NAPLEX Question of the Week: Lipids and TPNs

A common calculation is the subject of our question of the week!
NAPLEX Question of the Week: Lipids and TPNs

NM (5’9”, 180 lbs) is a 52-year-old male who is intubated in the intensive care unit due to hospital-acquired pneumonia. He is currently being treated empirically with vancomycin 1,250 mg IV Q12hr and Zosyn 3.375 mg IV Q8hr (4-hour infusion) and sedated with continuous, stable IV infusions of fentanyl 50 mcg/mL at 2 mL/hr and Diprivan 10 mg/mL at 15 mL/hr. Due to inability to tolerate enteral nutrition, the patient is to be started on total parenteral nutrition (TPN). He has a total caloric requirement of 30 kcal/kg/day and a protein requirement of 1.5 g/kg/day. His non-protein calories should come 30% from lipids and 70% from dextrose. Using ideal body weight for weight-based calculations, how many milliliters of a 20% lipid emulsion should be used to make the daily TPN? Round to nearest whole number. 

 

 

Answer with rationale: 

The correct answer is 57 mL.

Performing TPN calculations is an important skill for hospital pharmacists. Many patients in the hospital cannot consume their diet by mouth. In these situations, if the patient has a properly functioning gastrointestinal tract, enteral nutrition is typically preferred to help maintain this function. However, some patients may be candidates for parenteral nutrition due to gastrointestinal problems such as an obstruction or loss of motility.

A TPN contains three macronutrients: carbohydrates in the form of dextrose, proteins in the form of amino acids, and lipids. Each of these components forms the TPN from a stock solution. The total calories in the TPN is the sum of the calories provided by each of the three macronutrients. In this question, we are told that the patient has a weight-based total energy requirement of 30 kcal/kg/day, so we first multiply this by weight. We use ideal body weight throughout this problem because the patient’s actual body weight is less than 30% greater than his ideal body weight.

IBW = 50 + (2.3 x 9) = 70.7 kg

Total caloric requirement = 30 kcal/kg/day x 70.7 kg = 2,121 kcal/day

Protein requirements for patients vary based on stress level. Protein requirements are higher for patients who are critically ill than those who are not. Remembering that there are 4 kcal/g of protein, we can determine the calories that the protein requirement should provide.

Calories from protein = 1.5 g/kg/day x 70.7 kg x 4 kcal/g = 424.2 kcal/day

The portion of the total caloric requirement that does not come from protein is split between dextrose and lipids. In this example, 70% of the non-protein calories should come from dextrose while 30% should come from lipids. It is important to note that some patients may have lipids omitted in certain clinical situations.  We next solve for the amount of calories that the patient should receive from lipids.

Non-protein calories = 2,121 kcal/day - 424.2 kcal/day = 1,696.8 kcal/day

Calories from lipids = 1,696.8 kcal/day x 0.3 = 509.04 kcal/day

However, these lipid calories should not all be put into the TPN because the patient is already receiving a significant amount of lipids from propofol (Diprivan), which is formulated as a 10% lipid emulsion infusion. Lipids contribute 1.1 kcal/mL of the 10% emulsion. By subtracting these calories from the total lipids that the patient should receive, we obtain the portion of the total caloric requirement that should come from lipids in the TPN.

Calories from lipids in propofol = 15 mL/hr x 24 hr/day x 1.1 kcal/mL = 396 kcal/day

Calories required from lipids in TPN = 509.04 kcal/day - 396 kcal/day = 113.04 kcal/day

The lipid content of the TPN comes from a stock 20% lipid emulsion that contains 2 kcal/mL. We solve for the number of milliliters of stock solution needed by setting up a ratio.

2 kcal/1 mL = 113 kcal/X

X = 56.52 mL

We will have one more question of the week next week to round out 2021 before we do our 2 part series on preparing for success on the NAPLEX in 2022. 

Merry Christmas and Happy New Year!

Dr. Bland

Comments

Go to the profile of Grace  Anim
6 months ago

Awesome

Go to the profile of TK
TK
5 months ago

great example, would love to see all the calculation questions you submitted in one file instead of scattered. Thank you.