Assessment of Pain in Surgical Patient by PMD 200

1. Background The key to adequate pain management is assessing its presence, duration and severity, identifying those who require intervention, and evaluating the treatment efficacy. Pain relates to both the noxious input, via peripheral nerves, and to central modulation, thereby, integrating different modalities such as affect, experience or personality. Nociception refers to the peripheral and central nervous system (CNS) processing of information about the internal or external environment, as generated by activation of nociceptors. Typically, noxious stimuli, including tissue injury, activate nociceptors that are present in peripheral structures and that transmit information to the spinal cord dorsal horn (or its trigeminal homologue - the nucleus caudalis). From there, the information continues to the brainstem and ultimately the cerebral cortex, where the perception of pain is generated. The pain is a product of higher brain center processing, whereas nociception can occur in the absence of pain.

Objective assessment of nociception poses a multitude of challenges. First, sedated or unconscious patients by definition cannot experience pain. Under these circumstances, it is more accurate to assess the nociception level. A body of research suggests that not managing nociception can lead to central changes in pain pathways that predispose individuals to chronic pain states.

Current "gold-standard" pain assessment tools rely on self-reporting, requiring an individual both to process external information and to communicate this personal experience.

Circumstances exist where self-reporting is not possible, or where it is unreliable. In these situations, surrogate markers utilize changes in behavioral or physiological parameters. However, their use can be associated with considerable shortcomings. They may be unreliable, hampered by observational bias, or influenced by disease processes or pharmacological interventions. Developing an objective method of pain assessment therefore needs to ensure tools that are sensitive and specific to pain. They need to be observer-independent, not reliant on the patient's ability to communicate and not influenced by disease characteristics.

A novel measure of nociception is the nociception level (NoL) index. It is based on objective measures related to changes in the autonomic nervous system. The NoL index ranges from 0 to 100 and is based on a nonlinear combination of nociception-related physiologic variables, specifically heart rate, heart rate variability at the 0.15- to 0.4-Hz band power, photoplethysmograph wave amplitude , skin conductance level, number of skin conductance fluctuations, and their time derivatives.