FEN: Hypernatremia and Hyperosmolal States

Question 1

At what serum sodium does hyperosmolality occur?

Answer 1

145 mEq/L or greater

Question 2

Symptoms are related primarily to what physiologic change in reponse to hypernatremia?

Answer 2

Hypernatremia and resultant hyperosmolality causing fluid to shift out of brain cells into the extracellular compartment causing dehydration

Question 3

The symptoms of hypernatremia are related to the ___ of increase in plasma osmolality and the ______ of increase in plasma osmolality

Answer 3

Rate and degree

Question 4

Early symptoms such as lethargy, weakness and irritability tend to occur at what serum sodium?

Answer 4

145-158 mEq/L

Question 5

Late symptoms such as twitching, seizures, coma and death tend to occur at what serum sodium?

Answer 5

greater than 158 mEq/L

Question 6

Fatal symptoms of hypernatremia are often related to what phenomenon in the brain

Answer 6

Cerebral dehydration can cause cerebral vein rupture with subsequent intracerebral or subarachnoid hemorrhage

Question 7

Osmoregular in response to hypernatremia is achieved through what two physiologic mechanisms

Answer 7

1. ADH release (prevent loss of free water)

2. Thirst (increase intake of free water)

Question 8

Dysfunctions in thirst can caause hypernatremia: what two groups of patients are at an increased risk?

Answer 8

1. Adults with altered mental status who have an impaired thirst response or inability to ask for water/access to water
2. Infants

Question 9

Normal range of plasma osmolality?

Answer 9

275-290 mOsm/kg

Question 10

List two principle causes of hypernatremia

Answer 10

1. Loss of isotonic or hypotonic fluid

2. Administration of hypertonic saline or other form of Na+

Question 11

List five ways of isotonic of hypotonic fluid loss

Answer 11

1. fever
2. burns
3. infection
4. renal loss (e.g. diabetes insipidus)
5. GI loss

Question 12

Describe cerebral osmotic adaptation in the setting of hypernatremia

Answer 12

1. Brain cells take up many solutes, including Na+ and K+, thus limiting the osmotic gradient between the IC and EC fluid compartments
2. This prevents cellular dehydration, and it will increase the brain volume toward a normal value, despite hypernatremia.

Question 13

Why might some patients with chronic hypernatremia be asymptomatic?

Answer 13

Cerebral osmotic adaptation

Question 14

When treating hypernatremia, what is a safe rate of correcting serum sodium?

Answer 14

Serum sodium should be reduced slowly by no more than 0.5 mEq/L/hour and 12/mEq/L/day

Question 15

Explain why serum sodium should not be rapidly corrected

Answer 15

1. Because of osmotic adaptation, the brain volume is raised toward normal despite an elevated serum osmolality
2. Rapid reduction in plasma osmolality can cause an osmotic gradient, causing water to move into brain cells with subsequent cerebral edema.

Question 16

What is the ideal way to replenish free water deficit in patients with asymptomatic chronic hypernatremia?

Answer 16

Enteral free water

Question 17

What is the typical intravenous agent to replenish water deficit in hypernatremia?

Answer 17

IV D5W

Question 18

What causes of hypernatremia happen with concurrent Na+ and water depletion?

Answer 18

1. vomiting
2. Diarrhea
3. Diuretic-induced depletion

Question 19

If concurrent Na+ and water depletion occur, what IV fluid can be used to help replenish the Na+ loss?

Answer 19

IV D5W + 0.225% NaCl

Question 20

What to do in a patient with hypernatremia that has intravascular volume depletion

Answer 20

1. Restore intravascular volume to restore tissue perfusion
2. 0.9% NaCl is preferred crystalloid for fluid resuscitation, and contains relatively hypotonic sodium in a patient with hypernatremia.

Question 21

In patients with severe central diabetes insipidus, what is the complication with hypernatremia and what is a treatment option?

Answer 21

1. DI causes increased urine output and decreased urine specific gravity (basically peeing water)
2. May require desmoperssin (DDAVP) to replace insufficient or absent endogenous ADH.

Question 22

What is the estimated free Water Deficit in hypernatremia equation?

Answer 22

WD = TBW * ([Serum sodium]/[Normal sodium] - 1)

where Total Body Water is LBW * 0.6 in men and LBW *0.5 in women.

*the sodium ratio will always be a number greater than 1 in hypernatremia, typically around 1.1-.12, so typically the equation results in administering 10-20% of TBW.

Question 23

How to treat hypernatremia by replacing water deficit?

Answer 23

1. First calculate the estimated free water deficit
2. Give that water deficit over 48-72 hours. Divide total volume by hours to find out rate.
3. Serial sodium monitoring every 2-4 hours in acute phase should guide the rate so as not to exceed 0.5 mEq/L/hour or 12 mEq/L/day.