Derangements in Tonicity (II)

hyponatremia

-serum sodium is an excellent surrogate for serum osmolality, but not perfect

-normal serum osmolality = 290 ± 5 mOsm/kg

iso-osmolar

-lipids/proteins

pseudohyponatremia- indirect ion specific electrode

hyponatremia- hyper-osmolar

-unmeasured solutes

-”translocational” hypoNa

hyperglycemia and translocational hyponatremia

-in hyperglycemia can “adjust” [Na+] for the glucose level using a correction factor to better represent osmolality

-[Na+] decreases 1.6-2.4 meq/L for every 100 mg/dl increase in glucose above 100 mg/dl

hyponatremia- hypoosmolar

-increased free water intake

-decreased free water excretion

hypoosmolar hyponatremia- increased free water intake

-primary polydipsia, water intoxication

-ADH- absent

-Uosm: low (<100 mosm/L)

-note: increased water intake exacerbates every kind of hyponatremia

hypoosmolar hyponatremia- decreased free water excretion

-renal failure (severe)

-ADH: appropriate ADH- true volume depletion, sensed volume depletion (CHF, cirrhosis), renal/cerebral salt-wasting (rare); inappropriate ADH- SIADH or SIAD

-special cases (antidiuresis independent of ADH): low solute intake, thiazides, adrenal insufficiency (salt wasting + hypotension + increased CRH → ADH)

hyponatremia in edematous states

low solute intake

-urine must contain Osms

-Na+, K+, NH4+, conjugate anions, and urea

-”tea and toast”, beer potomania

-low protein intake = inability to generate large volume of dilute urine

-Uosm usually low (<200) compared to SIADH

thiazides

-impair urinary dilution

-do not affect medullary concentration gradient

-cause urinary loss of solutes

-enhance water permeability of CD (reabsorption), independent of ADH

-thiazide-induced hyponatremia associated with variant in SLCO2Aq, which encodes a prostaglandin transporter in the distal nephron (increased urinary prostaglandin E2)

adrenal insufficiency

diagnostic approach for hyponatremia

causes of SIADH

-tumors: bronchogenic carcinoma, mediastinal, lymphoma, pancreatic cancer, mesothelioma

-pulmonary: infections (pneumonia, lung abscess, TB), pneumothorax, broncoscopy

-CNS: head injury, neurosurgery, subdural hematoma, subarachnoid hemorrhage, meningitis

-drugs: carbamazepine, chlorpropamide, clofibrate, cyclophosphamide, desmopressin, “ecstasy”, NSAIDs, oxytocin, opiates, phenothiazines, SSRI, MAOIs, tricyclic antidepressants, vincristine

-miscellaneous: severe nausea, pain, postoperative, prolonged exercise

treatment of hyponatremia- symptomatic v asymptomatic

treatment of hyponatremia- renal failure, volume depletion, CHF, cirrhosis, SIADH

urea therapy

effectiveness of fluid restriction depends on

-urine/plasma electrolyte ratio

treating hyponatremia with fluids (hypertonic saline)

summary