Introduction

Respiratory failure and the need for mechanical ventilation brought about by a variety of medical, surgical and traumatic events makes the optimum nutritional requirements of this group of patients difficult to determine. Nonetheless, nutritional support is an important adjunct to the management of patients in the intensive care unit, mechanically ventilated patients being especially vulnerable to complications of under- or over-feeding. This chapter will consider the nutritional requirements, route and timing of nutritional support, and complications associated with feeding mechanically ventilated, critically ill patients.

Nutritional status and outcome

The metabolic response to critical illness, which features a rise in circulating levels of the counter-regulatory hormones and pro-inflammatory cytokines, is characterized by insulin resistance, increased metabolic rate and marked protein catabolism. The loss of lean body mass impairs function, delays recovery and rehabilitation and, at its most extreme, may delay weaning from artificial ventilation. The degree of catabolism and its impact on outcome depends on the duration and severity of the inflammatory response.

Anthropometric techniques routinely used to measure changes in body mass and composition are inaccurate in the presence of excess fluid retention and therefore the assessment and monitoring of the nutritional status in critically ill patients is difficult. A pre-illness weight and weight history may provide useful information on pre-existing malnutrition, but once admitted to the intensive care unit (ICU), acute changes in body weight largely reflect changes in fluid balance.