Anesthesiology 2001; 95:1356–61 © 2001 American Society of Anesthesiologists, Inc. Lippincott Williams & Wilkins, Inc. The Visual Analog Scale for Pain Clinical Significance in Postoperative Patients Carol A. Bodian, Dr.P.H.,* Gordon Freedman, M.D.,† Sabera Hossain, M.S.,§ James B. Eisenkraft, M.D.,ʈ Yaakov Beilin, M.D.‡ Background: The visual analog scale is widely used in re- able.” Its simplicity, reliability, and validity, as well as its search studies, but its connection with clinical experience out- ratio scale properties, make the VAS the optimal tool for side the research setting and the best way to administer the VAS describing pain severity or intensity.1 forms are not well established. This study defines changes in dosing of intravenous patient-controlled analgesia as a clini- There remain some outstanding questions regarding Downloaded from http://pubs.asahq.org/anesthesiology/article-pdf/95/6/1356/333546/0000542-200112000-00013.pdf by guest on 26 September 2021 cally relevant outcome and compares it with VAS measures of the use of the VAS in research studies. We address three postoperative pain. of these in this report. First, an optimal connection Methods: Visual analog scale measurements were obtained between VAS values and clinical experience outside the from 150 patients on the morning after intraabdominal sur- research setting is not well established. Certain differ- gery. On the same afternoon, 50 of the patients provided a VAS score on the same form used in the morning, 50 on a new form, ences between groups in their VAS scores or changes in and 50 were not asked for a second VAS measurement. score may have no clinical relevance, even if they Results: Visual analog scale values and changes in value were achieve statistical significance.2–6 Moreover, a patient’s similar for patients who were given a new VAS form in the perception of meaningful change in pain may depend on afternoon and those who used the form that showed the morn- the initial level of pain.7 The latter suggestion came from ing value. The proportions of patients requesting additional analgesia were 4, 43, and 80%, corresponding to afternoon VAS a study that used hypothetical assessments about a mean- scores of 30 or less, 31–70, and greater than 70, respectively. ingful pain reduction from various starting levels. In our Change from morning VAS score had no apparent influence on study, the patient’s request for additional analgesia pro- patient-controlled analgesic dosing for patients with afternoon vides a concrete definition of clinical relevance to help values of 30 or less or greater than 70, but changes in VAS scores interpret absolute values and changes in values of VAS of at least 10 did discriminate among patients whose afternoon values were between 31 and 70. scores. Conclusions: When pain is an outcome measure in research Second, when the VAS is administered repeatedly to studies, grouping final VAS scores into a small number of cate- the same patient, the question of whether or not earlier gories provides greater clinical relevance for comparisons than values should be visible to the patient on succeeding using the full spectrum of measured values or changes in value. forms has not been well answered.8,9 Third, another Seeing an earlier VAS form has no apparent influence on later values. issue to consider in measuring pain for research pur- poses is the possibility that the process of collecting data THE current investigation was motivated by discussions for research might itself influence the clinically relevant about planning studies to compare the effectiveness of outcome. We have included a control group in the cur- various analgesic agents in relieving postoperative pain. rent study to shed light on that question. The main issue was how to characterize the intensity of We address these three issues in the setting of postop- pain so that the study results would reflect clinically erative pain assessment in a series of patients who important differences between groups. agreed to use intravenous patient-controlled analgesia The visual analogue scale (VAS) is commonly used as (PCA) after intraabdominal surgery with general anesthe- sia. The reliability of VAS measurements as a measure of the outcome measure for such studies. It is usually pre- 10 sented as a 100-mm horizontal line on which the pa- pain intensity and of differences in VAS measurements as a measure of change in pain sensation for patients tient’s pain intensity is represented by a point between 11 the extremes of “no pain at all” and “worst pain imagin- experiencing mild to moderate pain has been demon- strated in postoperative patients. * Associate Professor of Biomathematical Sciences, † Assistant Professor of Anesthesiology and Obstetrics, Gynecology and Reproductive Sciences, ‡ Asso- Materials and Methods ciate Professor of Anesthesiology, § Programmer Analyst of Biomathematical Sciences, ʈ Professor of Anesthesiology. After approval by the Institutional Review Board of the Received from the Departments of Biomathematical Sciences, Anesthesiology, Mount Sinai School of Medicine (New York, New York), and Obstetrics, Gynecology and Reproductive Sciences, Mount Sinai School of written informed consent was obtained from patients Medicine of New York University, New York, New York. Submitted for publica- tion December 22, 2000. Accepted for publication July 26, 2001. Support was scheduled to undergo intraabdominal surgery with gen- provided solely from institutional and/or departmental sources. Presented in part eral anesthesia to participate in the current prospective, at the annual meeting of the American Society of Regional Anesthesia, Atlanta, Georgia, April 12, 1997. randomized study. All patients had agreed to the use of Address reprint requests to Dr. Bodian: The Mount Sinai Medical Center, Department intravenous PCA for their postoperative pain management. of Biomathematical Sciences, Box 1023, One Gustave L. Levy Place, New York, NY 10029-6574. Address electronic mail to: [email protected]. Individual article No changes from routine use of the PCA pump were reprints may be purchased through the Journal Web site, www.anesthesiology.org. made for the study. The pump was regulated to allow Anesthesiology, V 95, No 6, Dec 2001 1356 VISUAL ANALOG SCALE IN POSTOPERATIVE PATIENTS 1357 Table 1. Demographic Data and Morning VAS Scores New Form Group Same Form Group Control Group No. of patients 50 50 50 Age (yr) 49 (37–61) 49 (35–60) 53.5 (38–71) Height (cm) 170 (167–178) 167 (163–173) 170 (163–178) Weight (kg) 69 (60–80) 61 (55–72) 68 (57–80) Gender (% male) 62% 42% 52% Day 1 postoperative morning VAS score 38.5 (22–51) 34.5 (19–58) 49.5 (25–71) Data are presented as median and interquartile range, or percent. New form group ϭ patients marked a new visual analog scale (VAS) form in the afternoon; Same form group ϭ afternoon VAS markings were put on same form ϭ as morning markings; Control group no afternoon pain scores were obtained. Downloaded from http://pubs.asahq.org/anesthesiology/article-pdf/95/6/1356/333546/0000542-200112000-00013.pdf by guest on 26 September 2021 the patient to self-administer morphine sulphate in anesthesiologist unaware of their content. If the patient amounts between 1.0 and 1.5 mg (demand dose), de- was not satisfied with the degree of pain relief and pending on the patient’s age and weight. The demand wanted additional pain medication, the pump settings dose was limited to between six and eight doses per were increased (clinical change). If the patient com- hour, also depending on age and weight. None of the plained about side effects from the medication, such as patients received a basal infusion of morphine from the pruritus, the medication may also have been changed, PCA pump, and all patients were encouraged to ambu- but in the current study clinical change refers only to late, according to the routine of the Department of changes related to pain management, not to those re- Surgery. lated to side effects. On the morning of postoperative day 1, a research assistant presented each patient with a VAS for pain, Statistical Analysis 12 followed by a McGill pain questionnaire. The VAS All data were entered into an Excel database and con- consisted of a 100-mm horizontal line anchored at one verted to a SAS file (SAS/STAT User’s Guide, version 6; end with the words “no pain” and at the other end with SAS Institute Inc., Cary, NC) for statistical analysis. Ordi- the words “worst pain imaginable.” The research assis- nal and categorical data were compared using the chi- tant asked the patient to mark the line at the point that square test or the chi-square test for trend. The Mantel- best represented the intensity of his or her pain. The VAS Haenszel test was used to compare groups stratifying on numeric value is the distance in millimeters from “no quartiles of morning VAS scores, to control for group pain” to the point marked by the patient. differences in the morning scores. The Wilcoxon or Patients were randomly assigned to one of three Kruskal-Wallis test was used to compare continuous vari- groups defined in terms of the protocol for afternoon ables. Differences were considered significant at P Ͻ 0.05. pain assessment. A table of random numbers generated Logistic regression analysis was used to study the con- the randomization sequence, using a restricted random- nection between use of additional analgesia and VAS ization scheme to assure equal numbers in each group. scores, the fit being tested by chi-square goodness-of-fit Group assignments were sealed in opaque envelopes criteria. Estimates of the probability of requesting addi- and opened sequentially by the investigators. tional analgesia at specified VAS values were obtained On the afternoon of postoperative day 1, between 8 from the fitted logistic regression model as p ϭ inv and 12 h after the morning assessment, patients in the (1 ϩ eϪ(aϩbx)), where a is the value of the fitted inter- first group (new-form group) were asked to complete a cept, b the slope, and x the VAS measurement.