Understanding the metabolic response to critical illness: factors that cause patients to deviate from the expected pattern of hypermetabolism.

Resting energy expenditure is comprised by the sum of the metabolic needs of the peripheral tissues and individual organ systems, influenced by an interaction between thyroid hormone and the sympathetic nervous system. Multiple primary, secondary, and tertiary factors affect this sum to generate the total energy expenditure. Disease processes may increase energy expenditure through direct (stimulating the sympathetic nervous system, increasing oxygen delivery, raising body temperature, increasing motor activity) or indirect processes (uncoupling effect, inefficient metabolism, release of cytokine mediators). Not all patients demonstrate the classic hypermetabolic response. A number of factors involving treatment, weight, choice of predictive formula, timing of the metabolic study, nutritional support, and level of consciousness may contribute in a factitious manner to a hypometabolic response. In some patients, a true serial progressive pattern of inappropriate low metabolism may imply impending septic shock and identifies that patient at risk for organ failure and increased mortality. Serial indirect calorimetric studies may be needed in the intensive care setting to adequately establish patterns of response to critical illness.