Introduction

Pain is an unpleasant sensation to a noxious stimulus, a stressor that can threaten the individual’s homeostatic state. The physiological and behavioural adaptive responses to this stress have evolved to preserve life. However, the persistence of this stress response with its concomitant over-stimulation of the sympathetic nervous system, energy depletion and the anomalous hormonal imbalance may be life-threatening.

Effective pain control is an essential component of high-quality care of the surgical patient. Despite advances in the knowledge of pathophysiology, pharmacology, and techniques of post-operative pain control, numerous surgical procedures take place every year where patients continue to experience unnecessary discomfort.

As the fifth vital sign pain should be measured as frequently as other vital signs visible at the end of the bed. It should be recorded on the standard observation chart, and algorithms should be in place to set thresholds for intervention. Pain that becomes uncontrolled can alert the physician to a change in the pathology or progression of the underlying condition.

This chapter will cover: the transference of the pain signal (transduction, transmission, perception and modulation); the analgesic response (stress release of pro-opiomelanocortin cleaved into ACTH (adreno-corticotrophic hormone), and encephalins); the reflex escape response (the sympathetic nervous system and neuroendocrine system); and the effects of physiological changes.