Many patients experience unnecessary moderate to severe postoperative pain. A knowledge of pain physiology can help nurses to address this aspect of care
Sharon Wood MSc, Dip healthcare research studies, IHBC, RGN, is lecturer in nursing at University of Leeds
Wood S (2010) Post operative pain 1: Understanding the factors affecting patients’ experiences of pain. Nursing Times; 106: 42, early online publication.
Management of post operative pain is complex and multidimensional and effective management present challenges to nurses. Failure to control pain can result in long term complications including chronic post surgical pain. This first article, in a two part series, explores the physiological mechanism involved in the perception of pain and the role of psychological and environmental influences on how patients respond to it. Part 2, to be published next week explores the principles of patient assessment and management of post operative pain.
Keywords Post operative pain, Nociception, Chronic post surgical pain
- This article has been double-blind peer reviewed
- Nurses need underpinning knowledge about pain physiology to understand their patient’ experience of pain.
- Assessment of pain should be initiated from the preoperative phase where risk factors can be identified and a plan of care developed.
- Causes of pain following surgery should be assessed and appropriate interventions implemented and evaluated.
The effective management of post operative pain (POP) is a fundamental human right and should be the cornerstone of ethical, patient-centred nursing practice. There have been a plethora of recommendations, evidence-based guidelines, benchmarks and service developments for POP management over the past 20 years (Chandler et al, 2003; Clinical Standards Advisory Group, 1999; American Society of Anaesthesiologists, 1995; Agency for Health Care Policy and Research 1992; Royal College of Surgeons and College of Anaesthetists, 1990). Despite this, POP continues to be poorly managed with up to 67% of patients in the UK still experiencing unnecessary moderate to severe POP (Dolin et al, 2002).
What is post operative pain?
Pain is predictable following surgery and gradually decreases over time (Bϋyϋkyilmaz, 2010). POP is a highly individualised, complex, multi-dimensional experience which is influenced by the interaction of many factors including biological, psychological, environmental, and social (Australian and New Zealand College of Anaesthetists and Faculty of Pain Medicine (ANZCA), 2010). Definitions of pain are outlined in Table 1. If POP is not managed effectively the patients’ recovery can be adversely affected resulting in chronic post surgical pain (CPSP) and even death (ANZCA, 2010; Mei et al, 2010; Apfelbaum et al, 2003; Morrison et al, 2003).
Persistent unrelieved moderate to severe POP and other risk factors are associated with CPSP (ANZCA, 2010; Kehlet et al, 2006; Macrae, 2008). The effects of unrelieved post operative pain are outlined in Table 2. Prompt identification of POP is therefore essential and nurses must have an understanding of the nociception of pain if POP and CPSP are to be accurately identified and effectively managed. Table 3 lists the risk factors associated with CPSP.
Nociception is the term used to describe the neural processes by which a noxious substance or a tissue damaging event such as surgical incision is perceived as pain (Fig 1). This is described in four stages, transduction, transmission, perception and modulation. Nociception involves a complex interaction between the peripheral nervous system (PNS) and central nervous system (CNS) as well as an evaluation of patients’ pre and post operative psychological and environmental influences. The tissue damaged caused by surgery results in the nociceptive system operating in a ‘sensitised state’ to encourage behaviours that guard the wound from further damage thus promoting wound healing (Johnson, 2009) (Table 4).
Noxious stimuli associated with surgery are detected in the PNS by nociceptors of the A delta and C fibre peripheral sensory afferents. These are distributed throughout the body in the skin, muscles, joints and viscera and respond to a range of noxious stimuli that are associated with surgery. These are:
- Mechanical stimuli, for example, surgical incision, pressure from swelling, inflammation, extravasation;
- Thermal stimuli, for example, heat from inflammation;
- Chemical stimuli, for example, chemicals released in response to tissue damage, inflammation, ischaemia, infection and wound cleansing agents.
Transduction occurs when these noxious stimuli initiate the release of chemical mediators such as cyclo-oxygenase -2 [COX-2] and substance P. These chemical mediators activate and sensitise the nociceptors to noxious stimuli resulting in peripheral sensitisation (Table 4). This sensitised nociceptive state produces allodynia (pain due to a stimulus which does not normally provoke pain) and hyperalgesia (an increased response to a stimulus which is normally painful) (Johnson, 2009). Hyperalgesia is observed in the post operative patients as primary hyperalgesia at the site of the surgical wound and secondary hyperalgesia in the healthy tissue surrounding the surgical wound (Johnson, 2009). The intensity of the noxious stimuli and the chemical mediators are responsible for initiating an action potential in the nociceptor. The action potential is then converted to a nerve (pain) impulse.
The nerve (pain) impulse is transmitted along the A delta and C fibres from the PNS to the dorsal horn of the spinal cord, where the A delta and C fibres terminate (Figure 1). The nerve impulse transmits information about the noxious stimuli including;
- Intensity of pain: for example, sharp, severe associated with the A delta fibre and dull, aching associated with the C fibre;
- Location: for example, incision site, intravenous cannula site or wound drain site.
Various neurotransmitters such as glutamate and substance P, are released in the spinal cord in response to noxious stimuli. These neurotransmitters enable the transmission of the nerve impulse, across the spinal cord, along the ascending spinal pain pathways to the brain stem (ANZCA, 2010).
Perception is the process where pain becomes a conscious sensation. The brain does not have a single centre associated with pain but there is a complex interplay between many different centres depending on their activation by pre, intra and post operative neuronal, psychological, environmental and social influences. The level of this activation and interplay of the brain centres depends on patients’ individual pre and postoperative circumstances including:
- Fear of post operative pain;
- Previous pain experience;
- Emergency or elective surgery;
- Pre operative anxiety;
This interplay of information between the different centres of the brain is termed the ‘pain matrix’ and is not fully understood. The overall evaluation of this information will result in the patient’s own perception of their POP.
The transmission of the nerve impulse associated with noxious stimuli can be inhibited or facilitated via the peripheral A beta fibre (sensory cutaneous) stimulation and/or the descending modulatory pain pathway (DMPP) from the brain to the spinal cord. This is referred to as modulation (Fig 1).
Transcutaneous electrical nerve stimulation (TENS), massage, acupuncture and heat/cold are examples of non-noxious A beta fibre stimulation. If A beta fibre stimulation is strong enough nerve impulses will inhibit or partially inhibit the A delta and C fibre nerve impulse from travelling across the spinal cord. Consequently the perception of POP is inhibited or partially inhibited. There is limited evidence to support TENS, massage, heat/cold for effective POP management (ANZCA, 2010). If POP is persistent there may be changes in the function of the A beta fibres. This includes a change in the balance of mediatory and inhibitor chemicals at the receptor and fibre sites, thus the response to non-painful stimuli are altered, resulting in the non-painful stimuli being transmitted to the CNS as noxious information. This may result in the patient perceiving the non-noxious stimuli to be painful. This is described as allodynia.
Use of relaxation for POP management may initiate the inhibitory DMPP and result in the full or partial blocking of the nerve (pain) impulse (Fig 1). Relaxation stimulates the release of inhibitory chemicals in the spinal cord. These inhibitory chemicals will block or partially block the transmission of the pain impulse ascending to the brain, thus blocking or diminishing the perception of POP.
This is however not as straightforward as it appears. Patients have different levels of these inhibitory chemicals and this in turn is influenced by many factors such as time of surgery, type and intensity of noxious stimuli, length of time following noxious stimuli, genetics, age, gender, and culture. To compound this further, the effectiveness of relaxation for POP is also influenced by a range of additional factors including, environmental issues, psychological, social and spiritual needs. One patient may have a different response to relaxation compared to another patient. The evidence to support the use of relaxation for POP is weak (ANZCA, 2010).
The DMPP may also be involved with facilitating the nerve (pain) impulse within the CNS. This facilitation increases the intensity of POP the patient experiences, known as hyperalgesia (Johnson, 2009).This may be initiated by;
- Pre and post operative anxiety;
- Ineffective preoperative information about POP
- Unrelieved POP
- Repeated post operative treatments where pain is unrelieved such as wound dressing changes.
If facilitation persists then pathophysiological changes in the PNS and CNS may occur resulting in peripheral and central sensitisation (Table 4), which, if prolonged, may become irreversible resulting in CPSP (Samaraee et al, 2010).
Nurses need to have an underpinning knowledge of pain physiology to understand how their patients experience pain. They also need to be aware of and use the correct terminology when describing pain to other members of the healthcare team. Part 2 of this series explores the assessment and management of patients with post operative pain.
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