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Nephrology USMLE Step 3 > Section 3: Electrolytes > Flashcards

Section 3: Electrolytes Flashcards

1
Q

List the normal lab values for:

  1. Sodium (Na+)
  2. Potassium (K+)
  3. Chloride (Cl-)
  4. Bicarbonate (HCO3-)
  5. Magnesium (Mg2+)
  6. Calcium (Ca2+)
  7. Serum osmolality
  8. Blood urea nitrogen (BUN)
A
  1. 135 - 145 mEq/L
  2. 3.5 - 4.5 mEq/L
  3. 95 - 105 mEq/L
  4. 22 - 28 mEq/L
  5. 1.5 - 2.0 mEq/L
  6. 8.4 - 10.2 mEq/L
  7. 275 - 295 mEq/L
  8. 10 - 20 mg/dL
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2
Q

Normal arterial blood gas (ABG) levels:

  1. pH
  2. PCO2
  3. PO2
A
  1. 7.35 - 7.45
  2. 35 - 45 mmHg
  3. 90 - 100 mmHg
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3
Q

What is the implication of elevated serum sodium (hyperkalemia)?

A

It implies a free water deficit

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4
Q

List the 3 broad classes of causes of hypernatremia

A

Extrarenal water losses

Renal water losses

Iatrogenic

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5
Q

List the causes of hypernatremia from extrarenal water losses

A
  • Insensible losses
    • fever
    • tachypnea
    • mechanical ventillation
  • Sweat losss in hot environment
  • GI losses
    • osmotic diarrhea (e.g. enteral tube feedings)
    • acute infectious diarrhea
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6
Q

List the causes of hypernatremia from renal water losses

A
  • 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
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7
Q

List the causes of iatrogenic hypernatremia

A
  • Administration of hypertonic saline
  • Administration of sodium bicarbonate (NaCHO3)
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8
Q

Explain the pathophysiology and consequences of hypernatremia

A

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

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9
Q

Types of diabetes insipidus

A
  • 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.

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10
Q

Features common to both central and nephrogenic DM

A
  • 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.

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11
Q

List the specific diagnostic tests and Rx of Central diabetes insipidus

A
  • 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.

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12
Q

List the specific diagnostic tests and Rx of Nephrogenic diabetes insipidus

A
  • 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.

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13
Q
  • 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
A
  • High volume nocturia
  • Diabetes insipidus
  • True
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14
Q

Best initial test for diabetes insipidus

A

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
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15
Q

Comparison of CDI and NDI

A
  • 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

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16
Q

Rx of Hypernatremia

A
  • Fluid loss: correct the underlying cause of fluid loss
  • CDI: Replace ADH (vasopressin also known as DDVAP)
  • NDI:
    • Correct potassium and calcium
    • Stop lithium or demeclocycline
    • Give hydrochlorothiazide or NSAIDs for those still having NDI despite these interventions
17
Q

What is the effect of rapid correction of hypernatremia?

A

If sodium levels are brought down too rapidly, cerebral edema will occur. This is from the shift of fluids from the vascular space into the cells of the brain. Cerebral edema presents with worsening confusion and seizures

18
Q

CF of hyponatremia

A
  • Confusion
  • Disorientation
  • Seizures
  • Coma
  • There will be neither edema nor signs of dehydration

Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 7925-7928). . Kindle Edition.

19
Q

What is the first step in the management of hyponatremia?

A

The first step in the management of hyponatremia is to assess volume status to determine the cause and, therefore, the treatment.

20
Q

List the causes and Rx of hypervolemic hyponatremia

A
  • Congestive heart failure (CHF)
  • Nephrotic syndrome
  • Cirrhosis

These are managed by correcting/ managing the underlying cause.

Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 7930-7940). . Kindle Edition.

21
Q

List the causes of hypovolemic hyponatremia

A
  • Diuretics
  • GI loss of fluids
    • vomiting
    • diarrhea
  • Skin loss of fluids
    • burns
    • sweating

Correct the underlying cause and replace with normal (isotonic) saline. Remember to check serum sodium frequently.

Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 7930-7940). . Kindle Edition.

22
Q

Diagnosis and Rx:

  • Hyponatremia
  • Hyperkalemia
  • Mild metabolic acidosis
A

Addison’s disease

Aldosterone replacement

  • Fludrocortisone

Addison’s disease is from insufficient aldosterone production

23
Q

List the causes of euvolemic hyponatremia

A
  • Syndrome of inappropriate ADH release (SIADH)
  • Hypothyroidism
  • Psychogenic polydipsia

Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 7947-7968). . Kindle Edition.

24
Q

List the causes of SIADH

A
  • Any CNS abnormalities
  • Any lung disease
  • Medications such as sulfonylureas or SSRIs
  • Cancer

Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 7947-7968). . Kindle Edition.

25
Q

Features of SIADH secretion

A
  • Inappropriately high urine sodium (> 20 mEq/ L)
  • Inappropriately high urine osmolality (> 100 mOsm/ kg)
  • Low serum osmolality (< 290 mOsm/ kg)
  • Low serum uric acid
  • Normal BUN, creatinine, and bicarbonate

Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 7947-7968). . Kindle Edition.

26
Q

Rx of hyponatremia

A

Mild Hyponatremia (no symptoms)

Treat by restricting fluids

Moderate to Severe Hyponatremia (confused, seizures)

  • Saline infusion with loop diuretics
  • Hypertonic (3 percent) saline
  • Check serum sodium frequently
  • ADH blockers (conivaptan, tolvaptan)

Do not correct serum sodium more than 10 – 12 mEq/ L in the first 24 hours or more than 18 mEq/ L in the first 48 hours. Otherwise, you run the risk of central pontine myelinosis.

Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 7947-7968). . Kindle Edition.

27
Q

What is the risk of correcting serum sodium more than 10 – 12 mEq/ L in the first 24 hours or more than 18 mEq/ L in the first 48 hours.

A

Central pontine myelinosis

Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 7969-7975). . Kindle Edition.

28
Q

Rx for chronic SIADH (as from a malignancy)

A
  • Demeclocycline blocks the effect of ADH at the kidney
  • Conivaptan and tolvaptan are inhibitors of ADH at the V2 receptor of the collecting duct. Conivaptan raises sodium as an ADH blocker.

Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 7969-7975). . Kindle Edition.

29
Q

Hyperkalemia is predominantly caused by increased potassium release from tissues, such as muscles, or red blood cells, such as in rhabdomyolysis or hemolysis.

Aldosterone normally functions to excrete potassium from the body. If there is a deficiency or blockade of aldosterone, potassium levels will rise.

List the other causes of hyperkalemia

Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 7977-7982). . Kindle Edition.

A
  • Metabolic acidosis from transcellular shift out of the cells
  • Adrenal aldosterone deficiency, such as from Addison’s disease
  • Beta blockers
  • Digoxin toxicity
  • Insulin deficiency, such as from diabetic ketoacidosis (DKA)
  • Diuretics, such as spironolactone
  • ACE inhibitors and angiotensin receptor blockers, which inhibit aldosterone
  • Prolonged immobility, seizures, rhabdomyolysis, or crush injury
  • Type IV renal tubular acidosis, resulting from decreased aldosterone effect
  • Renal failure, preventing potassium excretion

Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 7983-7991). . Kindle Edition.

30
Q

Pseudohyperkalemia is an artifact caused by the hemolysis of red cells in the laboratory or prolonged tourniquet placement during phlebotomy.

Rx?

Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 7992-7994). . Kindle Edition.

A

Pseudohyperkalemia does not need therapy; you need only repeat the test.

Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 7992-7994). . Kindle Edition.

31
Q

Major clinical significance of hyperkalemia

A

Hyperkalemia can lead to cardiac arrhythmia

Potassium disorders are not associated with seizures or neurological disorders.

Remember: First peaked T-waves occur, then loss of the P-wave, and then the widened QRS complex occurs.

Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 7995-7998). . Kindle Edition.

32
Q

Rx of hyperkalemia

A

Severe Hyperkalemia (EKG abnormalities, such as peaked T-waves)

  • Administer calcium gluconate intravenously to protect the heart.
  • Follow with insulin and glucose intravenously.

Moderate Hyperkalemia (no EKG abnormalities)

  • Administer insulin and glucose intravenously.
  • Use bicarbonate to shift potassium into the cell when acidosis is the cause of the hyperkalemia or there is rhabdomyolysis, hemolysis, or another reason to alkalinize the urine.
  • Kayexalate (potassium-binding resin) is administered orally to remove potassium from the body. This takes several hours.

Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 8000-8008). . Kindle Edition.

33
Q

List the causes of hypokalemia

A
  • Dietary insufficiency can lead to hypokalemia
  • Increased urinary loss caused by diuretics
  • High-aldosterone states, such as Conn syndrome
  • Vomiting leads to metabolic alkalosis, which shifts potassium intracellularly, and volume depletion, which leads to increased aldosterone
  • Proximal and distal renal tubular acidosis (RTA)
  • Amphotericin from the RTA it causes
  • Bartter syndrome is the inability of the loop of Henle to absorb sodium and chloride. It causes secondary hyperaldosteronism and renal potassium wasting.

Hypokalemia leads to cardiac rhythm disturbance. The EKG will show “U-waves,” which have an extra wave after the T-wave indicative of Purkinje fiber repolarization. Hypokalemia can also cause muscular weakness from its ability to inhibit contraction; this effect can be so severe that rhabdomyolysis occurs.

Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 8009-8020). . Kindle Edition.

34
Q

Features of hypokalemia

A
  • Hypokalemia leads to cardiac rhythm disturbance. The EKG will show “U-waves,” which have an extra wave after the T-wave indicative of Purkinje fiber repolarization
  • Muscular weakness from its ability to inhibit contraction
    • this effect can be so severe that rhabdomyolysis occurs.
35
Q

Rx of hypokalemia

A
  • Therapy is to replace potassium. There is no maximum rate on oral potassium replacement; the bowel will regulate the rate of absorption.

Avoid glucose-containing fluids in cases of hypokalemia. They will increase insulin release and worsen the hypokalemia

In hypokalemia cases, IV potassium replacement must be slow so as not to cause an arrhythmia with overly rapid administration

Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 8022-8027). . Kindle Edition.

36
Q

List the causes of hypermagnesemia

A
  • Overuse of magnesium-containing laxatives
  • From iatrogenic administration
    • during premature labor when it is administered as a tocolytic.

It is rare to have hypermagnesemia without renal insufficiency

Hypermagnesemia leads to muscular weakness and loss of deep tendon reflexes

Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 8030-8043). . Kindle Edition.

37
Q

Rx of hypermagnesemia

A
  • Restricting intake
  • Saline administration to provoke diuresis
  • Occasionally dialysis

Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 8030-8043). . Kindle Edition.

38
Q

List the causes of hypomagnesemia

How does it present (CF)

A
  • Loop diuretics
  • Alcohol withdrawal
  • Gentamicin
  • Cisplatin

Hypomagnesemia presents with hypocalcemia and cardiac arrhythmias

Magnesium is required for parathyroid hormone release. This is particularly important in the management of torsades de pointes.

Fischer, Conrad (2012-09-22). Master the Boards: USMLE Step 3 (Kindle Locations 8030-8043). . Kindle Edition.

39
Q

List 3 important drug causes of hyponatremia

A
  • Hydrochlorothiazide
  • SSRIs (Paroxetin, Citalopram)
  • Carbamazepine (Tegretol)

Nephrology USMLE Step 3 (5 decks)

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