When to Initiate Parenteral Nutrition

When to Initiate Parenteral Nutrition

A 19 year old man presented to the surgical team complaining of abdominal pain. He underwent a laparoscopic appendicectomy and a perforated appendix was removed. He returned to the surgical ward and three days later was ready for discharge. Unfortunately he then developed worsening abdominal pain, fevers and breathlessness. He underwent a CT scan and this demonstrated multiple collections of infected matter within his abdomen in addition to bi-basal atelectasis. He was admitted to the intensive care unit for haemodynamic monitoring, oxygen therapy and broad spectrum antibiotics. He underwent three intra-abdominal washouts of infected material over an eight day admission. During this time he had attempted enteral feeding via a nasogastric tube but had very high gastric aspirates, with no absorption, as a result of a prolonged ileus. He was started on parenteral nutrition on day eight of his ICU admission.

When should parenteral nutrition be initiated in those that are failing to meet caloric targets with enteral feeding alone?


There are a number of guidelines and recommendations to guide clinicians in this decision; unfortunately some of them are conflicting.

What type of artificial nutrition should be used first line (and when)?

What strategies should be employed to improve the success rate of enteral nutrition?

How long should the clinical team persist with enteral nutrition before switching to parenteral nutrition?

What type of artificial nutrition should be used first line?

When considering nutritional support for critically ill patients, the use of enteral nutrition (EN) is strongly recommended over the use of parenteral nutrition (PN) 1. This statement is fully supported by other organisations including NICE 2, ESPEN 3 and ASPEN 4. Enteral nutrition is often recommended as being safer (compared to parenteral nutrition) from a septic complications point of view, although there is much less evidence suggesting a mortality difference 4. However, these conclusions have been based on studies of varying quality & varying doses of parenteral nutrition 5. They often include hypercaloric or excessive hypocaloric feeding, which is known to be detrimental, or use of PN in inappropriate patients 5. Enteral feeding can also be catastrophic if feeding tubes are misplaced. Woodcock et al showed that EN was associated with a higher mortality rate (often due to delivery related complications), therefore PN (when properly done) is as safe as EN 6.

Why is providing nutrition important? A caloric debt (the difference between caloric intake and resting energy expenditure) is associated with increased complications, including infections and ICU stay 7.

The EN vs PN debate should not really be a debate unless there is a partial failure of EN and you want to consider supplemental EN to meet targets 5.

There is general agreement also that enteral nutritional support should be started early (within 24-48 hours of admission), once resuscitation is complete 7.

What strategies should be employed to improve the success rate of enteral feeding?

A whole range of strategies can be employed to improve the chance of successful enteral feeding. I have listed the recommended ones below 1.

Prokinetics at the initiation of the feeding protocol

Acceptance of higher gastric residual volumes (250mls-500mls)

Post-pyloric feeding tubes

How long should the clinical team persist with enteral nutrition before switching to parenteral nutrition (if inadequate intake to meet targets)?

This area has been subject to significant transatlantic differences in the guidelines. The 2009 ESPEN 3 & ASPEN 4 guidelines are markedly different. The American guidelines 4 advise that in the patient who was previously healthy prior to critical illness with no evidence of malnutrition, use of PN should be reserved and initiated only after the first 7 days of hospitalisation (when EN is not possible). If there is evidence of malnutrition on admission and EN is not feasible, it is appropriate to initiate PN as soon as possible following admission and adequate resuscitation 4.

In Europe, it is recommended that all patients who are expected not to be on normal nutrition within 3 days should receive PN within 24-48 hours, if EN is contraindicated or if they can’t tolerate it 3. There is quite a difference in timing.

The Canadian guidelines 1 suggest that PN not be started at the same time as EN. They say there is insufficient data about when PN should be initiated in those not tolerating adequate EN. Their guidelines recommend weighing up risks and benefits, and do not start PN without trying all other strategies to make EN succeed 1.

Two recent studies have attempted to address this significant difference in recommended practice.

In 2011 the EPaNIC study 8 compared the ASPEN and the ESPEN guidelines for initiation of PN (day 2 vs day 8) for those not meeting caloric targets with EN alone. The group that received early PN (as per the ESPEN guidelines) met their energy targets much earlier but outcomes were better in terms of complications and length of ICU stay in the late (ASPEN) group.

In 2013 a second study compared supplemental PN (in addition to EN) vs EN alone at day 4 if EN was inadequate to meet goals 9. Starting supplemental PN to meet caloric needs on day 4 resulted in a fewer infections and reduced time on mechanical ventilation.

These two trials were well conducted 7 but with different outcomes concerning the earlier introduction of supplemental PN. The second trial only included those with a longer predicted length of stay whereas the EPaNIC trial had lots of cardiac and non-cardiac surgical patients (shorter stays) and excluded those that were malnourished.

An editorial nicely summarised the conclusions 7. “There is no rush to start PN when EN notmeeting targets. Supplemental PN should be considered 4-7 days after ICU admission and should probably only be given to the most severely ill patients.who have a substantial caloric debt. In patients with malnutrition, supplemental PN could be stared earlier” 7.

Learning points relevant to this case

Excessive early feeding is harmful, no matter the route of administration, because during the acute phase of critical illness, the body mobilises substrates from insulin dependent organ to match resting energy expenditure. Under these conditions, exogenous calories no longer inhibit the production of glucose by gluconeogenic organs; hence excessive nutrition in the acute phase of illness could induce occult overfeeding and its complications 7.

Excessive nutrition in the acute phase of critical illness is bad. Use the following NICE targets.

25–35 kcal/kg/day total energy

0.8–1.5 g protein (0.13–0.24 g nitrogen)/kg/day

If the patient is critically ill, initiate feeding at no more than 50% of the above target rate and build up to meet the full need over 24-48 hours. If malnourished, start even lower at 5 or 10kcal/kg/day and increase slowly with biochemical monitoring 2.

Remember to increase the feeding rate to keep up with requirements when the acute phase of illness ends.

Enteral feeding can also be dangerous – misplaced NG tubes can kill.

The real debate is not whether enteral nutrition is better than parenteral nutrition, but more importantly ‘when should EN be supplemented by PN, if EN is failing to meet targets?’ 5. A figure of 4-7 days has recently been suggested in most critically unwell patients but maybe earlier if very undernourished 7.

Figure 1 below is a nice summary of current recommended practice 7.

Figure 1: From Vincent & Preiser et al (2013) 7.


  1. Critical Care Nutrition. Canadian Clinical Practice Guidelines. Summary of Revisions to recommendations 2013 [online]. Available: http://www.criticalcarenutrition.com/docs/cpgs2012/Summary%20CPGs%202013%20vs%202009_2July2013.pdf
  2. National Institute of Health and Care Excellence. Clinical Guideline 32: Nutritional Support for Adults. 2006.
  3. Singer et al. ESPEN Guidelines on Parenteral Nutrition: Intensive care. Clinical Nutrition. 2009: 28; 387-400
  4. McClave et al. Guidelines for the Provision and Assessment of Nutrition Support Therapy in the Adult Critically Ill Patient: SCCM & ASPEN. Journal of Parenteral and Enteral Nutrition. 2009: 33(3); 277-316
  5. Trevor Smith [personal communication] ICM regional teaching, March 2014.
  6. Woodcock et al. Enteral vs parenteral nutrition: a pragmatic study. Nutrition. 2001: 17(1);1-12
  7. Vincent JL & Preiser. When should we add parenteral to enteral nutrition? The Lancet 2013: 381;354-355
  8. Casaer et al. Early vs late parenteral nutrition in critically ill adults. N Engl J Med. 2011: 365; 506-17
  9. Heidegger at al. Optimisation of energy provision with supplemental parenteral nutrition in critically ill adults. The Lancet. 2013: 381; 385-393

2 thoughts on “When to Initiate Parenteral Nutrition

  1. Great post. There have been a few important papers published since this 2014 review. These are covered by TBL @WICSBottomLine and include:

    CALORIES: Early nutritional support through the parenteral route is neither more harmful nor more beneficial than through the enteral route. Enteral feeding does increase episodes of vomiting and hypoglycaemia but with no evidence of harm or nosocomial infection. Daily calorific targets were rarely achieved in this study (< 40% in both groups). This reflects a possible deficiency in our feeding protocols and adherence within UK and highlights the need for this to be addressed.


    PermiT: This trial has not demonstrated a moderate survival benefit from permissive underfeeding with moderate caloric intake (around 50% of target calories) and maintenance of full protein requirement (1.2-1.5g per kg per day). A small survival benefit may exist but this study was not large enough to detect one. Permissive underfeeding with full protein requirement appears safe in critically ill patients.


    PYTHON: Allowing patients with severe pancreatitis three to four days to initiate oral intake appears to be safe and effective. This can reduce the discomfort, cost and complications associated with tube feeding in these patients. Although well-conducted this was a relatively small trial, and future data may change this picture



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