Dr. Epstein is Associate Professor of Medicine, Tufts University School of Medicine and Director, Medical Intensive Care Unit, Tufts-New England Medical Center, Boston.
Recent nutrition surveys in hospitals continue to suggest that upwards of 40-50% of patients, particularly those in the intensive care unit, have a moderate to severe degree of malnutrition, and it has been shown that this degree of malnutrition has a significant negative impact on clinical outcomes in the hospital. Given all the advances in modern medicine in the intensive care unit, why does malnutrition still play a key role in patient outcomes?
Dr. Epstein reports no conflict of interest.
One reason is that the patients are older and have long-standing chronic illnesses that interfere with nutrition. Another is that ICU patients are often mechanically ventilated, which makes it challenging to provide adequate nutrition for them. And, finally, acute illness, particularly of the severe degree that brings someone to the ICU, rapidly creates a state of malnutrition.
Patients with Respiratory Compromise
Malnutrition has a number of important consequences that are particularly significant for the patient in the ICU, specifically for the patient with respiratory failure. Respiratory muscle weakness is, by far, the most significant. We know that patients who are malnourished have both decreased respiratory muscle strength, as well as a decrease in respiratory muscle endurance. The lack of endurance is especially important when trying to wean the patient from mechanical ventilation.
Patients who are malnourished have an abnormal control of breathing. Studies have shown that these patients do not respond appropriately to the lack of oxygen in their bloodstreams, that is, they don't breathe enough. Malnourished patients are also more likely to get a respiratory tract infections, and that's in addition to the constant concern of a mechanically ventilated patient — a ventilator-associated pneumonia.
Can an aggressive nutritional support help? Systematic observations have demonstrated that we don't always do such a good job providing adequate nutritional support. But there is evidence to suggest that nutritional repletion helps the patient with respiratory failure, especially those who come to the ICU already malnourished, and can improve weaning from mechanical ventilation.
When to Start Nutritional Support?
It is essential that each patient be considered individually. Most studies seem to say that it is the moderately to severely malnourished patient who benefits most from aggressive nutritional support, whether that be by tube feeding or IV. There is considerably less evidence that a well-nourished patient is going to benefit from nutritional support.
In the ICU setting, we use as our marker for moderate to severe malnutrition an unintentional weight loss from illness that is greater than or equal to 10% of the usual body weight. This is a very practical way of identifying those patients whose malnutrition is severe enough to require aggressive nutritional support and is, surprisingly, accurate when compared to more sophisticated methods of body composition analysis.
Determining Appropriate Nutritional Support
When we begin nutritional support in the intensive care unit, it's frequently with a glucose intolerant patient (someone with pre-existing diabetes or because of the effects of critical illness) and, as a result, hyperglycemia (too much sugar in the blood) can occur. The hyperglycemia can be controlled and when it is patients do much better: shorter duration of mechanical ventilation and, most importantly, decreased mortality. In order to achieve tight control of blood sugar, more than 90% of the patients required in one study a continuous infusion of insulin.
Overfeeding can result in too much carbon dioxide production. To remove the gas from the patient's blood, ICU doctors would need to raise the ventilation rate for the patient on the mechanical ventilator, a task that may be quite difficult if the patient has emphysema or is already, for some other reason, on a high ventilation protocol. Therefore, as a consequence of this concern, avoiding overfeeding carbohydrate calories is important since carbohydrates potentially produce the most CO2. In response to this concern, researchers developed feeding formulas that provided a greater proportion of calories as fat rather than carbohydrate. But as long as overfeeding is avoided, and in particular, if extra carbohydrate is not given by feeding tube or IV, specialized low-carb formulas are not necessary.
Immunonutrition and Immunomodulatory Formulas
The immunomodulatory formulas (sometimes referred to as 'immunoenhancing formulas'), of which there are several on the market, contain the conventional nutrients necessary for meeting the nutritional needs of the patient, but in addition contain supplemental quantities of specific nutrients in large quantities to convey pharmacologic actions. These so-called 'nutraceuticals' include RNA, glutamine, arginine, and omega-3 fatty acids. There now exist many controlled clinical studies which demonstrate that a variety of immune- and inflammation-associated functions are modified by the administration of these immunomodulatory formulas. Nevertheless, given the complexity of the immune response as well as both survival benefits and risks associated with the inflammatory response, it has not been entirely clear in what settings and in what patients the modulation of the immune system actually improves clinical outcomes. At the present time, the data would not support generalized use of immunonutrition in all critically ill patients.
Supplemental Omega-3 Fatty Acids and Biotrauma
Acute respiratory distress syndrome (ARDS) and acute lung injury are markedly inflammatory states. There are now very good data to show that mechanical ventilation may not only cause physical injury to the lung in the conventional sense("barotrauma") but may also lead to a heightened inflammatory state. The term we use for this is "biotrauma."
The omega-3 fatty acid supplemented formulas have been studied in the setting of ARDS and it has been demonstrated that these formulas can reduce the heightened inflammatory response. There is also one large randomized controlled trial in patients with ARDS that showed that these formulas, compared to conventional ones, are associated with a decrease in the number of days on mechanical ventilation, a decrease in organ failure and a decrease in the length of stay in the ICU. So observations to date suggest that omega-3 fatty acid-supplemented formulas might hold some genuine benefit to patients with ARDS, although we need some confirmatory studies that substantiate their benefit, since a single clinical trial is often not enough.