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Nutrition support in critically ill patients: Parenteral
nutrition
Author: David Seres, MD
Section Editor: Polly E Parsons, MD
Deputy Editor: Geraldine Finlay, MD
All topics are updated as new evidence becomes available and our peer review process is complete.
Literature review current through: Dec 2019. | This topic last updated: Jul 24, 2019.
INTRODUCTION
Parenteral nutrition (PN) support refers to the provision of calories, amino acids, electrolytes,
vitamins, minerals, trace elements, and fluids via a parenteral route. Access, prescribing,
monitoring, and complications of parenteral nutrition are reviewed here.
The goals, outcomes, indications, contraindications, and nutritional requirements for parenteral
nutrition are discussed separately. (See "Nutrition support in critically ill patients: An overview".)
The American Society of Parenteral and Enteral Nutrition recommendations for managing
shortages relevant to components of parenteral nutrition can be accessed at the following site:
http://www.nutritioncare.org/Professional_Resources/Drug_Shortages_Update/.
INDICATIONS AND CONTRAINDICATIONS
The indications and contraindications for parenteral nutrition are described elsewhere. (See
"Nutrition support in critically ill patients: An overview", section on 'Patient selection'.)
INITIATION
In order to initiate parenteral nutrition, appropriate access must be obtained and the prescription
(ie, composition and infusion rate) must be determined. Guidelines suggest that when tolerance
to enteral nutrition is evident, parenteral nutrition should be weaned and discontinued when >60
percent of the patients' needs are met enterally, although there are no data to support this
practice [1]. Our practice is to wean parenteral nutrition proportionate to the amount of enteral
nutrition being delivered, thus meeting but not exceeding the daily goals.
Access — Parenteral nutrition given for more than a few days must be delivered via a central
venous catheter because its high osmotic load is not tolerated by peripheral veins [2,3].
Parenteral nutrition may be given via a peripheral vein if it is significantly more dilute, so called
peripheral parenteral nutrition (PPN). Because of the ease of obtaining central access in the
hospital, the large volume usually required to administer PPN, and the lack of benefit from
short-term parenteral nutrition, PPN is rarely indicated. (See 'Peripheral parenteral nutrition'
below.)
Choosing among a peripherally inserted central catheter (PICC) or a tunneled central venous
catheter (TCVC; eg, Hickman catheter, Groshong catheter, or implanted infusion port) should be
individualized and depend upon duration of need for TPN, local expertise, ability of the facility or
individual to take care of the device, and presence of other risk factors for catheter-related
blood stream infections (CRBSI; eg previous sepsis with CVC in place). The American Society
for Parenteral and Enteral Nutrition (ASPEN) have issued clinical guidelines to describe best
practices in the selection and care of central venous access devices (CVADs) for the infusion of
home parenteral nutrition in adult patients [4]. In the absence of guidelines specific to the
critically ill, similar principles can be used when selecting CVADs for hospitalized critically ill
patients in whom TPN is indicated. ASPEN guidelines use an arbitrary cutoff of 30 days to
distinguish short-term from long-term TPN use.
●
In general, we agree that patients in whom short-term administration of TPN is desirable or
intended that TPN be delivered through a PICC. Alternatively, while not preferable, TPN
may be administered through a subclavian, internal jugular, or femoral central venous
catheter if TPN is only needed for very short periods (eg, a few days) or if a PICC is not
feasible or reasonable. Tradition teaches that the femoral site is least desirable due to an
increased risk of CRBSIs [5], and is consequently not preferred. (See 'Bloodstream
infection' below.)
●
For long term administration of TPN, ASPEN guidelines prefer a TCVC. However, the
rationale for this choice is based upon expert opinion and observational studies that
suggest infection rates may be lower with TCVC rather than randomized trials that validate
this finding [6-9]. In fact, one systematic review suggested that rates were no different
between TPN delivered through a PICC compared with a TCVC (relative risk 0.40; 95% CI
0/19-0.83) [6].
●
When possible, a single lumen central venous catheter should be used for the infusion of
parenteral nutrition. If a multiple lumen central venous catheter is used it should have one
port dedicated solely for the infusion of parenteral nutrition. In addition, catheter
manipulations should be minimized. These interventions may decrease the infectious
complications associated with parenteral nutrition [10,11]. For patients who have an
existing CVC, a new CVC is not typically required unless there has been septicemia during
the life of the existing line.
Indications, insertion, and complications of central venous catheters are discussed separately.
(See "Complications of central venous catheters and their prevention" and "Overview of central
venous access" and "Catheter-related upper extremity venous thrombosis".)
Prescription — Guidelines that describe safe practices for parenteral nutrition have been
published by professional organizations [11-15]. These recommend that parenteral nutrition be
prescribed by a multidisciplinary team of physicians, nutritionists, pharmacists, and nurses [11].
Parenteral nutrition should not be prescribed by clinicians nor compounded by pharmacists
without specific training because it is complicated and requires advanced knowledge about
issues such as nutrient metabolism and solute compatibility [14,15].
Parenteral nutrition is an admixture of solutions containing dextrose, amino acids, electrolytes,
vitamins, minerals, and trace elements. Lipid emulsion may be infused separately or added to
the mixture. However, mixing all three, a so-called total nutrient admixture, or 3-in-1 parenteral
nutrition, is favored by most experts [16]. The exact composition and infusion rate are tailored to
the nutritional and fluid needs of each patient. (See "Nutrition support in critically ill patients: An
overview", section on 'Nutritional requirements'.)
Dextrose — Dextrose-containing stock solutions are available in a variety of concentrations,
most commonly 40, 50, and 70 percent. The percentage of calories that is contributed by
dextrose is titrated according to individual factors, such as severity of illness, the caloric needs
of the patient, and the patient's ability to tolerate fluid volume.
The caloric contribution of dextrose in medical solutions is 3.4 kcal/gm, which differs from
dietary carbohydrate (4 kcal/gm). The reason for the difference is that water contributes to the
weight of the dextrose-hydrate that is used to prepare parenteral nutrition.
Amino acids and electrolytes — Amino acid solutions contain most essential and
nonessential amino acids. The exceptions are arginine and glutamine. While these are thought
to be conditionally essential in critical illness, due to blockade of their metabolic pathways in
catabolism, research does not support their supplementation. The caloric contribution of amino
acids is approximately 4 kcal/gm. The buffer used in amino acid solution contains electrolytes,
usually in small quantities. Amino acid solutions with large amounts of electrolytes have been
used infrequently because they limit the degree to which parenteral nutrition can be customized.
However, with the increasing use of premixed parenteral nutrition with a variety of available
electrolyte content, customization may not be as necessary. It is occasionally necessary to use
amino acid solutions with further electrolyte restriction (usually these are 15 percent amino acid
solutions), particularly in patients with renal failure when electrolytes and phosphate levels are
difficult to control.
The amino acid stock solutions come in concentrations ranging from 5.5 to 15 percent. Higher
concentrations are useful for minimizing volume and electrolytes delivered to patients. Enteral
protein, peptide, and arginine supplementation is discussed separately. (See "Nutrition support
in critically ill patients: Enteral nutrition", section on 'Composition'.)
Enrichment of parenteral nutrition with branched chain amino acids has been studied, but there
is insufficient evidence of benefit to recommend this approach. A meta-analysis of four
randomized trials (202 patients) that compared branched chain amino acid-enriched parenteral
nutrition to standard parenteral nutrition found a trend toward decreased mortality among
patients who received branched chain amino acids (24 versus 36 percent, relative risk 0.58,
95% CI 0.26-1.28) [17]. However, the meta-analysis was limited by methodological differences
among the trials and an imprecise estimate of the magnitude of effect. Other meta-analyses
have found no difference in infection rate, ICU length of stay, or hospital length of stay.
Lipids — Lipids are provided as an emulsion that may be infused separately or added to the
mixture (total nutrient admixture [TNA] or three-in-one). In the United States, most lipid
emulsion consists of long-chain omega-6 triglycerides derived from soybean and safflower oils
and then emulsified using egg phospholipids and glycerin. Recently, mixtures of several types
of lipids (eg, refined olive, soybean, and fish oil emulsions), have been approved and are
available for use in the US. We observe improvement in liver enzymes in many patients with
parenteral nutrition-related hepatic dysfunction when switching from soy-base to mixed lipid
emulsions. But we also note this when the percent of fat is decreased and when total calories
are decreased. To date, randomized trials have not conclusively demonstrated any clinical
meaningful benefits among patients receiving parenteral nutrition, nor justify the added expense
of using mixed fat emulsions for all patients [18-22] (see "Intestinal failure-associated liver
disease in infants", section on 'Fish oil-based lipid emulsions'). The caloric contribution of a
typical lipid emulsion is 10 kcal/g or 2 kcal/mL in 20 percent emulsion and 11 kcal/g or 1.1
kcal/mL in 10 percent emulsion. Dietary fat provides 9 kcal/g, but there is a contribution of
calories from the emulsifiers used to create the lipid suspension for intravenous administration.
Use of intravenous fat emulsions should be done with care in patients with prior allergy to eggs
as allergic cross-reactions have been reported. But these are very rare.
Vitamins and trace elements — Patients receiving parenteral nutrition must receive
adequate vitamins and minerals to prevent deficiencies. For most patients, a unit dose of a
standard multivitamin and multi-trace element solution will suffice to provide minimum daily
requirements. It is advisable that patients with cholestasis not receive copper or manganese, as
these are excreted in bile. A total bilirubin of 2 is often used as a cut off to restrict these, and it is
often necessary to order the remaining trace elements individually and then follow levels. There
are also concerns about clearance of some vitamins in anephric patients. It is not our practice to
restrict either vitamins or minerals in these patients, and to monitor levels if patients are on
parenteral nutrition for extended periods.
Therapeutic vitamin and mineral supplementation — It is not our practice to supplement
vitamins and minerals above the recommended daily amounts, unless a deficiency is
documented. It should be noted that a deficiency is not the same as a low level. Rather, a
deficiency occurs only when a lack of a substance causes an undesired health outcome that is
reversed or prevented by supplementation. For example, levels of vitamin D, which is protein
bound, drop in critical illness. But even though there is a correlation between vitamin D levels
and ICU mortality, supplementation of vitamin D has no effect. [23,24]. The decrease in vitamin
D levels is due to the decrease in carrier protein levels, much in the same way albumin
decreases. Despite the incorrect use of the term "deficiency" in a vast literature, low vitamin D
levels in this context does not constitute a deficiency.
Numerous clinical trials have examined the effect of antioxidants on critically ill patients when
given as single nutrients (eg, selenium) or as a combination of nutrients (selenium, copper, zinc,
vitamin A, vitamin C, vitamin E, and N-acetylcysteine). The trials administered the antioxidants
in various ways, including as a separate intravenous infusion, as a component of parenteral
nutrition, as a component of enteral nutrition, and orally.
A meta-analysis of 15 randomized trials (1647 patients) found that critically ill patients who
received vitamins and trace elements, either alone or in combination, had a lower mortality rate
than patients who did not receive vitamins or trace elements (20 versus 27 percent, relative risk
0.76, 95% CI 0.65-0.88) [25]. Similar meta-analyses showed improvement in the duration of
mechanical ventilation, but no differences in infectious complications, hospital length of stay, or
ICU length of stay.
Few of the trials looked specifically at patients receiving parenteral nutrition. Rather, most
enrolled a heterogeneous sample of patients receiving either parenteral or enteral nutrition and
did not look for differences in the effects of the vitamins and trace elements in these subgroups.
Given their safety, it seems reasonable to provide vitamins and trace elements to most critically
ill patients, regardless of the type of nutrition support that they are receiving. The optimal
mixture of vitamins and trace elements is yet to be determined. Enteral vitamin and trace
element supplementation is discussed separately. (See "Nutrition support in critically ill patients:
Enteral nutrition", section on 'Vitamins and trace elements'.)
Glutamine — Glutamine is a precursor for nucleotide synthesis and an important fuel source
for rapidly dividing cells, such as gastrointestinal epithelia. Despite evidence that indicates that
parenteral glutamine may be beneficial to patients who are receiving parenteral nutrition [2628], glutamine is only available in the United States as a non-sterile powder and is not generally
recommended for patients who are receiving parenteral nutrition. Enteral glutamine
supplementation is discussed separately. (See "Nutrition support in critically ill patients: Enteral
nutrition", section on 'Glutamine'.)
MONITORING
Routine monitoring of parenteral nutrition includes measurement of fluid intake and output, as
well as selected laboratory studies. It is reasonable to measure serum electrolytes, glucose,
calcium, magnesium, and phosphate daily until they are stable, or more frequently when the
patient is at high risk for or exhibiting refeeding syndrome. It is similarly reasonable to measure
aminotransferases, bilirubin, and triglyceride at least once each week during treatment. More
frequent measurements should be performed in the period immediately after parenteral nutrition
is initiated, or after changes in the composition. Frequency of monitoring should be adjusted to
the individual patient’s acuity, stability, and risks for deficiencies. General guidelines have been
published [13]. (See "Anorexia nervosa in adults and adolescents: The refeeding syndrome".)
COMPLICATIONS
Patients who receive parenteral nutrition support are at risk for infection, adverse metabolic
effects, and complications related to venous access. Clinical practice guidelines, protocols, and
oversight by multidisciplinary teams have been shown to reduce the complications and total
cost of parenteral nutrition [11,25,29].
Bloodstream infection — Access to the central venous system is necessary when TPN is
indicated such that TPN is typically administered through a peripherally inserted central venous
catheter (PICC) or a tunneled central venous catheter (TCVC) (see 'Access' above). Patients
receiving parenteral nutrition are at increased risk of acquiring a bloodstream infection (bacterial
and fungal), compared to patients who have central venous catheters but are not receiving
parenteral nutrition [30]. This risk is likely to have decreased since new government
reimbursement policies were initiated which preclude payment for prolonged hospital stay due
to catheter-related sepsis [31].
Among patients receiving parenteral nutrition, factors that are independently associated with
bloodstream infection include poor patient hygiene, insertion of the central venous catheter
under emergent circumstances, and, to a lesser extent, the severity of illness and duration of
central venous catheterization [32]. Conversely, proper hand hygiene and maximal barrier
precautions during insertion of the central venous catheter are associated with fewer
bloodstream infections [32]. Diagnosis and treatment of catheter-related bloodstream infection
is discussed separately. (See "Intravascular non-hemodialysis catheter-related infection: Clinical
manifestations and diagnosis" and "Intravascular non-hemodialysis catheter-related infection:
Treatment".)
Metabolic effects — Parenteral nutrition is associated with metabolic complications, including
hyperglycemia, serum electrolyte alterations, macro- or micro-nutrient excess or deficiency,
refeeding syndrome [33,34], Wernicke's encephalopathy [35], and hepatic dysfunction. Routine
monitoring of serum glucose, electrolytes, and volume status may minimize the impact of such
complications.
Hyperglycemia is particularly common among patients who receive parenteral nutrition. In a
meta-analysis of six randomized trials (264 critically ill patients with acute pancreatitis), the
incidence of hyperglycemia was approximately two times greater for patients who received
parenteral nutrition than patients who received enteral nutrition [36]. Both groups had similar
nutrient intake. The relationship between hyperglycemia and clinical outcomes in critically ill
patients is discussed in detail elsewhere. (See "Glycemic control and intensive insulin therapy in
critical illness", section on 'Effects of hyperglycemia'.)
Refeeding syndrome is a potentially fatal condition resulting from rapid changes in fluids and
electrolytes when malnourished patients (eg, anorectics, alcoholics) are given oral, enteral, or
parenteral feedings [34]. Patients with ongoing electrolyte losses (eg, from diarrhea, vomiting,
fistulas) are at increased risk of refeeding syndrome. It is defined primarily by manifestations of
severe hypophosphatemia (including respiratory failure, cardiovascular collapse,
rhabdomyolysis, seizures, and delirium), but hypokalemia, hypomagnesemia, and Wernicke’s
encephalopathy also occur. Refeeding syndrome is described in greater detail elsewhere. (See
"Eating disorders: Overview of prevention and treatment", section on 'Refeeding syndrome'.)
Venous access — Parenteral nutrition requires venous access, which is associated with
potential complications. Examples include bleeding, vascular injury, pneumothorax, venous
thrombosis, arrhythmia, and air embolism. The complications of venous access for parenteral
nutrition are not different from access for other purposes, and are described separately. (See
"Complications of central venous catheters and their prevention" and "Catheter-related upper
extremity venous thrombosis" and "Air embolism".)
PERIPHERAL PARENTERAL NUTRITION
Peripheral parenteral nutrition (PPN) is a type of parenteral nutrition that can be delivered
through a peripheral intravenous catheter because it has an osmolarity lower than that of
conventional parenteral nutrition (<900 mOsm). To deliver adequate nutrients, either a large
volume and/or a high fat formulation must be used. Frequent replacement of intravenous
access is usually necessary. Despite an osmolarity lower than conventional parenteral nutrition,
PPN is still quite hyperosmolar and irritating to the peripheral veins. PPN is rarely prescribed
because of the uncertain clinical benefit of short-term parenteral nutrition.
SOCIETY GUIDELINE LINKS
Links to society and government-sponsored guidelines from selected countries and regions
around the world are provided separately. (See "Society guideline links: Nutrition support
(parenteral and enteral nutrition) in adults".)
SUMMARY AND RECOMMENDATIONS
●
Parenteral nutrition is a mixture of solutions that contain dextrose, amino acids,
electrolytes, vitamins, minerals, and trace elements. Lipid emulsion may be infused
separately or added to the mixture (ie, a total nutrient or three-in-one admixture). (See
'Prescription' above.)
●
Parenteral nutrition should be prescribed by a multidisciplinary team with advanced
knowledge about how to tailor the composition and infusion rate to the needs of each
patient, rather than by untrained individual clinicians. Guidelines exist that describe safe
practices for parenteral nutrition. (See 'Prescription' above.)
●
For critically ill patients receiving parenteral nutrition, we suggest the inclusion of
multivitamins and trace elements (Grade 2B). They are typically already included as a
component of the parenteral nutrition, in the multivitamin, and multi-trace element solutions
that are usually given in unit doses. The optimal mixture of nutrients has not been
determined. (See 'Vitamins and trace elements' above.)
●
Routine monitoring of parenteral nutrition includes measurement of fluid intake and output,
as well as selected laboratory studies. (See 'Monitoring' above.)
●
Patients who receive parenteral nutrition support are at risk for infection, adverse metabolic
effects, and complications related to venous access. (See 'Complications' above.)
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