Easy parenteral nutrition calculation

Carbohydrates generally make up about 50% to 60% of the caloric prescription, at 3.4 calories/g of dextrose. Protein generally provides about 15%....
Mahitab Hany, BS Pharm, PGDipClinPharm, PGDipClinNutrition

BACKGROUND ㅡ Parenteral nutrition (PN) prescriptions typically provide from 1 to 3 liters of fluid per day depending on the assessment of maintenance fluid requirements. A general rule of thumb is 30 mL/kg. 

     Carbohydrates generally make up about 50% to 60% of the caloric prescription, at 3.4 calories/g of dextrose. Protein generally provides about 15% to 25% of the calories at 4 calories/g of amino acid. The current US Food and Drug Administration (FDA)-approved lipid emulsions are primarily made up of omega-6-rich oils, which have been shown to be proinflammatory and potentially immunosuppressive. Hence, lipid provision should be limited to 20% to 25% of calories (at 10 calories/g of IV lipid). Novel IV lipids made up of olive oils or fish oil are available (SMOFlipid).

EQUATIONS FOR BMR

The Harris-Benedict equation (HBE) is based on studies of healthy volunteers; it is the oldest and most widely used equation for determining basal metabolic rate (BMR). The Mifflin-St. Jeor equation may be more accurate than the HBE for determining caloric needs (when compared to indirect calorimetry). This equation is recommended for obese noncritically ill patients (although precision goes down with increasing obesity). Penn State equation is the Mifflin-St. Jeor equation modified with age, body temperature, and minute ventilation for eucaloric feeding. This equation is recommended for obese critically ill patients. 

     Hence, once the BMR is determined, an estimate of the additional caloric expenditure from activity and metabolic stress (from disease or acute illness) is factored in to determine the overall daily caloric requirement. Stress factors can range from 10% in routine patients to 100% in severe burn patients.

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Figure 1; Percent change in metabolic rate due to injury.
Table (1). Equations for determining basal metabolic rate.
Harris Benedict Equation
Men = [13.75 × weight (kg)] + [5.00 × height (cm)] — [6.78 × age (y)] + 66.5

Women = [9.56 × weight (kg)] + 11.85 × height (cm)] — [4.68 × age (y)] + 655.1
Mifflin-St. Jeor Equation
Men = (9.99 × weight) + (6.25 × height) — (4.92 × age) + 5

Women = (9.99 × weight) + (6.25 × height) — (4.92 × age) —161
Example: A 45-year-old man presents a diverticular abscess with ileus. He is 6 ft tall (182.9 cm) and 176 lbs (80 kg).
BMR (using Mifflin-St. Jeor) = [9.99 × 80] + [6.25 × 182.9] — [4.92 × 45] + 5 = 1726
Calorie Requirement = 1726 (BMR) × 1.25 (i.e., 25% activity factor) × 1.1 (i.e., 10% stress factor) = 2473 calories/day

TPN PRESCRIPTION AND MONITORING

It is advisable to start with a lower volume and concentration of dextrose when initiating PN to avoid metabolic complications. One liter of a 10% dextrose solution is a good starting point. Subsequently the volume, and dextrose, lipid, and protein concentrations are increased as needed in the metabolically stable patient to the eventual goal caloric and fluid provision. 

     Blood sugar levels should be closely monitored and maintained below 180 mg/deciliter (dL), and abnormal electrolyte levels should be corrected, especially potassium, phosphate, and magnesium, before starting or advancing to the goal solution. Additionally, any patient who is at high risk for refeeding syndrome should receive IV thiamine replacement prior to initiation of total parenteral nutrition (TPN) to prevent development of Wernicke’s encephalopathy.

In general, PN lipid infusion should not exceed 1 g/kg/day, and dextrose infusion should be less than 5 mg/kg/min. Eventually, the PN solution can be cycled nocturnally (generally between 10 and 16 hours) in long-term PN patients, once glycemic control is achieved. Once stable, patients can have lab monitoring decreased to once or twice weekly.

TPN CALCULATION

EXAMPLE OF TPN CALCULATION


This assumes a triple mix (i.e., mix of carbohydrate, lipid, and amino acids in one bag). Some institutions will hang lipid separately. Other institutions do not allow for customized solutions and have a limited selection of solutions from which the clinician chooses to get close to the calorie and protein needs of the patient.

REFERENCES