A few basic points about TPN

TPN (total parenteral nutrition) is a vital part of treating patients with malnutrition or inadequate intake, and it confused the hell out of me as a med student. I only spent one day with the surgical nutrition team at my hospital in medical school, but it was invaluable.

How do you calculate how much TPN a patient needs?

  • Resting energy expenditure (REE) tells you how much the person needs per day at baseline. This can be calculated with the Harris-Benedict equation. It’s usually about 25 kcal/kg/day.
  • REE alone can be supplied by dextrose.
  • The DEE (daily energy expenditure)=1.3*REE.

How much do different types of nutrition supply? (carbs vs lipids vs proteins, etc)

  • TPN is usually given as 1-2 L per day. This equates to about 41 (the number will vary) cc/hr. “Can this result in fluid overload?” you may wonder. I don’t know of any reports of such a thing, but you should be careful to reconcile volume status. You can reduce the amount of volume given by giving a more concentrated formula (1 calorie, 2 calorie, etc).
  • Protein usually 1 g/kg/day
  • Emulsion lipids usually 2x/wk

What should you be aware of when starting a patient on TPN?

  • Especially if transitioning off tube feeds, you want to be cautious about how much you give a patient
  • Initially, it’s okay to “underfeed.” Better underfeed than put someone at risk for refeeding syndrome.
  • It’s also okay to “underfeed” a little if the patient is obese.

How can you tell if a patient is getting adequate nutrition on TPN?

  • The MREE (measured resting energy expenditure). It’s used to assess the response of a patient who has been on TPN for a while—if dropping, you may want to give more TPN, if increasing, you may want to cut back a little bit. Usually a respiratory therapist measures MREE with a mask for about 10 minutes. Usually, it’s less than the DEE.

How do you monitor a patient on TPN?

  • TPN labs: baseline BG, basic chemistry, albumin and prealbumin, retinol-binding protein, Mag/phos/Ca, zinc,etc. Get these for 3 days to make sure TPN is adequately addressing BG, electrolytes, etc.
  • Albumin is pretty worthless as a nutritional lab because it’s an acute phase reaction protein (inflame). Prealbumin is variable.
  • Strict I/Os, daily weights. You can also measure output by how much diarrhea a patient is having.

Myths about TPN:

  • TPN is expensive. At my hospital, it cost $17/day.
  • “if the gut is good, use it.” This is a saying many surgeons are fond of. It means, if a person is able to tolerate PO intake, you should feed them enterally. However, enteral feeding has more complications, like aspiration and intolerance. By contrast, the risks of TPN are usually gastric residues or diarrhea.
  • Pre-existing chemo ports can be used for TPN. Avoid this because the rates of infection go way up if you leave a port line open for a week at a time. Use a PICC line while the patient is in-hospital.
  • Enteral feeding and TPN can’t occur at the same time. PO intake can always be used for comfort feeds, like someone who has a fistula and leaks out what they eat within minutes. But in this type of patient, you’d also want to make sure that they get TPN to provide adequate nutrition. but also gets TPN for actual nutrition.
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