Section 3: Electrolytes Flashcards
List the normal lab values for:
- Sodium (Na+)
- Potassium (K+)
- Chloride (Cl-)
- Bicarbonate (HCO3-)
- Magnesium (Mg2+)
- Calcium (Ca2+)
- Serum osmolality
- Blood urea nitrogen (BUN)
- 135 - 145 mEq/L
- 3.5 - 4.5 mEq/L
- 95 - 105 mEq/L
- 22 - 28 mEq/L
- 1.5 - 2.0 mEq/L
- 8.4 - 10.2 mEq/L
- 275 - 295 mEq/L
- 10 - 20 mg/dL
Normal arterial blood gas (ABG) levels:
- pH
- PCO2
- PO2
- 7.35 - 7.45
- 35 - 45 mmHg
- 90 - 100 mmHg
What is the implication of elevated serum sodium (hyperkalemia)?
It implies a free water deficit
List the 3 broad classes of causes of hypernatremia
Extrarenal water losses
Renal water losses
Iatrogenic
List the causes of hypernatremia from extrarenal water losses
- Insensible losses
- fever
- tachypnea
- mechanical ventillation
- Sweat losss in hot environment
- GI losses
- osmotic diarrhea (e.g. enteral tube feedings)
- acute infectious diarrhea
List the causes of hypernatremia from renal water losses
-
Osmotic diuresis (urine Osm > 300 mOsm/L)
- Glucose
- Urea (e.g. enteral tube feedings)
- Mannitol
-
Central diabetes insipidus (inadequate ADH)
- Head trauma
- Post-neurosurgical (craniopharyngioma, transphenoidal surgery)
- Neoplastic (primary or metastatic)
- Sarcoidosis
- Histiocytosis X
- Meningitis/encephalitis
- Idiopathic
-
Nephrogenic diabetes insipidus (inadequate renal response to ADH)
- Electrolyte disorders (hypercalcemia, hypokalemia)
- Drugs (lithium, demeclocycline)
- Recovery phase of acute renal failure
- Post uinary obstruction
- Chronic renal disease
List the causes of iatrogenic hypernatremia
- Administration of hypertonic saline
- Administration of sodium bicarbonate (NaCHO3)
Explain the pathophysiology and consequences of hypernatremia
Sodium concentration increases as water is lost. Water shifts out of cells to establishe osmotic equilibrium, and brain cells shrink. The patient may become progressively lethargic, even comatose. Intracranial bleeding may arise, especially in children. The dehydrated shrunken brain “hangs” by the meninges in the skull, which can tear the delicate bridging veins
Types of diabetes insipidus
- Central: failure to produce antidiuretic hormone (ADH) in the brain
- Nephrogenic: insensitivity of the kidney
- hypokalemia
- hypercalcemia
- lithium
Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 7894-7897). . Kindle Edition.
Features common to both central and nephrogenic DM
- Low urine osmolality
- Low urine sodium
- Increased urine volume
- No change in urine osmolality with water deprivation
Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 7901-7922). . Kindle Edition.
List the specific diagnostic tests and Rx of Central diabetes insipidus
- Prompt decrease in urine volume with administration of vasopressin (DDAVP)
- Prompt increase in urine osmolality with DDAVP
- Treat with DDAVP or vasopressin
Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 7901-7922). . Kindle Edition.
List the specific diagnostic tests and Rx of Nephrogenic diabetes insipidus
- No change in urine volume with DDAVP
- No change in urine osmolality with DDAVP
- Correct underlying cause, such as hypokalemia or hypercalcemia. Thiazide diuretics are used in other cases.
Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 7901-7922). . Kindle Edition.
- What is the first clue to the presence of DI
- Increased urine volume despite dehydration and hyperosmolality of blood suggests —-
- True or false: sodium disorders cause CNS problems
- High volume nocturia
- Diabetes insipidus
- True
Best initial test for diabetes insipidus
Water deprivation test. The patient is prevented from drinking, the observing the urine output and urine osmolality
- With DI, urine volume stays high and urine osmolality stays low despite vigorous urine production and despite developing dehydration
- A “positive” water deprivation test means urine volume stays high despite withholding water
Comparison of CDI and NDI
- Both CDI and NDI have polyuria and nocturia
- Urine osmolality is low in both CDI and NDI
- Both CDI and NDI have a positive water deprivation test
- CDI responds to ADH but NDI does not
- ADH level is low in CDI and high in NDI
A “positive” water deprivation test means urine volume stays high despite withholding water