SCOPE OF THE PROBLEM All chronic pain begins as acute pain. The physiologic transition from acute to chronic pain syndromes results from physiologic and psychosocial transitions that are not well defined. Generally speaking, acute pain becomes chronic pain when the physiologic insult that caused the original pain has resolved or reached a static or progressive state. A large number of emergency department (ED) patients have underlying chronic pain syndromes. It is very common in the U.S. population, with rates as high as 40% noted in one review. The key step to the prevention of chronic pain may be the adequate treatment of acute pain. Acute pain serves an adaptive purpose in that it stimulates protection of the injured area via recruitment of tissue repair mechanisms as well as physiologic responses such as increased blood pressure and respirations. As the injury heals, these adaptive purposes become maladaptive, as limited movement causes a decreasing range of motion, prolonged stress response causes impaired immune response, hypercoagulable states, and vegetative symptoms (Table 21-1). It can be difficult to determine at what point pain changes from adaptive to maladaptive, or from acute to chronic. Acute and chronic pain call for different treatment approaches, both in terms of the approach to the patient and to the medications used. Themaladaptive components of chronic pain can push patients toward behaviors that can hinder their recovery (lack of abnormal use of painful extremities, inactivity) and impact their mood, social interactions, and lifestyle (Figure 21-1).