NAPLEX Question of the Week: Fluids and Sodium
Ready for a calculation involving sodium?
A 40 year old male presents to the emergency department (ED) after a friend witnessed 3 seizures. His past medical history is significant for hypertension and alcoholism. While waiting to be seen by the physician, the nurse witnesses another seizure. The ED physician orders fosphenytoin 1500 mg PE IV in normal saline (final concentration 10 mg PE/mL) and one liter normal saline (NS). A CBC, CMP, EEG, and alcohol level are ordered.
While other lab results were unremarkable, the CMP revealed a sodium level of 115 mmol/L. The admitting hospitalist orders a 100 mL bolus of 3% hypertonic saline to be given over 10 minutes along with scheduled levetiracetam 1000 mg in 100 mL normal saline twice daily.
Assuming the patient received 2 doses of levetiracetam and all other ordered medications and fluids, how many mEq of sodium did this patient receive? Round your answer to the nearest whole number.
Answer with rationale:
The correct answer is 259 mEq.
Electrolyte imbalances are common and can be serious. Patients with hyponatremia may be asymptomatic or develop symptoms varying in severity such as nausea, headache, disorientation, seizures, brain herniation, and even death. When treating hyponatremia, it is important to determine the underlying cause. Some causes may include medications (i.e. thiazide diuretics), diarrhea, sweating, polydipsia, syndrome of inappropriate antidiuretic hormone (SIADH), and poor solute intake (i.e. beer potomania, tea and toast syndrome, etc.). Labs such as serum osmolality, urine osmolality, and urine sodium may be helpful for further work-up in finding the underlying cause. Additionally, treatment goals differ between acute vs. chronic hyponatremia. This is especially because it dictates how quickly sodium levels can be corrected. Patients with acute hyponatremia are at risk for cerebral edema while overly rapid correction of chronic hyponatremia puts patients at risk for osmotic demyelination. The serum sodium correction limit is usually no more than 8-12 mmol/L within 24 hours depending on risk factors for osmotic demyelination.
Fluids are often used to correct sodium levels, and it is crucial to know the amount of sodium in different types and concentrations of fluid. Pharmacists can play a significant role in the management of electrolyte imbalances and correction with these fluids. Additionally, IV medications are usually in fluids, and these are often overlooked (sometimes referred to as “hidden fluids”).
Below is a list of sodium composition of common fluids as well as an ampule of sodium bicarbonate:
0.45% sodium chloride = 77 mEq/L
0.9% sodium chloride = 154 mEq/L
3% sodium chloride = 513 mEq/L
Lactated Ringer’s = 130 mEq/L
Plasma-Lyte = 140 mEq/L
8.4% sodium bicarbonate = 1 mEq/mL (~50 mEq/amp)
Here is the calculation for the total amount of sodium this patient received:
Fosphenytoin 1500 mg PE IV in 0.9% sodium chloride (concentration 10 mg PE/mL)
1500 mg PE / 10 mg pE/mL = 150 mL = 0.15 L NS
154 mEq/L x 0.15 L = 23.1 mEq
1 L 0.9% sodium chloride = 154 mEq
100 mL bolus of 3% hypertonic saline = 0.1 L
513 mEq/L x 0.1 L = 51.3 mEq
Levetiracetam 1000 mg in 100 mL NS x 2 doses
100 mL x 2 = 200 mL = 0.2 L NS
154 mEq/L x 0.2 = 30.8 mEq
23.1 + 154 + 51.3 + 30.8 = 259.2 mEq rounded to the nearest whole number = 259 mEq