Nutritional Support in Critical Care

Nutritional support has become a routine part of the care of the critically-ill patients and leads to better clinical outcome, lower infection rate and reduced hospital stay

"The critically ill patient often utilise fat better than carbohydrate as an energy source, so it may be advantageous to provide more than 30 per cent fat, which is the content in our normal diet" - Dr Vikram Mahajan Senior Consultant Indraprastha Apollo Hospital New Delhi

As many as 40 per cent of adult patients are seriously malnourished when admitted to a hospital. Acute illness further exacerbates patients' poor nutritional status by increasing their metabolic rate and impairing the allocation of nutritional substrates. The consequences of malnutrition especially in the critically ill leads to increased morbidity and mortality, prolonged hospital stay, impaired wound healing, defective muscle function (reduced respiratory and cardiac function), immuno-suppression with increased risk of infection. Nutritional support plays a vital role in preventing the above and has become a routine part of the care of the critically ill patients and leads to better clinical outcome, lower infection rate and reduced hospital stay.

The role of nutrition in the critically ill continues to develop. Here, we shall review the following issues:

• Optimal route.
• Quantity.
• Micronutrient components and Immuno-nutrition.

Route of Administration

All methods of delivering artificial nutrition carry risks and these must be minimised and the potential benefits considered.

There is certainly consensus that enteral nutrition should be considered before the parenteral route. Enteral route is more physiological and has lots of advantages over parenteral route. Functional and structural integrity of the gastrointestinal mucosa is maintained. It is relatively non-invasive, cheap and may reduce the risk of infectious complications, associated with parenteral nutrition. So, it is the preferred route and should be started when nutrient intake is inadequate for one to two days.

However, enteral nutrition may be associated with some problems like risk of micro-aspiration in ICU, risk of dangerous misplacement or displacement of the feeding tube. High gastric aspirates may be present in patients on opioids, sepsis and with electrolyte imbalance. It may frequently result in under-nutrition unless protocols are used to avoid to ready cessation of feeds. Acceptance of gastric residual volumes of 200-250 ml and the early use of prokinetics are key elements of such protocols. Head-up tilt of 30-450 should be used whenever possible to facilitate enteral nutrition and to prevent microaspiration. Feeding tubes like the standard Ryle's tube of 14 F or 16 F are not preferred since it not only causes more discomfort but also enhances gastro oesophageal reflux and increased chances of aspiration. Fine bore size 8F or 10F flexible tubes with stylet is to be used.

Although the enteral route should be considered before the parenteral route. However, parenteral nutrition may not be as harmful as often assumed to be.

Certain patients, particularly in the emergency surgical setting, cannot be fed by the enteral route because it is neither possible nor safe. There is also a substantial group of non-surgical patients who cannot tolerate the required quantity in the first few days and have persistently high gastric residual volumes together with significant pre-existing nutritional deficits. These patients should be fed parenterally to provide adequate nutrition and not be deprived of essential nutrients in order to avoid parenteral nutrition.

When using the parenteral route, one must be aware of all the possible complications and try to minimise them. Infection and bacteremia is the most dreaded complication and should be prevented by strict aseptic precautions in not only inserting but also handling the central venous catheter line at all times. Also, the line should be dedicated to feeding only and not used for other drug/ fluid administration or monitoring the CVP. Other complications are ones related to catheter insertion, thrombosis, venous perforation, air embolism and line displacement. Parenteral nutrition is also associated with metabolic complications like hyperglycaemia, hypoglycaemia, electrolyte imbalance, hypertriglyceridemia, metabolic acidosis and trace element deficiencies. Frequent monitoring of the blood chemistry will prevent the metabolic complications. The use of insulin to maintain tight glycaemic control is now common place in critical care and essential when parenteral nutrition is administered. Underfeeding with lipid-free, hypo-caloric solutions does not prevent hyperglycaemia or reduce infective complications.

Quantity of Support

The overall aim of nutritional support is to provide patients with their macro-nutrients (carbohydrates, fat and proteins) and micro-nutrients (trace elements, vitamins). The estimation of a patient's requirements is an essential component of nutrition support, ensuring that the patient's nutritional needs are met without significant over or underfeeding. In everyday hospital practice, several different equations are used, often without an adequate understanding of their origins and limitations. This can lead to significant variation in energy provision, which could have serious implications for patient care. As a first step of working, start with estimating total fluid requirements. As a rule of thumb this will be between 30 and 40 ml/kg/day or 1 ml water per calorie for an adult, but this will need to be supplemented should total losses be excessive.

Also, it should be remembered that requirements will need to be modified according to individual patient needs and specific disease processes.

Carbohydrate, Fat & Proteins

Thirty to 70 per cent of the total calories can be supplied as carbohydrate in the form of glucose but fructose and sorbitol are also used. 20 to 50 per cent is to be given as fat. Each gram of fat can yield 9.1 kcal. The critically ill patient often utilise fat better than carbohydrate as an energy source, so it may be advantageous to provide more than 30 per cent fat which is the content in our normal diet. Essential fatty acids like linoleic acid are also an important content of the fat whose deficiency leads to cardiac dysfunction and increased susceptibility to infection. The protein intake should match the rate of catabolism. The protein requirement during normal metabolism is 0.8-1.0 gm/kg and in hypermetabolism is 1.2-1.6 gm/kg. The goal is to maintain a positive balance of four to six gms by providing enough non-protein nitrogen.

Vitamins and trace elements are an essential component of a patient's daily requirement and are necessary for normal metabolism, cellular function and several enzyme systems.

It is certainly better to underfeed rather than attempt to match a calculated energy requirement, particularly when this is high in sepsis or trauma. This has been shown in a study (by Krishnan JA, Parce PB, Martinez A, Diette GB, Brower RG. Caloric intake in medical ICU patients: consistency of care with guidelines and relationship to clinical outcomes. Chest 2003; 124: 297-305), when patients who received between 33 per cent and 65 per cent of calculated requirements (according to American College of Chest Physician guidelines) had better outcomes in terms of mortality and duration of ventilation compared with those receiving greater than 65 per cent. Failure to deliver at least 25 per cent of calculated requirements is associated with significant increases in infection and mortality.

Immuno-nutrition

Immuno-nutrition is a relatively new concept in critical care feeding to which there is a growing body of evidence reporting benefits. Glutamine, arginine, fish oils and ribonucleotides, as well as a host of anti-oxidants including Vitamins C and E, selenium, and other trace elements have been the focus of immuno-nutrient research. There are a number of important considerations in evaluating their use.

Glutamine: The rationale for glutamine replacement during critical illness is clear as it is an important fuel for enterocytes and lymphocytes and has a role in nucleotide synthesis. This helps to maintain gut mucosal integrity and cellular immune function. As a consequence, translocation of enteric bacteria is reduced and infective complications less frequent. Glutamine is also a precursor for glutathione which has important anti-oxidant actions. Overall, its benefit gas been reviewed in a meta-analysis by Heyland et al. It has shown to be beneficial in parenteral nutrition in general ICU patients and possibly beneficial in elective surgical patients and in enteral nutrition in burns/ trauma patients.

Arginine: Arginine supplementation has been shown to be beneficial in cancer patients but may actually be harmful in the critically ill. It may lead to haemodynamic instability, immunosuppression, cytotoxicity and organ dysfunction. Most studies have shown significant increase in mortality in patients with sepsis. Therefore, arginine supplementation is not recommended in septic ICU patients.

Fish oils: The use of omega-3 fatty acids results in release of mediators which reduce platelet aggregation and leukocyte chemotaxis. This may be helpful in patients with acute lung injury on enteral feeds containing omega-3 fatty acids and reduce mortality rates. This has been clinically shown in a study but still the evidence base is limited.

Anti-oxidants: The body has a number of endogenous anti-oxidant defence mechanisms like superoxide, dismutase, catalase, glutathione peroxidise and reductase (with zinc and selenium as co-factors) as well as vitamins E and C. Many patients in the ICU will already be deficient in these due to chronic health, smoking and poor diet. These patients are then subjected to increased consumption of scavengers and losses of anti-oxidants and decreased intake of vitamins and minerals when free radical production is at its highest. In many studies the use of anti-oxidants has shown to be of benefit in the general ICU patients and the evidence seems more dramatic for selenium.

Prebiotics/Probiotics/Synbiotics: The intestine normally contains millions of microbes whose optimal function depends on the supply of fermentable fibres and gastrointestinal secretions delivered to the colonic bacteria. In ICU, the combination of antibiotics, reduced intake of normal dietary substrates and the un-physiological use of tube feeding regimes lead to sub-optimal gastrointestinal function and altered gut flora. We need to provide new flora (probiotics) and also food for the flora (prebiotics). A combination of probiotic micro-organisms and pre-biotic carbohydrates is called synbiotic food. New studies have shown that addition of prebiotics/ probiotics/ synbiotics to enteral nutrition may not have effect on infectious complications, but may be associated with significant reduction in ICU mortality and may also reduce diarrhoea.

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