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Measuring acute pain in the prehospital setting
  1. P A Jennings1,2,
  2. P Cameron2,
  3. S Bernard1,2
  1. 1
    Ambulance Victoria, Melbourne, Victoria, Australia
  2. 2
    Monash University Department of Epidemiology and Preventive Medicine, Melbourne, Victoria, Australia
  1. Mr P A Jennings, Locked Bag 9000, Ballarat Mail Centre, Victoria, Australia 3354; paul.jennings{at}rav.vic.gov.au

Abstract

Severe pain is a common presenting symptom for emergency patients. One major challenge in the management of severe pain is the objective measurement of pain. Due to the subjective nature of pain, it can be very difficult for clinicians to quantify pain intensity and measure the qualitative features of the pain experience. A number of measurement tools have been validated in the acute care setting, with some appropriate for use in the prehospital setting. This paper reviews the characteristics required of a prehospital acute pain measure and appraises the relative utility of a number of currently used pain measures. At present, the verbal numerical rating scale appears the most appropriate pain measure to administer in the prehospital setting for adult patients as it is practical and valid. Either the Oucher scale or the faces pain scale is suitable for prehospital care providers to assess pain in children.

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Pain is a common presenting symptom for emergency patients. In North America, between 20% and 31% of patients transported by ambulance have moderate to severe pain,1 2 and pain is the main complaint in 70–86% of emergency department (ED) presentations.3 4 Whereas pain is a common presentation, it often remains undertreated.25 The difficulty associated in accurately assessing pain may be a major barrier to the appropriate provision of analgesia in the prehospital environment.1

There are a number of pain measurement tools available to the clinician to evaluate the intensity of a patient’s perception of their pain. Due to the subjective nature of pain, it can be very difficult for clinicians and researchers to quantify pain intensity and measure the qualitative features of the pain experience. Many pain measurement tools have been rigorously tested and validated in the hospital environment; however, only a few are appropriate for use in the prehospital setting.

The physiological mechanisms of pain are uncertain. The pain severity perceived by the individual is then dependent on a number of cognitive and experiential factors specific to the individual. Many pain measures are unidimensional; measuring only pain severity and not taking into account the many other complex factors, the net summation of which dictates an individual’s perception of pain. Multidimensional pain measures combine behavioural, contextual and physiological information. Although several multidimensional measures exist, their utility in the prehospital environment is uncertain due to their length and complexity. Moreover, the majority of pain measurement tools rely on patient self-report, rather than physiological measures.6

The prehospital environment brings with it a number of difficulties and barriers to patient assessment. The prehospital environment also has loud ambient noise and dim lighting that increase the challenges of accurate patient assessment before hospital arrival. Furthermore, the time constraints in patient assessment combined with the requirement for concurrent emergency management of a patient in pain contribute to these difficulties and render long and complex measures of pain inappropriate.

In order for pain measurement tools to be considered useful in the prehospital setting, several requirements must be satisfied. Prehospital pain measurement tools should: be quick and simple to administer; not rely upon specific equipment or recording documents; accurately report pain intensity and identify trends; be based on patient self-report; be applicable to all individuals taking into account psychological or emotional state, cultural background and age;3 be validated, consistent and reproducible with high intra and interrater reliability.

This paper aims to examine pain measurement within the prehospital setting, including the tools available to the clinician involved in the assessment and management of the patient with acute severe pain. The strengths, weaknesses and prehospital utility of these tools will be appraised.

METHODS

A review of the literature was conducted using several electronic medical literature databases, including Medline, the Cumulative Index of Nursing and Allied Health Literature (CINAHL) and EMBASE from their earliest record to the end of April 2008. This literature search included peer and non-peer reviewed journals and was combined with emergency treatment and emergency medical services subject headings to focus the search to more relevant literature. The medical subject headings: pain, emergency medical services, emergency medical technicians, pain measurement were combined and further limited to articles or abstracts written in English and human studies. All types of articles were considered for inclusion, and were included if the content was potentially relevant to prehospital acute pain measurement.

DISCUSSION

The objective of pain measurement is to quantify accurately the intensity of the patient’s pain experience and to be able to identify any changes over time and with treatment interventions. A reliable objective pain measurement tool is particularly important as clinicians’ perceptions have been shown to be inaccurate when used to quantify the intensity of pain being experienced by a patient. Clinicians have a tendency to underestimate pain7 and this underestimation increases with increasing clinical experience.8

Unidimensional rating scales

Given the time and logistical challenges of the prehospital environment, unidimensional tools are usually recommended as a trade-off to more complex but thorough measures.4 Table 1 lists the various pain measurement tools and summarises the strengths and weaknesses in relation to their applicability to the prehospital environment.

Table 1 Summary of pain measures

Heart rate, blood pressure and respiratory rate

One objective measure of pain is the measurement of heart rate; however, this can be affected by a number of factors beyond pain intensity, such as fever, anxiety and medications. Bossart et al9 compared the change in heart rate with the patient’s self-reported pain intensity (using a 100-mm visual analogue scale; VAS) in 975 ED patients. They found poor correlation (Pearson’s ρ 0.08; 95% CI 0.03 to 0.13) between the change in pain intensity and the change in heart rate.9 Another study investigating the association of heart rate, blood pressure and respiratory rate with pain intensity, Marco et al10 retrospectively examined 1063 cases of adults presenting to an ED with pain (including nephrolithiasis, myocardial infarction, small bowel obstruction, fracture, burn, crush injury, stab wound, amputation, corneal abrasion and dislocation). They found there were no clinically significant differences in mean vital signs across the individual pain scores, as demonstrated by overlapping confidence intervals across pain scores, concluding that there were no clinically significant associations between self-reported triage pain scores and heart rate, blood pressure, or respiratory rate.

Visual analogue scale

The VAS is commonly used for the rapid assessment of pain severity and consists of either a 100-mm horizontal or vertical line anchored by perpendicular stops. Horizontal lines are preferred as the scores are believed to be more normally distributed.11 The descriptors “no pain” and “worst pain ever” are placed at either end of the line. Patients place a mark on the line representative of their pain severity. The clinician then measures from the “no pain” anchor to the intersection of the mark drawn by the patient and reports the pain intensity. This provides a pain rating score out of a possible 100. The validity and reliability of this pain measure has been consistently demonstrated in a variety of settings.1115

Given the visual nature of this measure, reliability can be reduced when applied to patients who are visually impaired. Likewise, cognitive impairment and lack of understanding of the task or an inability to follow instructions can impact on the accuracy of the measure.16 The VAS has been found to be less accurate in the elderly17 and young children.18 This measure relies on the immediate availability of the pain scale for a patient to place their mark. Lord and Parsell19 reported that approximately one quarter (26%) of paramedics considered that VAS was too cumbersome and the VAS device was often lost or difficult to locate when required. It is possible that the use of portable electronic information system tablets may improve the application of pain measures requiring patient interaction or complex calculations; however, a pain scale that does not require any equipment would be logistically superior.

Verbal numerical rating scale

The verbal numerical rating scale (VNRS) is a commonly used and easily applied measure. It requires the patient being asked to rate the intensity of their pain from an 11-point scale where 0 is considered “no pain” up to 10 considered the “worst possible pain”. The benefits of this scale are that it is quick to administer and provides a quantitative rating of the patient’s perceived pain intensity. The VNRS has been validated in patients aged 13 years and older2 and has been found to be comparable in accuracy to the VAS.20 Interestingly, although the VNRS has been found to perform as well as the VAS in assessing changes to pain, patients consistently score their pain higher on the VNRS.21 One of the clear benefits of the VNRS is that it does not require any associated equipment, unlike the VAS, which is particularly beneficial in the prehospital setting. One potential barrier to the use of the VNRS is difficulty in translating instructions to the patient if there is a language barrier.16 The VNRS has been demonstrated to be valid and reliable in the young and elderly patient.22 In particular, the VNRS may be superior to the VAS in the elderly patient with chronic pain.23

Adjective response scale

The adjective response scale (ARS) consists of between three and five ranked pain verbal descriptors, “none”, “slight”, “moderate”, “severe” and “agonising”. This scale has been evaluated for its validity, reliability and ease of use and strong correlations were found between the ARS and the VAS. One of the limitations of the ARS is the relative lack of discrimination with a five-point scale. Also, like the VNRS, there are potential barriers with language and cross-cultural issues.

Verbal descriptor scale

The verbal descriptor scale is very similar to the ARS and is based on the present pain index from the McGill pain questionnaire (MPQ). In this measure, a verbal descriptor scale ranges from 0 being “none” to 5 being “excruciating” pain, and the patients choose a descriptor of their pain as it is experienced “right now”. The verbal descriptor scale has been validated in both younger people and in the elderly with chronic pain.22

The faces pain scale

There are several versions of the faces pain scale, which are predominantly used for assessing pain in children. Alternative measures are required as children have limited cognitive ability and are unable to use most of the adult scales.4 These include the Wong and Baker faces pain scale (WBFPS)24 and the Bieri scale.25 The WBFPS uses six pictures of faces, whereas the Bieri scale utilises seven. For no pain, the Bieri scale has a neutral face, whereas the WBFPS has a smiling face. In both scales, the faces in between show steadily increasing intensities of pain. The faces pain scale has been validated for use in children aged 5 years and older in non-acute settings and requires clear instructions and presentation. As the measure is pictorial, translation in terms of multicultural use is not necessarily required and it could be used in cross-cultural studies, but there is no evidence to support this at this time.16

Colour analogue scale

The colour analogue scale is similar to the VAS and is anchored by the descriptors “no pain” and “worst pain”. The patient slides a marker to the point on the thermometer that best describes the pain they are currently experiencing, with a colour shaded from a bright, dense colour to white. The reverse side of this instrument has a numerical rating scale so that a number from 1 to 10 can be assigned to the individual assessments. Bulloch and Tenenbein26 have validated the colour analogue scale and found it to be valid in the measurement of acute pain in children in the ED when compared against the seven-point faces pain scale.

The Oucher scale

The Oucher scale is another pain scale used to measure pain in children and combines pictures much like the faces pain scale with a vertical VAS. It has previously been validated in children 3–12 years of age.6 Although not yet validated in the emergency or prehospital setting, the Oucher scale appears promising in the prehospital environment.6

Although the applicability of these scales for use with young children makes them appealing to prehospital care providers, they still rely on ready access to the pictorial scale available at the time of pain assessment.

Multidimensional rating scales

Multidimensional pain measures take into account a range of pain dimensions beyond solely pain intensity. A multidimensional assessment aims to explore and measure physical, psychological, social, cultural and spiritual components of pain. They are far more complex to administer, require specific training in their use and interpretation, are time consuming and rely on the availability of assessment records. For these reasons, the potential utility of the multidimensional pain scale in the prehospital environment is low.

The McGill pain questionnaire

The MPQ is the most well-known multidimensional scale, and has been shown to be valid, reliable and consistent. The MPQ relies on the patient to choose a range of descriptors. The descriptors fall into four major groups: sensory, affective, evaluative and miscellaneous. A value is given to each of the descriptors based on the position of this descriptor within a word set and the sum of the values provides a pain rating index. The MPQ includes the present pain intensity based on a scale of 0 to 5; 0 being “no pain” and 5 being “excruciating pain”.23 The MPQ is administered by reading a list of descriptors to a patient and asking them to choose a descriptor that best describes their pain at the moment. The MPQ is reported to take 15 minutes to administer and score, and given this, it has limited application within the prehospital environment and questionable application within the emergency medicine setting. There has only been one study using the MPQ in an emergency care setting.27

A short form of the MPQ has been developed that takes approximately 5 minutes to complete. The short form was made up of only words from the original questionnaire that at least 33% of patients used. This yields 15 terms, 11 from the sensory domain and four from the affective domain. Patients are asked to describe how each of the 15 words applies to their current pain on a four-point scale none: mild, moderate and severe. The short form MPQ shows promise in situations in which the standard MPQ would be considered to take too long to administer, the VAS is believed inadequate, yet qualitative information is required.28 It is definitely worthy of further examination in the prehospital setting.

Descriptor differential scales of pain intensity and pain affect

This scale was developed by Gracely et al29 in 1978, with the aim of reducing bias and having the ability to assess separately the sensory intensity and “unpleasantness” of pain or pain affect, essentially being able to use what Gracely and colleagues29 defined as a cross-modality-matching procedure.23 The descriptor differential scale utilises two forms that measure separately the sensory intensity and pain affect. This scale has been shown to be sensitive in the experimental pain setting, but not in the acute clinical setting.11 Some recent validation studies have shown that the descriptor differential scale is sensitive to small changes, perhaps even more sensitive than the VAS, and they were able to confirm the ratio scale properties of the tool.11 Although possessing a benefit of being multidimensional, this measurement tool would be far too time consuming for the prehospital environment and the requirement for a score sheet is also a disadvantage.

The brief pain inventory

This interference scale is another self-reported measure that is multidimensional. It aims to measure two dimensions: pain intensity and pain interference. The pain interference scale uses an 11-point numerical scale; 0 being “no interference” to 10 being “interferes completely”, and it aims to assess the interference as a result of pain in seven areas: general activity, mood, walking ability, normal work including outside the home and housework, relations with other people, enjoyment of life and sleep.23 This scale has demonstrated excellent psychometric properties and has been shown to be valid and reliable. It is well suited to chronic pain, and has been used previously in studies relating to cancer, osteoarthritis and neuropathic pain.23 Its applicability to acute pain and the prehospital/emergency environment is yet to be established.

CONCLUSION

For a pain measurement tool to have a place in the prehospital setting, it must be quick to administer, have satisfactory inter and intrapersonal reliability, not rely upon specialised equipment or records and be an accurate, valid and reproducible measure of pain.

Currently, the VNRS appears to be the most appropriate pain measure to administer in the prehospital setting for adult patients, as it is practical and valid. The Oucher scale or the faces pain scale appear most suited in children aged more than 3 or 5 years, respectively; however, these rely on an additional instrument that may be difficult to locate immediately when required in this setting.

Although multidimensional pain measures take into account a range of pain dimensions beyond solely pain intensity, they are complex to administer, require specific training in their use and interpretation, are time consuming and rely on the availability of assessment forms. For these reasons, the use of multidimensional pain scales in the prehospital environment is not recommended.

REFERENCES

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Footnotes

  • Competing interests: None.

  • Contributors: PAJ had the idea for the article. PC and SB provided advice regarding the focus of the article. PAJ undertook the literature search and wrote the article. PC and SB critically reviewed the article and provided further advice. PAJ is the guarantor.

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