul></ul></ul></ul><ul><ul><ul><ul><li> 1/3 stay in ECF, 2/3 goes into ICF </li></ul></ul></ul></ul><ul><ul><li>More hypotonic solutions can be used but hemolysis is a risk </li></ul></ul>
  32. 34. Calculation of Water Deficit- ICF <ul><li>ICF  assess current vs expected ICF and ECF volumes </li></ul><ul><ul><ul><li>70kg pt, sodium increase 140  160mmol/l, ECF normal on physical examination, usual ICF 30L and ECF volume 15L. </li></ul></ul></ul><ul><ul><ul><ul><li>No of effective osmoles in ICF: </li></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>ICF volume X 2(plasma [Na + ]) </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>8400 mOsm </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>After water loss, assume no change in effective osmoles in ICF </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>New effective osmolality is 320 (160X2) </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>New ICF volume  8400/320=26.25L </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Water deficit  3.75L </li></ul></ul></ul></ul></ul>
  33. 35. Short form of Calculation of ICF Water deficit <ul><li>ICF volume (normal) X effective osmoles (normal) </li></ul><ul><li>= </li></ul><ul><li>ICF volume (abnormal) X effective osmoles (abnormal) </li></ul>
  34. 36. Calculation of Water Deficit- ECF <ul><li>Change in ECF volume- </li></ul><ul><ul><li>Not reflected by plasma [Na + ] </li></ul></ul><ul><ul><li>Reflected by clinical assessment of vascular and interstitial volume </li></ul></ul><ul><ul><li>Plasma [Na + ] X estimated ECF volume </li></ul></ul><ul><ul><li>140X15L= 2100mmol </li></ul></ul><ul><ul><li>160X15L= 2400 mmol </li></ul></ul><ul><ul><li>300mmol of Na + is needed to achieve Na + balance </li></ul></ul>

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