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Gastrointestinal, Liver and Nutritional Alterations 513

Also, patients not receiving enteral feeding can have
181
detectable glucose in aspirates. This is further con- TABLE 19.5 Components of TPN solutions
founded by the presence of blood, which is closely
associated with glucose values >20 mg/dL; conse- Component Implication
quently, any blood in the respiratory tract could con-
181
tribute to a false-positive result. These findings led Carbohydrate ● This should supply approximately 70% of the
patient’s non-protein to prevent protein
to the consensus that glucose monitoring in respira- catabolism needs. 190
tory secretions should also be abandoned. 179 ● Insulin production will usually be increased to
● Measurement of pepsin in tracheobronchial secre- maintain normoglycaemia, although some
tions has been used in an animal study suggested that patients, such as those with diabetes, may
the detection of pepsin, a component of gastric secre- require an insulin infusion to maintain
normoglycaemia.
tions, may be useful in determining pulmonary aspi- ● Glucose solutions over 25% are hyperosmolar
ration. 182 however, further investigation in acutely ill and may cause thrombophlebitis.
patients receiving enteral feeding is necessary. ● Glucose solutions mixed with lipids can
protect the vein from thrombophlebitis,
PARENTERAL NUTRITION provided osmolality is <800 mOsmol/kg. 189
The appropriate use of PN in the context of critical illness Lipids ● More energy-dense than carbohydrate
continues to be debated. 183-185 EN is the preferred method (9 kcal/g), and should provide 30–40% of the
non-protein energy.
of nutritional support because it is less expensive and is ● Available as a 10% (1 kcal/mL) or 20%
associated with fewer infectious and metabolic complica- (2 kcal/mL) solution.
tions than PN. However, it is not uncommon for critically ● Necessary to maintain cell wall integrity,
ill patients to have difficulty in meeting daily caloric prostaglandin synthesis and the absorption of
intake 34,71 and this may necessitate supplementation of lipid-soluble vitamins. 191
enteral nutrition with PN or the sole provision of nutri- ● Isotonic, so can be given via a peripheral line if
tional support through parenteral means (as TPN). For necessary.
patients who are unable to be fed by the enteral route Nitrogen ● A balance of crystalline amino acids supplying
and who were healthy prior to ICU admission, with no 0.2 g nitrogen per kg body weight is required
evidence of protein-calorie malnutrition, then it is recom- to achieve a nitrogen balance in most patients,
mended that PN be initiated after 3–7 days 186 of hospi- although some patients may utilise more
nitrogen.
talisation. 187 The lack of agreement on the efficacy of PN ● Some critically ill patients may utilise more
means that the use of this therapy varies both within and nitrogen and thus have higher requirements.
between countries. 58,186,187
Electrolytes ● Content varies in amino acid solutions, so
PN solutions contain carbohydrates, lipids, proteins, elec- content in relation to patient requirements
trolytes, vitamins and trace elements. PN, whether sup- needs to be considered.
plementary or complete, provides daily allowances of ● Monitoring of electrolyte status is essential,
nutrients and minerals. The components of PN are listed particularly serum phosphate levels if the
amino acid solution used is phosphate-free.
in Table 19.5. The addition of vitamins and trace ele- ● The balance between chloride and acetate is
ments to PN solutions is necessary, particularly as water- monitored, as administration of additional
soluble vitamins and trace elements are rapidly depleted sodium or potassium may result in acid–base
(see Table 19.6). Glucose is the primary energy source in imbalances.
PN solutions. Concentrations of 10–70% glucose may be
used in PN solutions although the final concentration of
the solution should be no more than 35%. The high TABLE 19.6 Trace elements in TPN 192
concentration of PN solutions can cause thrombosis so
PN is normally infused via a central venous catheter Trace element Action
(CVC). Peripheral administration can be considered
when the final solution concentration is 10–12%, 188 but Zinc Wound healing
is not usually used in the context of critical illness because Iron Haemoglobin synthesis
high volumes of PN would be required to meet caloric Copper Erythrocyte maturation and lipid metabolism
requirements. 189
Manganese Calcium and phosphorus metabolism
Catheter insertion, ongoing care and replacement are
similar to that with any other CVC. A dedicated CVC, or Cobalt Essential constituent of vitamin B12
lumen of a multilumen CVC, should be used for PN. 191,193 Iodine Thyroxine synthesis
Manipulation of the CVC and tubing should be avoided Chromium Glucose utilisation
to minimise infection of the catheter.
Routine monitoring of the patient’s fluid balance, glucose, STRESS-RELATED MUCOSAL DISEASE
biochemical profile, full blood count, triglycerides, trace
elements and vitamins is necessary. The patient is also The reported incidence of stress-related mucosal damage
assessed for signs of complications associated with the is variable 194 and complicated by definitions of end
administration of PN (see Table 19.7). points, difficulty in measuring the end points, and the