1. Your patient has a serum Na of 125 and a urine osmolality of 600. You plan to give them 1 L of normal saline. Which direction would you expect the serum sodium to change assuming the urine osmolality remains constant?
c) stay the same
d) cannot be determined
2. Your patient has decreased urine output, a serum sodium of 129, but appears total body volume up. He is otherwise asymptomatic. What is the treatment of choice?
a) 3% hypertonic saline administration
b) Bolus of normal saline
d) Free water restriction
3. Which of the following would lead to decreased serum ionized calcium levels?
a) Increased vitamin D intake
b) Increased PTH levels
c) Decreased calcitonin levels
d) Massive blood transfusion
ANSWERS & EXPLANATIONS
1. B. The urine osmolality is 600 and the normal saline you are giving your patient has an osmolality of approximately 300 (308 to be precise). Hence, they will be able to excrete the 300 osm of solute in 1/2L of free water, leading to effectively, retention of 1/2L of free water. Hence, despite the fact that the serum Na is 125 and the sodium content of normal saline is 154 mEq/L, your liter of normal saline will lead to further hyponatremia (assuming the urine osmolality remains consistent). Your patient likely has increased ADH secretion (potentially SIADH given the low serum osmolarity)
2. D. Your patient appears to have SIADH. Free water restriction is the treatment of choice for an asymptomatic patient. You might consider 3% hypertonic saline if your patient was having CNS symptoms or was at risk for cerebral edema. Conivaptan is an ADH antagonist that may provide useful for treating patients with excess ADH activity although it is not currently commonly used.
3. D Massive blood transfusion can cause decreased levels of serum ionized calcium due to the anticoagulant, citrate, used to keep blood products from clotting. Citrate chleates calcium, a cofactor required throughout the coagulation cascade.