Acute pain is generally accepted as being of recent onset and limited short duration. It usually has a temporal (follows immediately after surgery/trauma) and causal (has a known cause) relationship to injury or disease. The intensity of acute pain is greatest at the onset of injury but with healing pain intensity reduces. The International Association for the Study of Pain (IASP) offered this definition of pain 'an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage' . McCaffery  offered 'pain is what the experiencing person says it is and exists when experiencing person says it does'. What is lacking in the IASP definition is the acknowledgement that patients can communicate their pain and should be involved with the assessment and management of their pain. Conversely what is lacking in the McCaffrey definition is the acknowledgement that some patients cannot express their pain e.g. those who are sedated, the very young, those with dementia or learning difficulties. Pain assessment therefore needs to be tailored to the individual and we need to use valid and reliable tools to help patients’ share with us their pain in ways that we can understand and can use to plan effective pain relief.
Acute pain is usually associated with an underlying physiological (labour pain) or pathological (postoperative pain) process. It may be recurrent, with or without a background of ongoing chronic pain, (e.g. sickle cell disease, rheumatoid arthritis). Particularly after surgery, patients will be subjected to degrees of pain and we need to be able to assess this pain, commence pain strategies preoperatively if possible and implement strategies to minimise the pain so that the patient is able to deep breath, cough and mobilise comfortably postoperatively. The duration of severe pain after surgery tends to be relatively short-lived and patient requirements for pain relief usually reduce significantly after 48-60 hours post operation. However, patients who do have severe pain for longer must be believed and investigated for any problems that may maintain the severity of the pain. We also need to be aware of the importance of evaluating our pain management strategies to ensure that we have 'got it right'. Getting it right involves not only patient satisfaction outcomes, but a reduction in pain scores with an improvement in morbidity and mortality while considering and managing risks.
The degree of postoperative pain experienced by individuals varies enormously. There are many reasons that people will have different levels of pain postoperatively such as the site of the operation. If you consider the sites where muscles are made to move then it is relatively easy to work out that thoracic or upper abdominal surgery is more painful than procedures involving the head, neck or limbs. Thoracic and abdominal incisions produce the most severe pain particularly when the peritoneum or pleura have been breached and the favoured approach for optimal analgesia is usually neuraxial blockade. Other factors include the pharmacokinetic and pharmacodynamic variations between patients; the Minimum Effective Analgesic Concentration (MEAC, the lowest plasma concentration associated with an analgesic effect) varies fourfold between patients (Austin et al 1980). Individual beliefs and values of both the patient and the postoperative staff will impact on the pain experience. Older patients become more sensitive to the side effects of opioids therefore the dose to achieve a desired effect may be reduced. There are many other reasons why people feel different levels of pain and there are resources within the acute pain domain if you want to explore these further.