Nutrition in ICU
2. The effects of malnutrition
Critically ill patients with low energy reserves and increased metabolic demands can ill afford to suffer further nutritional depletion. The effects of malnutrition are:
• impaired immune function and increased risk of sepsis
• poor wound healing,
• muscle wasting including cardiac and respiratory muscles
• impaired ventilatory drive, leading to increased time on a ventilator
• low mood
Fig 1. Simplified diagram of normal metabolism.
Goals of nutritional support in ICU
detection of pre-existing malnutrition
satisfying energy requirements
optimisation of patients’ metabolic status
fluid and electrolyte management
reduction of morbidity
Effects of starvation on the gut
In patients already malnourished, lack of enteral feeding leads to:
• mucosal atrophy
• loss of integrity of the gut mucosal barrier
• increased permeability
• decreased absorptive capacity
• reduction in mucosal blood flow
• reduction in hormonal regulation of digestion
• reduction in gut immunity
• loss of cell architecture
• increased bacterial adherence
• pathogenic transformation of bacteria
• decreased production of bile salts
• atrophy of lymphoid tissue
Pathogenic bacteria may enter the circulation through damaged mucosa, contributing to organ damage through sepsis, or may be aspirated into the lungs past the endotracheal tube causing ventilator-associated pneumonia.
Even if full enteral feed is not tolerated by the gut, a background amount of 10 mls/hour may be helpful in preventing these negative effects.
Metabolic effects of starvation
In the initial few days glycogen stores are used, and there is some gluconeogenesis from amino acids in muscle. Thereafter free fatty acid oxidation results in energy production from ketones. After fat reserves are depleted, muscle bulk diminishes rapidly. Insulin secretion decreases as a response to low levels of carbohydrates and the metabolic rate decreases. Eventually essential amino acids are used for glucose production and cell death ensues. This leads to depletion of intracellular electrolytes, especially magnesium, potassium and phosphate. The imposition of critical illness on this situation results in even more severe depletion of body tissue and electrolytes.