The stress response to pain is thought to trigger pathophysiological mechanisms leading to hyperglycaemia, cortisol secretion, catecholamine release and the secretion of anti-diuretic hormones [10, 11]. This is evidenced by studies demonstrating a correlation between pain scores and plasma levels of stress hormones in the post-operative period [12] and studies of perioperative analgesia, indicating a less exaggerated stress response when regional anaesthesia and effective pain relief are used [13, 14]. It is thought that activation of the autonomic system, as seen in the surgical stress response, is in part due to pain [15]. Therefore, pain may cause an amplified stress response in patients who already have a severe inflammatory burden, resulting in greater physiological disturbance.

Pain can have an impact on ventilation. Inadequate analgesia in spontaneously ventilating patients predisposes them to atelectasis, and subsequently hospital-acquired pneumonia and hypoxaemia. Although the majority of evidence comes from surgical rather than medical patients, there is a correlation between high pain intensity and poor respiratory outcomes [16, 17]. It also impacts on mechanically ventilated patients. Merely assessing pain has been shown to correlate with a reduction in the duration of mechanical ventilation [18] and a decrease in nosocomial infections [19]. Respiratory and musculoskeletal physiotherapy is important for improving mobility, weaning from mechanical ventilation and global functional recovery. Pain has been shown to be a barrier for initiating and adherence to such programmes [20].

Wound healing may also be improved, indirectly, by providing effective pain relief. Subcutaneous partial pressure of oxygen is increased post-operatively in those with adequate analgesia [21] and therefore could reduce the incidence of wound infection [22].

Chronic pain is a well-recognised complication of acute pain. Both medical and surgical patients who recall experiencing pain whilst on ICU show a higher incidence of chronic pain [23]. The incidence of post-ICU chronic pain is between 30 and 40 % at 6 months and can significantly impact on both physical and emotional rehabilitation [24]. This incidence is higher than other chronic post-interventional pain states such as post-thoracotomy pain [25] and post-mastectomy pain [26]. ARDS and sepsis are also associated with the development of chronic pain in large cohort studies [24, 27].

Changes in excitability in peripheral nociceptors (peripheral sensitisation) or in the dorsal horn and/or brain (central sensitisation) through exposure to repeated stimuli and mediators of tissue injury are thought to contribute to the development of chronic pain along with a reduction in the descending inhibitory pathways. Furthermore, long-term structural and functional damage to the neurones predispose the patient to developing neuropathic pain [28].

Acute psychological disturbances such as anxiety and depression, as well as feelings of helplessness and nightmares have all been associated with inadequate analgesia [3] in the critically ill. Long-term follow-up of ICU patients has also revealed the correlation between acute pain and the development of post-traumatic stress disorder (PTSD) [29].

Failing to provide a patient with the basic right of comfort whilst ill could be seen as a breach of their human rights. Whilst controversial, there could be a legal right to effective analgesia, and certainly physicians should consider whether inadequately addressing pain is against both the Hippocratic oath and Declaration of Geneva.

There is also a financial incentive for the health care provider to deliver appropriate analgesia; the I-SAVE study showed nearly a $1000 saving per ICU hospitalisation when patients received a protocolised analgesic and sedation regimen [30]. A reduction in total length of hospital stay, reduction in duration of mechanical ventilation and total usage of analgesic and sedative agents were also observed in a group of trauma patients, after a protocolised analgesic and sedation plan was implemented [31].

Traditionally, it was assumed that the ability to perceive pain relied on a level of consciousness to enable interpretation of the nociceptive input. Painful stimuli in the periphery ascend, via the thalamus, to the somatosensory cortex (primary areas) and other higher order (secondary) areas such as the amygdala and insula, which coordinate a behavioural reaction to this stimulus. However, functional MRI has shown activation of both primary and secondary cortical areas, even when the patient clinically has a severely deranged level of consciousness [32]. Inconsistent voluntary and non-voluntary behavioural responses are also seen in patients in the minimally conscious state, indicating some degree of pain perception [33]. Therefore, an altered level of consciousness cannot be interpreted as an inability to perceive pain, but it does make its assessment challenging.

Consequently, guidelines for the assessment of pain on the ICU take this into account [4], suggesting pain is assessed regularly in all patients, recommending certain validated tools for specific circumstances.

Pain, a subjective experience, may not correlate to the perceived severity of an insult as it is based on prior experience, pain thresholds, emotional state, comorbidities and physiological response to the injury. Consequently, the ‘gold standard’ for assessing pain is self-reporting, where the patient commonly evaluates pain intensity using a predetermined scale of numbers or words. In those able to communicate in ICU, an enlarged version of the numerical rating scale (NRS) has been shown to be the most valid and feasible method [34].

In ICU, a self-report of pain is often limited by an inability to communicate. Patients may have an altered level of consciousness as a result of an underlying pathology, delirium or sedative medication. In these circumstances, pseudo-objective tools are useful, and the ACCM guidelines [4] recommend the use of either the Critical Care Pain Observation Tool (Table 5.2) or the Behavioural Pain Score (BPS) (Table 5.3) [35, 36] after subjecting a number of behavioural tools to psychometric analysis for reliability, validity and feasibility.