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1. PHYSIOLOGY AND PSYCHOLOGY
OF ACUTE PAIN
1.1 APPLIED PHYSIOLOGY OF PAIN
1.1.1 Definition of acute pain
Pain is defined by the International Association for the Study of Pain (IASP) as ‘an unpleasant
sensory and emotional experience associated with actual or potential tissue damage, or
described in terms of such damage’ (Merskey & Bogduk, 1994). However, the inability to
communicate verbally does not negate the possibility that an individual is experiencing pain
and is in need of suitable pain‐relieving treatment. This emphasises the need for appropriate
assessment and management of pain when caring for unconscious patients, preverbal or CHAPTER 1
developmentally delayed children, and individuals with impaired communication skills due to
disease or language barriers, as well as those who do not possess a command of the
caregiver’s language (Craig, 2006). Even individuals with native command of language and
cultural skills can face difficulty in communicating the complexities of the pain experience
(Craig, 2009).
Acute pain is defined as ‘pain of recent onset and probable limited duration. It usually has an
identifiable temporal and causal relationship to injury or disease’. Chronic pain ‘commonly
persists beyond the time of healing of an injury and frequently there may not be any clearly
identifiable cause’ (Ready & Edwards, 1992).
It is increasingly recognised that acute and chronic pain may represent a continuum rather
than distinct entities. Increased understanding of the mechanisms of acute pain has led to
improvements in clinical management and in the future it may be possible to more directly
target the pathophysiological processes associated with specific pain syndromes.
Section 1.1 focuses on the physiology and pathophysiology of the transmission and
modulation of painful stimuli (ie nociception). Psychological factors that impact on the
experience of pain are outlined in Section 1.2. However, in individual patients, biological,
psychological and environmental or social factors will all interact. An integrated
biopsychosocial approach to management that also considers patient preferences and prior
experiences is encouraged.
1.1.2 Pain perception and nociceptive pathways
The ability of the somatosensory system to detect noxious and potentially tissue‐damaging
stimuli is an important protective mechanism that involves multiple interacting peripheral and
central mechanisms. The neural processes underlying the encoding and processing of noxious
stimuli are defined as ‘nociception’ (Loeser & Treede, 2008). In addition to these sensory effects,
the perception and subjective experience of ’pain’ is multifactorial and will be influenced by
psychological and environmental factors in every individual.
Peripheral nociceptors
The detection of noxious stimuli requires activation of peripheral sensory organs (nociceptors)
and transduction into action potentials for conduction to the central nervous system.
Nociceptive afferents are widely distributed throughout the body (skin, muscle, joints, viscera,
meninges) and comprise both medium‐diameter lightly myelinated A‐delta fibres and small‐
diameter, slow‐conducting unmyelinated C‐fibres. The most numerous subclass of nociceptor
Acute pain management: scientific evidence 1
