Nutritional Issues and Practical Aspects of Nutritional Support

Although nutritional support has always had a role in the treatment of critically ill patients, it has evolved over the last 50 years with improvements in line care, enteral access, enteral formulas, and intestinal rehabilitation. Over this same time frame, many aspects of critical care have evolved with advances in ventilator support, resuscitation, fluid management, glycemic control, and sepsis management. However, despite our deeper understanding of catabolic illness and advances in critical care, a standard approach to the nutritional support of the critically ill patient remains elusive. In fact, a number of recent publications have presented conflicting outcomes with increased mortality and complications in patients who developed progressive caloric and protein deficits, as well as those who were treated with aggressive nutritional support (1–4). To develop an effective clinical approach, it is imperative to put this conflicting literature in perspective. Older studies evaluating the outcome of nutritional support interventions in critically ill patients may be less relevant now as there have been advances in critical care therapeutic interventions. In addition, the recent studies that have been done across mixed intensive care units (ICUs), age groups, and disease processes must be interpreted carefully, as their results would be expected to differ from those studies performed in more selective populations. A “cookbook” or “one-size-fits-all” method is not the optimal approach in the modern ICU. Nutrition support must be handled similarly to other critical interventions such as pressor and ventilator therapy, if we are to expect the best outcomes. To gain perspective on the current conflicting literature, it is helpful to review the development of nutritional support that has led up to this point in time, beginning in 1919 with the publication of the Harris–Benedict equations for resting caloric requirements for men and women that has led up to this point (Fig. 150.1) (5). After this equation, the next major development was the definition of essential amino acids by William Rose in 1930. Over the next 30 years, protein hydrolysates, dextrose, and lipid emulsions were developed as life-saving modalities in patients with intestinal failure. Initially, parenteral proteins were hydrolyzed with the larger peptides dialyzed off (5). These solutions contained the amino acid glutamine in the form of dipeptides along with trace elements. Later with the development of crystalline amino acids, glutamine, tyrosine, cysteine–cystine, and trace elements were not included, which led to deficiencies. Micronutrient deficiencies were described in the 1970s and led to the development of balanced vitamin and trace mineral solutions. In contrast with trace mineral formulations, multivitamin solutions have been upgraded over the years. However, these solutions have been developed to prevent deficiencies in stable home parenteral nutrition patients, and have not been specifically adapted to meet the needs of the critically ill patient (6). The amino acid deficiency of glutamine was addressed later with the development of the L-alanine–Lglutamine dipeptide solution in the early 1980s by Furst et al. (7). However, the instability of this formula, its cost, volume requirements, and controversial indications have prevented its routine use. Wretland’s development of long-chain triglyceride emulsions from soybean oil, using egg yolk phospholipid as the emulsifier resulted in total parenteral nutrition therapy in Europe in 1962; these emulsions were not available in the United States until 15 years later. Currently, there are many new lipid formulations since Wretland’s first emulsion was developed, some with pharmacologically beneficial effects (5) (8). Unfortunately, because of FDA restrictions, these new formulations are not readily available in the United States. When it was demonstrated in the 1960s that these infusions could be used to support children and adults with intestinal failure (9), it became easy to use this method whenever it was problematic to enterally feed patients because of access or functionality. The concept of “hyperalimentation” resulted in overfeeding and hyperglycemia with their inherent complications. Indeed, many of the complications attributable to parenteral nutrition in the 1980s were actually due to overfeeding, not the parenteral nutrition itself. These reported complications along with the published benefits of enteral support, led to the near abandonment of parenteral nutrition in favor of the more physiologic enteral route. However, difficulties associated with the delivery of adequate enteral nutrition in critically ill patients once again led to the worldwide progression toward severe malnutrition in the critically ill. As parenteral nutrition may be life saving in selected patients, its use early on in critical care is again under investigation. The timing, route, and indications for combination therapy must be determined based on individual patient nutritional status, metabolic conditions, and anticipated course, as nutritional support in the critically ill patient has evolved to be a therapeutic metabolic intervention with the goal of optimization and manipulation of the stress response and its inherent catabolism.