Derangements in Tonicity (I)

why does tonicity matter?

1) cells/enzymes function optimally at physiologic osmolality- cytoplasm contains particles in solution

2) rapid shifts of water in/out of cells causes injury (especially neurologic)

definitions- osmole, osmolality, osmolarity, meq, tonicity

osmolarity v tonicity

-osmolarity describes solution (units of concentration)

-tonicity describes behavior of solution (unitless)

-hypoosmolar solutions are always hypotonic

-isoosmolar and hyperosmolar solutions may be hypo, iso, or hypertonic

IV fluid therapy- fluid, sodium concentration, osmolarity, tonicity, use

why does tonicity matter?- increase and decrease in plasma sodium

intra/extracellular osmolality key concepts

-intracellular osmolality = extracellular osmolality (at steady state)

-outside world (illness, fluids, diet, etc) affects extracellular osmolality

-changes in extracellular osmolality rapidly equilibrate with intracellular by the movement of water- raising extracellular osmolality → water exits cells; lowering extracellular osmolality → water enters cells

-intracellular osmolality is largely dependent on potassium

-extracellular osmolality is largely dependent on sodium

osmolality and serum sodium

-osmolality depends on total body exchangeable sodium + potassium

-serum sodium is an excellent (though imperfect) surrogate for osmolality

osmolality and relationship to sodium and potassium in serum and urine

interpreting lab tests- serum osmolality, serum sodium concentration, urine sodium, urine osmolality

why does tonicity matter?- cell shrinking v swelling

morbidity of hyponatremia

-mild chronic hyponatremia is associated with falls, unsteadiness, and attention deficits

osmotic demyelination syndrome

-complication of acutely raising serum osmolality (usually in setting of correction of severe hypoosmolality/hyponatremia)

-rare but morbid: mortality 40-50%

-biphasic:

1) initially- encephalopathy, seizures → recovery

2) days later- dysarthria and dysphasia, flaccid quadriparesis, oculomotor abnormalities, “locked-in syndrome”

hypernatremia

-always implies hyperosmolality

-three components to consider: water intake, water loss, solute intake

hypernatremia pathway

renal water loss- diabetes

-diabetes mellitus (“sweet”) = osmotic diuresis

-diabetes insipidus (“tasteless”) = water diuresis

diabetes insipidus

hypernatremia diagnostic approach

adaptation to hypernatremia

-chronic hyperNa leads to intracellular retention of “idiogenic osmoles”

-idiogenic osmoles raise intracellular osmolality to limit cell water loss

-can be generated as early as 4 hours after acute hypertonic challenge

correction of hypernatremia

correction of hypernatremia in infants

-infants at risk of cerebral edema and seizures with rapid correction of hypernatremia/dehydration

-seizures virtually nonexistent if corrected <0.5 mEq/L/h

correction of hypernatremia in adults

treatment and limits of correction of severe hypernatremia

summary