Flashcards in Week 2: Disorders of water and serum sodium concentration: polyuria and hypernatremia Deck (16):
Distinguish between abnormal total body water balance and disorders of total body sodium balance.
-Abn TBW balance: causes hypo or hypernatremia. Regulated by ADH
-Abn total body sodium balance: causes volume contraction or expansion. Regulated by RAAS.
What are causes of non-osmotic ADH release?
-Large decreases in intravascular volume
-emotion, CHF, liver disease, hypoxia, nicotine, morphine
-nausea/vomiting, pain, stress, AngII, hypoglycemia, hypotension, hypovolemia
Define polyuria and polydipsia.
-polyuria=urine output>3 liters/day=loss o free water
-high frequency urination, not large volumes
-polydipsia is intake>3 liters/day
What are causes of polyuria?
-osmotic: uncontrolled DM, hyperosmolar ionic contrast, mannitol, electrolytes
-water diuresis: psychogenic polydipsia, central diabetes insipidus, nephroenic diabetes insipid is
Define diabetes insipidus.
-passing of tasteless urine because of relatively low sodium content
What are causes of central diabetes insipidus?
Decreased ADH production due to hypothalamic or high pituitary stock lesion
-granulomatous disease (TB or sarcoidosis)
-autosomal dominant due to mutations in vasopressin gene-rare
What are causes of nephrogenic diabetes insipidus?
Inability of kidney to respond to ADH
Sickle cell disease
Recovery from ATN
How do we distinguish between causes of water diuresis?
1. Water deprivation test
-measures changes in body weight, urine output, and urine composition when fluids are withheld
-When 2 sequential urinary osmolalities vary by less than 30 mOsm/kg or when the weight decreases by more than 3%, or Posm reaches 300, give DDAVP
2. response to exogenous ADH-desmopressin (DDAVP)
-primary polydipsia: Uosm>750
-CDI: Uosm should increase >50% within 2 hours
-complete NDI: no increase
Describe principles of treatment of polyuria.
1. pyschogenic polydipsia: decrease water consumption
2. CDI: give ADH+adequate water intake
3. NDI: increase proximal water absorption by giving low sodium diet and a thiazide diuretic+adequate water intake
4. osmotic diuresis: remove osmolar load
What are effects of thiazide diuretics in treatment of NDI?
Reduce free water excretion by
1) Mild volume contraction which increases water reabsorption in Proximal Tubules and decreases water excretion.
2) Impair urinary dilution by increasing expression of aquaporin2 in collecting duct.
What is the pathogenesis of hypernatremia?
-decreased water intake +increased water loss
-water loss may be from renal, extrarenal, GI, sweat
-CDI or NDI
-no access to water
-hospitalized patients receiving IV hypertonic solutions
-When serum [Na]>145 mEq/L
Clinical manifestations of hypernatremia?
when serum Na rises>158 mEq/L
Describe compensatory mechanisms in hypernatremia.
-Increased serum osmolality--> shift of water from ICF to ECF
-brain shrinkage--> increase intracellular organic osmoles (1-3 days)
-leads to increased intracellular osmolality, leads to ECF-->ICF of water, and normalization of brain volume
Describe diagnosis of hypernatremia.
Failure to concentrate the urine suggests that there is renal free water loss due to diabetes insipidus or osmotic diuresis.
1. If Usom800 mOsm/kg
-decreased water intake +
-insensible H20 loss or
-GI water loss
How do you calculate free water deficit?
1. Estimate total body water (TBW) = ~50% of lean body weight (Kg) in females and 60% in males.
2. Serum [Na+] x TBW
= 140 mEq/L x TBW[Na+]140 mEq/L
3. TBWhypernatremia – TBW[Na+]140 mEq/L
Serum [Na+]=149. Weight 70kg