The Pain Visual Analog Scale: Is It Linear or Nonlinear?

Abstract

To the Editor:-I have spent some time considering the conclusions reached by Aubrun et al. 1 in their study investigating the relationship between the pain visual analogue scale (VAS) and morphine requirements in the postanesthesia care unit. They demonstrated that the Hill equation could be used to derive a sigmoid relationship between the pain VAS and morphine dosage, and concluded that this relationship is not linear. In their discussion, they referred to an earlier study in which I and colleagues had found a linear relationship between pain intensity and the pain VAS, 2 and they seemed to suggest that their study contradicted our earlier results. The results of the Aubrun et al. study could have been readily predicted from their methodology, in that the Hill equation is a standard approach used to generate a sigmoid curve using population pharmacokinetics. This approach generates a typical dose-response curve. Such a curve could be derived for the dose of a hypotensive drug and blood pressure, or the dose of a sedative drug and hypnotic state. Yet, the effect of interest-be it blood pressure, hypnotic state, or pain intensity-can still be linear phenomena. The results of the study data analyzed by the authors do not preclude the possibility that the pain VAS has ratio scale properties and is linear. In fact, if the pain VAS is linear then it would have a sigmoid relationship with morphine dosage. I have one other concern regarding the conclusions drawn by the authors. In their Methods, they arbitrarily defined "severe pain" as when the patient received a total dose of intravenous morphine greater than 0.15 mg/kg. Because they found an association between an initial VAS score of 70 or greater (VAS 70) and morphine dosage greater than 0.15 mg/kg, they concluded that VAS 70 was indicative of severe pain. But VAS 70 itself may not signify severe pain. Opioid dose-response has substantial interindividual variability, with a five-to tenfold range in requirements. Patients may fail to respond to a specific dose not because they have greater pain intensity but because they have had a lesser response to that dose.

The end of the line? The Visual Analogue Scale and Verbal Numerical Rating Scale as pain assessment tools in the emergency department

“…The second assumption is that they incorporate the same level of measurement. There is debate in the literature on this point—that is, whether the VAS and VNRS are ratio, interval or ordinal scales in all circumstances 26 27 29 30. In keeping with a range of other studies, we have adopted the simpler approach of treating both scales as being measured at the same level 19 20 24 25.…”

Validation of a Transient Pain Monitor in Healthy Volunteers

“…18 Thus, the tPM parameters (especially the E max ) could have displayed a better linearity than VAS, but this was not confirmed here, although a better reliability to the intensity of stimulation with the tPM parameters was suggested by the lower Bmismatching rate[ at the 2 higher levels of stimulation, with the 2 tPM parameters. It must be added that, as for ethical reasons, the maximal intensity of stimulation was limited, very high pain intensities (ie, pain scores on VAS 980/100) were not frequently reached; then, the loss of linearity of the VAS scores, which has been reported at levels of intense pain, 6 could have been masked in the present experiment.…”

“…Painful stimulation was generated with an infrared CO 2 laser stimulator, a system able to generate predictable levels of punctuate pain, after the amount of energy delivered. 12 We worked under the hypothesis that the reported pain intensity would display a linear relation to the intensity of stimulation in the conditions of the study because (i) such linearity is suggested for VAS in the range of moderate pain (30Y50/100) 6 and (ii) the cross-modality matching of force of handgrip to warmth on the arm following a Stevens power function exponent is close to 1 (0.96). 13 …”

Quality improvement project to reduce perioperative opioid oversedation events in a paediatric hospital

“…Good pain control was defined as a score of ≤3 on the Faces, Legs, Activity, Cry, Consolability tool (FLACC),18 Visual Analog Scale (VAS),19 or Neonatal Infant Pain Scale (NIPS),20 or a score of ≤5 on the Children's Hospital of Eastern Ontario Pain Scale (CHEOPS) 21. Additionally, patients had to reach an appropriate level of sedation (or wakefulness) before they could be transferred to the floor.…”