Clinical evaluation of mild analgesics: the measurement of clinical pain

Abstract

1 Simultaneous subjective estimates of pain intensity, using both visual analogue (VAS) and categorical measurements, have been carried out in patients with postoperative pain and patients with chronic pain due to cancer. The categorical scale selected had demonstrated a power function relationship to intensity in cross-modality tests in the laboratory. In our hands, the relationship of the analogue and categorical parameters was also best described in terms of a power function (y = 1.39 x^2.261, where y = VAS and x = pain category).

2 When the population was divided into those patients who had postoperative pain and those who had chronic cancer pain, the responses of both groups also fit power curves (y = 0.99x^2.99 in postoperative pain and y = 1.19x^2.14 in chronic cancer pain). These curves diverged at the upper end of the scales where patients with chronic cancer pain tended to rate `strong', `severe' and `excruciating' pain as less intense on the analogue scale than did the postoperative patients. This is presumptive evidence that the two groups are interpreting their pain differently in the light of differing pain experiences. Concurrent VAS and categorical pain data are thus able to provide some insight into differences in interpretation of pain by a variety of patient groups. Similar analyses in terms of age, sex and pain aetiology are proposed.

3 Comparisons of VAS scores for individual pain categories at various times before and after drug administration demonstrate some downward vertical movement (slippage) in the categories after drug administration. VAS measurements seem to be more sensitive to smaller changes in effect than are the categorical measurements.

4 Increased sensitivity of VAS over categorical measurements was demonstrated in a twin crossover assay of oral zomepirac and intramuscular morphine in a limited population of 20 postoperative patients. The endpoints were peak and total relief as measured either by a VAS or a five-point categorical scale. Relative potency estimates were consistent for all parameters indicating oral zomepirac to be about one-sixth as potent as intramuscular morphine, but only VAS data gave finite 95% confidence limits in this small group of patients.