Stressing the obvious? An allostatic look at critical illness.

Stress plays a crucial role in coping with extrinsic insults through modulating the autonomic nervous system, the hypothalamic-pituitary-adrenal axis and the cardiovascular, metabolic, and immune systems. The allostatic model of maintaining "stability through change" allows the body to respond to a challenge by adjusting to a new steady-state and terminating it once the danger has passed. However, unrelenting stress can lead to decompensation with development of pathologic illness. With sufficient activation the response may become more damaging than the stressor itself. Two types of "allostatic overload" are described: type 1 is an essentially protective response triggered by changes in environment, food supply, or physiologic status where energy demand exceeds supply. The response aims to reduce this imbalance by modifying behavior and intrinsic body systems to direct the animal into a survival mode. Type 2 overload occurs when there is sufficient or excess energy consumption; however, this situation does not trigger an escape or survival response. A clear analogy may be made to critical care where excess stress affects metabolic, hormonal, and immunoinflammatory responses and contributes to the development of organ failure. Ongoing stress also compromises recovery so it is incumbent upon caregivers to reduce stress, be it induced by tissue hypoxia, catecholamine infusion, sleep deprivation, pain, anxiety, and/or excess noise.