Chapter 328: Enteral and Parenteral Nutrition

INTRODUCTION

There are three kinds of specialized nutritional support (SNS): (1) optimized voluntary nutritional support, which is used when a patient’s barriers to adequate nutrition can be overcome by special attention to the details of how their food is constituted, prepared, served, and its consumption monitored, (2) forced enteral nutrition (EN), in which a liquid nutrient formula is delivered through a tube placed in the stomach or small intestine, and (3) parenteral nutrition (PN), in which a nutritionally complete mixture of crystalline amino acids, dextrose, triglyceride emulsions, minerals, electrolytes, and micronutrients is infused directly into the bloodstream.

When does a hospitalized patient need SNS? When SNS is indicated, how should it be provided? This chapter reviews the physiological principles that underlie the correct use of SNS, and provides practical information about the diagnosis and management of nutritional disorders in adult hospitalized patients.

The management of in-hospital nutritional disorders follows 3 steps: (1) screening and diagnosis; (2) determination of the severity and urgency of treating a diagnosed nutritional disorder in its overall clinical context; and (3) selection of the modality of SNS, its composition, and the details of providing it. To follow these steps effectively, physicians require a general understanding of nutritional physiology, nutrient requirements, the pathophysiology and diagnosis of the nutritional disorders, and familiarity with the indications, advantages, risks, and administration of the different kinds of SNS. Because most physicians are incompletely trained in clinical nutrition, they must collaborate with clinical dietitians and specialized pharmacists when ordering EN and PN.

NUTRITIONAL PHYSIOLOGY

(See Chaps. 325, 326, 327)

Energy

Total energy expenditure (TEE) is comprised of resting energy expenditure (REE, ~24 kcal/kg normal adult body weight/day), activity energy expenditure (~12 kcal/kg in healthy sedentary individuals) and the thermic effect of food (10% of TEE). The TEE of a healthy adult is ~36 kcal/kg. REE can be measured by indirect calorimetry or estimated using a variety of predictive equations that input weight, height, age, sex, and sometimes disease-related factors. Fever and some forms of critical illness increase REE. Prolonged semi-starvation normally triggers an adaptive reduction in REE, voluntary physical activity, and the thermic effect of food. Broadly speaking, a patient’s TEE identifies the amount of dietary energy they have to consume and metabolize to maintain their existing store of body fat (and protein). The amount of energy they actually require may be less than TEE (as in obesity therapy) or greater than TEE (when rehabilitating nutritionally depleted patients).

Protein and Amino Acids

Protein is an essential nutrient because whole body protein turnover—a continuous process of protein synthesis and breakdown to its constituent amino acids—is associated with obligatory amino acid catabolism to ...