Fluids, Electrolytes, Acid-Base Disorders, and Nutrition Support

Patient Case

Questions 1–3 pertain to the following case.

A 55-year-old woman (70 kg) admitted to the ICU for pneumonia and respiratory failure develops a serum

sodium of 125 mEq/L on her fifth day of hospital admission. Her other laboratory values include a serum

potassium of 4.6 mEq/L, chloride (Cl) 100 mEq/L, total carbon dioxide (CO2) content 24 mEq/L, BUN 20 mg/

dL, serum creatinine (SCr) 1.1 mg/dL, and glucose 167 mg/dL. She currently receives a 1-kcal/mL, 62 g of

protein/L enteral feeding formula at 60 mL/hour and a 5% dextrose in 0.45% sodium chloride infusion at 25

mL/hour. Her fluid balance has ranged from +300 to +600 mL/day during the past 3 days. She has no evidence

of any significant amount of edema. Her measured serum osmolality is 265 mOsm/kg, urine osmolality is 490

mOsm/kg, and urine sodium is 67 mEq/L.

Which is the most likely etiology for the patient’s hyponatremia?

Which change in the enteral feeding formula would be best for this patient?

Potassium homeostasis overview

98% intracellular

Total body stores: 35–50 mEq/kg in normal healthy adults; 25–30 mEq/kg if significantly

undernourished

Normal serum concentration: 3.5–5.2 mEq/L

serum potassium will decrease by around 0.6 mEq/L [range 0.4–1.3 mEq/L]) (J Am Soc Nephrol.

2011;22(11):1981-1989), and vice versa. This occurs because of potassium exchanging with hydrogen

by the H+/K+-ATPase pump.

Average daily requirement: About 0.5–1.5 mEq/kg

Kidney is primary route of elimination.

Losses can be extensive with severe diarrhea or body fluid drainages (see Table 4).

Magnesium status can influence potassium homeostasis (J Am Soc Nephrol 2007;18:2649-52; Crit

Magnesium serves as a cofactor for the Na-K-ATPase pump.

ii.

Magnesium closes potassium channels in distal nephron.