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

Answer: D

Hyperglycemia and other causes of non-hypotonic

hyponatremia have been excluded. Urine osmolality

is greater than 100 mOsm/kg, which rules out psy-

chogenic polydipsia, and a large amount of hypotonic

fluids were not being given. Urine sodium was greater

than 30 mEq/L, and the patient did not receive diuretic

therapy or have kidney disease. The patient appeared

to be normovolemic without evidence of significant

edema (expansion of the ECF compartment). Because

the patient also has pneumonia (a common cause of

SIADH), all of these factors indicate that the patient has

hyponatremia caused by SIADH (Answer D is correct).

Answer A is incorrect because the serum glucose con-

centration is not high enough to cause hyponatremia;

nor has the patient received mannitol, glycine: nor is

the patient hypertriglyceridemic. Answer B is unlikely

because there was no evidence of adrenal insufficiency

given in the case. Answer C is impossible because the

patient did not have a traumatic brain injury.

Fluid restriction is the most appropriate treatment of

SIADH (Answer B is correct). The “vaptans” may also

be considered; however, this was not a choice. Answer A

is incorrect because giving salt tabs may result in wors-

ened fluid overload and edema. Answer C is incorrect

for the same reason as Answer A. Answer D is incorrect

because the severity of hyponatremia and lack of symp-

toms does not warrant the emergency use of hypertonic

saline.

Answer: D

The best way to fluid-restrict an enterally fed patient is

to use the most concentrated formulas, which are the

2-kcal/mL formulations that are specifically designed

for patients with congestive heart failure. Unfortunately,

protein intake may be inadequate with the use of these

formulations in certain populations, and supplemental

protein may have to be provided. Answers A, B, and C

are incorrect because they do not result in the appropri-

ate therapeutic decision to reduce fluid intake.

Answer: B

These dosages should be selected as the correct answer

because they follow the dosing guidelines given in

this chapter (unlike the doses given in answers A, C,

D). Given that the patient is NPO with intolerance to

anything by mouth, oral options (Answers A and D) are

not practical. If the patient had a history of significant

recent weight loss or if he did not respond adequately

to these doses, the dose could be increased. Answer

C is not correct as the dosage of phosphorus is exces-

sive. Supplemental potassium and phosphorus would be

added to the PN solution, in addition to daily intrave-

nous doses of potassium and phosphorus.

Answer: B

Because it takes about 48 hours for serum magnesium to

redistribute, the next day’s serum magnesium concentra-

tion is falsely elevated. In general, it will take 4–5 days

to replete this patient’s magnesium deficiency (presum-

ably caused by chronic alcohol ingestion). Thus, answer

B would be the best option for this patient. Given that

the patient is NPO, oral dosing options (answer A) are

less desirable. Answer C is incorrect because it is too

aggressive a dosage given the current serum magne-

sium concentration of 2.0 mg/dL (despite it being falsely

elevated). Supplemental magnesium would be added to

the PN solution in addition to daily doses of intravenous

magnesium sulfate if he remained low or in the low-

normal range or if he was also hypocalcemic (because

hypomagnesemia can elicit hypocalcemia secondary to

end-organ resistance to parathyroid hormone). Answer

D is incorrect as the serum magnesium concentration is

falsely elevated to within the normal range but has not

fully redistributed yet. It would be anticipated that the

concentration will decrease the following day since it

takes multiple days to replenish magnesium stores.

Answer: C

Although critical illness (Answer D) and fluid resuscita-

tion therapy may have been a factor in the development

of his hypocalcemia, massive blood transfusion is the

most profound cause. Citrate, added to the blood as

an anticoagulant, readily binds calcium and can cause

hypocalcemia. Previous studies have shown that hypo-

calcemia is common when patients are given more than

5 units of blood at a time (Answer C is correct). Answer

A is incorrect because a low-normal serum magnesium

concentration of 1.8 mg/dL is unlikely to contribute to

the pathogenesis of hypocalcemia. A serum magnesium