Parenteral Nutrition: Feeding Into the Vein
The first rule of feeding a sick patient is almost a reflex: if the gut works, use it. Enteral nutrition is cheaper, safer, and keeps the bowel alive. But sometimes the gut cannot be used at all — it is too short, too obstructed, too leaky, or too inflamed to absorb anything. For those patients, medicine does something that would have seemed impossible a century ago: it bypasses the digestive tract entirely and drips a complete diet — sugar, protein, fat, salts and vitamins — straight into a great vein near the heart. It is life-saving. It is also one of the most dangerous things you can hang on a drip stand, and the reason a whole team watches the patient every single day.
A 34-year-old man is three weeks out from emergency surgery for a strangulated bowel. Most of his small intestine had to be removed — he now has short bowel syndrome, and what remains cannot absorb enough to keep him alive. He has not eaten in a fortnight, and his weight is falling. A tunnelled central line runs into a large vein above his heart, and through it hangs a single large bag: a pale, faintly milky fluid that is his entire diet for the day — glucose, amino acids, a lipid emulsion, electrolytes, vitamins and trace elements, mixed to his exact needs. Each morning the nutrition team redraws his bloods, adjusts the bag, and inspects the line. Twice already they have caught trouble early: a creeping blood sugar, and a low phosphate on the first day of feeding. In this one patient you can see the whole discipline — the gut that failed, the vein that replaced it, and the relentless monitoring that keeps the rescue from becoming the injury.
When the gut can't be used — the indications
Parenteral nutrition is the deliberate exception to "if the gut works, use it." The whole logic of clinical nutrition favours the enteral route — feeding through the gut, whether by mouth or by tube — as covered in the Enteral nutrition chapter. Parenteral nutrition (PN), and its complete form total parenteral nutrition (TPN), is reserved for the patient whose gut is non-functioning or inaccessible and who cannot be fed adequately any other way. The classic settings are intestinal failure and short bowel syndrome (not enough absorptive surface left); a prolonged ileus or mechanical obstruction that stops anything passing; a high-output intestinal fistula that would only leak more if fed; severe malabsorption that no enteral formula can overcome; and severe mucositis or a hostile gut after chemotherapy or radiation. The unifying idea is simple: PN is used when enteral feeding is impossible, unsafe, or insufficient — never as the easy option, always as the last resort.
What's in the bag — a complete diet, dissolved
A PN bag is a manufactured meal. The macronutrients are three: glucose as the main energy source; amino acids to supply protein and spare the patient's own muscle; and a lipid emulsion — a milky suspension of fat droplets — that delivers a dense dose of calories and the essential fatty acids the body cannot make. To these are added electrolytes (sodium, potassium, calcium, magnesium, phosphate), and — crucially — vitamins and trace elements. This last point is easy to forget and dangerous to miss: because IV feeding bypasses the whole digestive tract, it also bypasses the normal dietary intake of micronutrients. Nothing gets in unless you put it in. Vitamins (including thiamine, folate and the fat-soluble A, D, E, K) and trace elements (zinc, copper, selenium, chromium) must be added to every regimen, as the Trace elements and Vitamins chapters explain — omit them and the patient will, over weeks, develop a textbook deficiency. Finally the bag carries water and is balanced for volume. The mixture is tailored to the individual patient and adjusted daily as their bloods change.
Think of normal eating as a factory with a full supply chain: the gut is the loading dock, breaking down deliveries and letting in only what's needed, in the right order, with quality control at every step. Parenteral nutrition tears down the loading dock and pipes raw materials straight onto the factory floor. It works — the factory runs — but every safeguard the dock provided is now yours to supply by hand. You must pre-measure every ingredient (including the trace ones nobody thinks about), keep the pipes scrupulously clean, and check constantly that nothing is building up or running short, because there is no gut left to buffer your mistakes.
The route: why full TPN needs a central line
A full TPN mixture is extremely concentrated — a heavy load of glucose and amino acids makes it strongly hypertonic, with a high osmolarity. Push that into a small peripheral vein in the arm and it burns: the vessel becomes inflamed and thromboses within hours (thrombophlebitis). Peripheral veins simply cannot tolerate the osmolarity of full feed. So hypertonic TPN must go through a central venous catheter, whose tip sits in a large, high-flow central vein (near the heart) where blood volume dilutes the feed almost instantly. A dedicated lumen is reserved for the nutrition so nothing else contaminates it. Peripheral parenteral nutrition (PPN) exists, but only as a compromise: a lower-concentration, lower-calorie feed for short-term use when a central line isn't warranted. If a patient needs full nutrition for more than a few days, they need central access.
- PN is for the non-functioning or inaccessible gut — the deliberate exception to "if the gut works, use it."
- Classic indications: intestinal failure/short bowel, prolonged ileus or obstruction, high-output fistula, severe malabsorption or mucositis.
- The admixture = glucose + amino acids + lipid emulsion + electrolytes + vitamins + trace elements + water.
- Vitamins and trace elements MUST be added — IV feeding bypasses normal dietary intake of micronutrients.
- Hypertonic full TPN needs a central line with a dedicated lumen; peripheral PN is only a short-term, low-concentration option.
- The bag is tailored to the individual and adjusted daily as bloods change.
The price, part 1 — catheter complications
A permanent line into a central vein is a permanent door for bacteria. The central catheter that makes TPN possible is also its single most feared hazard. A central-line-associated bloodstream infection (CLABSI) — bacteria tracking down the line into that high-flow central vein — can seed a full-blown sepsis within hours. This is a leading cause of morbidity in long-term PN, and it is why the line is handled with almost ritual asepsis, why a lumen is kept dedicated to the feed alone, and why any unexplained fever in a PN patient is treated as line sepsis until proven otherwise. Management overlaps directly with the Antimicrobials chapter — empirical broad-spectrum antibiotics, blood cultures from the line and a peripheral vein, and sometimes removal of the catheter itself. The other catheter problem is mechanical: central-vein thrombosis around the line, which can occlude the vessel and threaten future access. Both are the price of the door you had to leave open.
The price, part 2 — metabolic complications
Feeding a starved body too fast is its own emergency. The most feared metabolic danger sits right at the start of feeding: refeeding syndrome. In a chronically starved patient, the sudden glucose load triggers a surge of insulin that drives phosphate, potassium and magnesium rapidly into the cells — and the plasma levels can crash to fatal lows, precipitating arrhythmias, weakness and death. This is the classic setting for the syndrome, and it is why the golden rules apply: start low and go slow, give thiamine before feeding, and monitor and replace phosphate, potassium and magnesium closely (the full mechanism is in the Refeeding syndrome chapter). Beyond that, the sheer glucose load commonly causes hyperglycaemia — the pancreas cannot keep pace with a continuous IV sugar infusion, and many patients need insulin to control it, linking to the Endocrine section. Electrolyte derangements of every kind appear as the daily balance shifts. And the lipid emulsion carries its own signature: hypertriglyceridaemia if the fat is infused faster than the body can clear it, which is why triglycerides are checked and the lipid dose is capped.
The single most examined trap in PN is the direction of the electrolyte shift in refeeding. Students expect a starved patient to be low on everything and reach to "feed them up" fast. But it is the feeding itself — the insulin it releases — that drops the phosphate, potassium and magnesium, because they rush into cells that are suddenly building tissue again. The sickest, most malnourished patient is therefore the one you feed most cautiously. Give thiamine first, start at a fraction of the target calories, and let the numbers guide you up. Fast feeding is not kindness here; it is the mechanism of harm.
The price, part 3 — hepatobiliary and long-term effects
Feed the vein long enough and the liver begins to protest. PN-associated liver disease is a recognised complication: early on, a cholestasis (a rise in liver enzymes and bilirubin as bile flow slows), and with prolonged use a fatty liver (steatosis) and, in the worst cases, progressive fibrosis. Part of the cause is the constant IV nutrient load the liver must process without the normal rhythm of eating; the lipid emulsion contributes too. Alongside it, the gallbladder suffers from disuse: with no food entering the gut, the gut hormones that normally squeeze the gallbladder fall silent, bile stagnates, and sludge and gallstones form — the same cholestatic theme that connects this section to the Liver chapter. Two further long-term prices round out the picture: micronutrient deficiencies if vitamins or trace elements are ever omitted from the regimen (zinc, copper, selenium and the vitamins each have their own deficiency syndrome), and metabolic bone disease — a demineralising loss of bone seen after months to years of PN. None of these is a reason to withhold life-saving nutrition; each is a reason to monitor for it and to get the patient back onto the gut as soon as it can be used.
Daily at the start: electrolytes (sodium, potassium, calcium, magnesium, phosphate) and glucose — the refeeding and hyperglycaemia watch. Regularly: liver function tests (LFTs) for cholestasis, triglycerides for the lipid load, and full blood count. Ongoing: weight and fluid balance to judge whether calories and volume are right, and daily inspection of the line and its site for infection. As the patient stabilises, bloods are spaced out. Behind all of it stands the multidisciplinary nutrition team — dietitian, pharmacist, specialist nurse and physician — who write the regimen, adjust the bag, and guard the line. Their standing goal is the same every day: wean back to enteral or oral feeding the moment the gut can take it.
- Catheter risks: central-line bloodstream infection/sepsis (strict asepsis, dedicated lumen) and central-vein thrombosis.
- Refeeding syndrome is the classic PN emergency — insulin-driven fall in phosphate, potassium, magnesium; give thiamine, start low.
- Hyperglycaemia from the glucose load often needs insulin; the lipid can cause hypertriglyceridaemia.
- Hepatobiliary: PN-associated cholestasis and steatosis, plus gallbladder sludge/stones from gut disuse.
- Long term: micronutrient deficiencies if vitamins/trace elements are omitted, and metabolic bone disease.
- Monitor daily electrolytes/glucose, then LFTs, triglycerides, weight/balance and line care — and wean to the gut as soon as possible.
- Feeding a severely malnourished patient at full calories from day one — the fast glucose load precipitates refeeding hypophosphataemia; and giving glucose without thiamine first risks Wernicke's encephalopathy.
- Infusing hypertonic full TPN through a peripheral cannula — the osmolarity destroys the vein (thrombophlebitis); full feed needs a central line.
- Dismissing a fever in a PN patient as a minor bug — any unexplained fever on a central line is line sepsis until proven otherwise, and needs cultures and antibiotics.
A severely malnourished patient with short bowel syndrome is started on total parenteral nutrition at full target calories. On the second day he becomes weak and confused, and bloods show a sharply low phosphate, potassium and magnesium. What is the underlying mechanism?
- Parenteral nutrition feeds glucose, amino acids and lipid — plus electrolytes, vitamins and trace elements — straight into a vein when the gut is non-functioning or inaccessible; it is the exception to "if the gut works, use it."
- Hypertonic full TPN needs a central line with a dedicated lumen; vitamins and trace elements must always be added because IV feeding bypasses normal intake.
- The complications cluster into catheter (line sepsis, thrombosis), metabolic (refeeding, hyperglycaemia, hypertriglyceridaemia) and hepatobiliary (cholestasis, steatosis, gallstones).
- PN is life-saving but high-risk — it demands daily monitoring by a nutrition team and weaning back to the gut as soon as possible; it is the classic setting for refeeding syndrome and trace-element deficiency.
- NICE Clinical Guideline CG32 — Nutrition support for adults: oral nutrition support, enteral tube feeding and parenteral nutrition.
- ESPEN Guidelines on Clinical Nutrition — parenteral nutrition and home parenteral nutrition.
- ASPEN (American Society for Parenteral and Enteral Nutrition) — clinical guidelines for parenteral nutrition ordering, monitoring and complications.
- Rang & Dale's Pharmacology — nutrition, vitamins and trace elements.
- BNF (British National Formulary) — parenteral nutrition, intravenous nutrition preparations.
- Mehanna HM, Moledina J, Travis J. Refeeding syndrome: what it is, and how to prevent and treat it. BMJ.
