NUTRITIONAL REQUIREMENTS OF SURGICAL PATIENTS

Knowledge of basic substrate-endocrine relationships is essential for the rational planning of perioperative nutritional support. Failure to consider these relationships leads to the use of empirical therapy, to the use of inappropriate intravenous solutions, and to the development of complications, such as hyperglycemia, hypoglycemia, metabolic acidosis, and fatty infiltration of the liver.

Normal organ function depends on normal cellular function. Proper nutrient substrates must be delivered to the cells to maintain healthy cells and normal organ function. The ultimate aim of guidelines for nutritional support therapy should be to maintain cellular homeostasis. Besides uptake of appropriate nutritional substrates, use by the cells depends on substrate delivery by cardiac output and on microcirculatory blood flow.

The choice of appropriate nutrient solutions depends on the knowledge of changes in the energy substrate-endocrine relationships that occur in the perioperative period.^[3]

2888

General Principles

The role of nutrition in the post-trauma or surgical patient is generally overlooked. Often, concern with the physical injuries and surgical procedures causes the physician to forget what is actually occurring within the individual. Healthy people eat to replenish energy stores and to maintain health and well-being. Although there are charts and suggestions for proper nutrition, most healthy adults consume enough (frequently too much) food to maintain normal structure and function of their organs. This implies that the nutrients delivered by the circulatory system are sufficient to meet the individual organ's needs. Nutrients provide structural building blocks for cells and fuels for the production of energy. Both functions are vital to the maintenance of normal cell structure and function.

In the normal individual, a balance between those processes involved in anabolism and catabolism is maintained such that the concentration of plasma metabolites remains fairly constant over a wide range of physiologic insults. After meals, anabolism predominates with the storage of energy substrates, which include glycogen, to sequester excess glucose mainly in liver and muscle; triglycerides from ingested fatty acids and de novo synthesis from glucose and stored in adipose tissue; and proteins necessary for normal function of all tissues ( Fig. 77-1 ). Between meals and under pathologic conditions, the body draws on these energy stores to maintain normal structure and function ( Table 77-1 ). The resulting catabolism is a normal response in periods in which nutrient intake could be compromised. However, although it is a normal response to starvation, in trauma and sepsis, this catabolic phase leads to further complications unless it is corrected. Although trace elements, vitamins, and minerals are also nutrients that contribute to the overall well-being of the organism, this discussion concentrates on the macronutrients (i.e., carbohydrate, fat, and amino acids). The state of nutritional assessment is based more on

Figure 77-1 Flow of nutrients in the fed state.