15 Na Disorders

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• both results lead to brain symptoms

Hyponatremia

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Plasma osmolality

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• albumin minor contributor, not in equation, more important for oncotic pressure

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• substances interfere with Na measurement

• triglycerides

• post-TURP

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• low osmolality: unknown cause

• Low usine osm: post TURP, beer potomonia

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Urinary Sodium

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• in reality, no normal levels because varies

• intake equals excretion

• urine Na < 10: extrarenal including CHF, cirrhosis, nephrotic syndrome

• urine > 20: renal including AKI, CKD

• Patients with SIADH are typically euvolemic; therefore, urine sodium concentration is typically elevated (>40 mEq/L), unlike in patients with hypovolemia.

Urinary Osmolality

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• ADH controls above 3 tests

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• if body responding appropirately

• urinary Na may vary with dietary intake

Causes

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HF

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• high Uosm because ADH high

Renal Failure

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• renal failure: concentrated urine even at baseline. Can't excrete water

Diuretics

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• hyponatremia common with thiazides

loop diuretic effect:

• decreased Na absorption, increased osm at CD, decreased Na/water absorption

• interstitial high osm eliminated, lower driving force to remove water

• result: very hard to reabsorb water and become hyponatremic

thiazide:

• Na blocked, increased osm at CD, decreased water/Na absorption

• medullary osm intact: continue to maintain ability to absorb free water

• result: excrete Na but absorb water = hyponatremia

ADH

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• reason why athletes drink Gatorade and not water

• hypothyroidism: high ADH with low thyroid

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• no crackles, ankle edema

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• stroke, brain bleeds, tumor

• any kind of pulmonary diseases, small cell lung cancer

• Inappropriately wet head: cyclophosphamide can cause hyponatremia due to SIADH

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• clinical euvolemia: absence of signs

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Psychogenic

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Diets

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• common theme: little Na ingestion

• kidney must maintain minimum osm

• pt on restricted diet can only excrete 10 water

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Summary

Volume and Osm

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• hypervolemic: physical exam signs

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• low Uosm: kidney response normal

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• measure UNa to differentiate

ADH and Osm

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• red

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Treatment

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• acute hyponatremia: correct as fast as can (e.g. surgery causes low Na)

• chronic hyponatremia: correct slow

• high risk: alcoholics, liver disease, malnutritioned, hypokalemia

• 10 meq correction 1st day

Hypernatremia

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DI

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• hypernatremia happen in central lesion

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• won't raise bp

• thiazide, endomethacin (NSAID), amiloride

Treatment

• calculate free water deficit

$$Water\ deficit = \frac{[Na]-140}{140} \times TBW$$