Örebro University School of Medicine Degree project, 30 ECTS 2016-06-06
Differences in the experience of postoperative pain
amongst women compared to men after laparoscopic
gastric bypass surgery. A cohort study.
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Version 2
Author: Markus Hartwig, bachelor of medicine Supervisor: Erik Stenberg, MD
Abstract
Introduction
Women seem to experience more postoperative pain than men. Few studies has, however, addressed this issue amongst obese patients undergoing bariatric surgery.
The aim of this study was to evaluate whether there are any differences in the experience of postoperative pain in women compared to men after laparoscopic gastric bypass (LGBP) surgery.
Methods
Using data from a prospective register (Painout), 134 women and 55 men who had undergone LGBP surgery were selected. The primary outcome was experience of pain when being the worst since surgery. Pain was evaluated as mean value of the visual analogue scale (VAS), and categorically as prevalence of severe pain (VAS 7-10). Multivariate analyses were conducted to standardize for body mass index (BMI), age and pre-existing pain.
Results
The mean VAS rating of pain was 4.9 (95% CI 4.3-5.6) among men and 5.9 (95% CI 5.5-6.3) among women, p=0.017. Among women, 48.5 percent experienced severe pain, among men 32.7 percent, p=0.047.
In both the linear and logistic regression analyses there was a statistical significant difference in worst pain since surgery and prevalence of severe pain related to the prevalence of pre-existing pain (adjusted beta coefficient 1.37, 95% CI 0.21 – 2.53, p=0.021; adjusted OR 2.82, 95% CI -1.00 – 7.93, p=0.049, for the presence of pain compared to no pain, respectively), and to the differences in age (adjusted beta coefficient -0.07, 95% CI -0.11 – -0.04, p<0.001; adjusted OR 0.96, 95% CI 0.93 – 0.98, p=0.002, per year, respectively). No statistical significant differences in postoperative pain between gender were seen in the multivariate analyses.
Conclusion
We where able to show that women indeed do experience more pain than men after LGBP surgery. These results, however, seem to be caused not by gender itself, but instead by younger age and a higher occurrence of preoperative pain among women.
Introduction
Pain is defined as an unpleasant sensory and emotional experience associated with actual or potential tissue damage [1]. Pain is a very common symptom associated with surgery, and there are evident relations between postoperative pain and increased immune deficiency, reduced wound healing and the occurrence of chronic pain [2,3]. Reducing the occurrence of postoperative pain has been shown to result in earlier mobilization after surgery and earlier discharge from hospital [4].
The prevalence of moderate to severe postoperative pain has been reported being 30-70 percent depending on the type of surgery [5,6], and, in America, several studies [7,8] have shown a prevalence of postoperative pain closer to 80 percent, in which the majority of the patients reported moderate to severe pain.
There are several different instruments for measuring pain. Many of these offer a quantified rating of the individual’s pain [9]. The most common rating scales include visual analogue scale (VAS), numeric rating scale (NRS) and verbal rating scale (VRS) [10]. VAS is the most commonly used rating scale in pain research and consists of a 10-centimetre ruler, marked from 0 to 10, where one end correspond to no pain and the other end to worst pain ever. The patient marks on the scale the level that best correspond to the patients’ level of pain [9]. The reliability of VAS for measuring acute pain is high [11], and the scale can be used both visually and verbally [9]. The result of pain rating according to VAS and NRS can be categorized into three groups: mild, moderate, and severe pain [12].
Women seem to experience more pain in the postoperative period than men [13-19], however, in a few previous studies [20,21] this difference was not seen.
In a study performed by Aubrun et al. [16] including 4317 adult patients (≥ 18 years), of which 73 % had undergone orthopaedic surgery, women reported a higher initial pain rating according to VAS than did men. Women also required higher amounts of morphine. In elderly patients (≥ 75 years) there was no statistical significant difference between sexes. Also among adolescents (12-18 years), girls reported higher grade of postoperative pain than boys [13]. The differences in pain rating between sexes tend to be more evident concerning mild to moderate pain, whereas the difference in occurrence of severe pain is not as evident [13,17]. The differences in pain rating seem to be of highest magnitude in the immediate postoperative period [14,16]. Morin et al. [21] demonstrated that women experience maximum pain
intensity in the afternoon, while men experience maximum pain in the evening, although the intensity of pain did not differ between the two groups. Besides female gender, there are several other known risk factors that may contribute to postoperative pain, the most prominent being younger age [5,22-27], preoperative or chronic pain [22-26,28,29] and obesity [5,24]. However, not many studies are considering these factors when comparing postoperative pain in men and women.
Obesity is considered a pandemic disease rapidly increasing [30], and today, bariatric surgery is considered being the most effective treatment option [31,32]. In 2013 the total number of bariatric surgery procedures was 468 609 worldwide, and the most common procedure in Europe being Roux-en-Y gastric bypass (RYGB). Of all bariatric surgery procedures worldwide, 95.7 percent was carried out laparoscopically [31].
In Sweden, the number of bariatric surgery procedures has had an increasing course. Until 2002 approximately 700 procedures were performed every year, while later on there has been a substantial increase, partly because new research that supported the intervention was
published in 2002 [33].
In 2014 there were 6800 bariatric surgery procedures performed in Sweden at 43 different institutions. The main procedure was RYGP (81.5 percent). Of all bariatric surgery, 97.3 percent was performed laparoscopically [34].
Previous studies [32,35-38] have shown a substantial majority of women undergoing bariatric surgery, comprising 63-85 percent of the patients. In Sweden, 75.6 percent of the patients undergoing bariatric surgery in 2014 were women [34].
Aim
Since bariatric surgery is an increasing surgical procedure of which the vast majority of patients are women, the aim of the present study was to evaluate whether there are any differences in the experience of postoperative pain in women compared to men after laparoscopic gastric bypass surgery.
Material and methods
Study design and Setting
In the present study data from Painout, a research project financed by the EU for accessing a quality register for postoperative pain treatment, have been used. The patients included in the project rated different parameters, such as postoperative pain, nausea, anxiety and their level of satisfaction with the pain treatment in a patient outcome questionnaire. For each patient questionnaire a process formula was linked, which, besides from demographic data, also contained data concerning surgical procedure, comorbidities, type of anaesthesia and medications given pre-, intra- and postoperatively in the recovery room and at the ward. All data were collected the day after surgery, in 12 to 24 hours after the surgical procedure. A requirement for participation in the register was that the patients should have been treated at the ward for at least six hours after surgery.
Örebro University Hospital (USÖ) participated in the project, and, during the year 2010 to 2012, data was collected from a total of 2649 patients that had undergone elective surgery.
Participants
We wanted to study a group of patients that had undergone the same type of surgery. For the present study, patients who had undergone laparoscopic gastric bypass (LGBP) surgery were selected. The laparoscopic gastric bypass procedures in the present study were all performed in a uniform method as described by Lönroth and Olbers [39]. 209 patients, 148 women and 61 men originally comprised this cohort. Seven patients had an additional operation at the same time and were excluded. An additional thirteen patients had not filled out the patient questionnaire correctly regarding the pain variables, leaving blank fields in either of the fields for worst pain since surgery, least pain since surgery or percent of time with severe pain since surgery. After exclusion this resulted in a final cohort of 189 patients, 134 women and 55 men.
Figure 1. Study flow chart.
Outcome
The variables analyzed in our study were based on the following questions in the patient outcome questionnaire:
1. Indicate the worst pain you had since surgery.
2. How often were you in severe pain since surgery? Estimate the percentage of time you experienced severe pain.
The level of pain was rated according to VAS, where 0 corresponded to no pain and 10 to worst pain possible. The percentage of time with severe pain was rated from 0 percent to 100 percent in steps of ten, with 0 corresponding to never in severe pain and 100 to always in severe pain.
Apart from the above-described variables, the prevalence of severe pain (defined as VAS 7-10) at any time after surgery was compared between men and women.
The analysis in our study consists of a group comparison of men and women in their ratings of the above-mentioned pain related variables. The patients included in the Painout register rated their postoperative pain at one time only, and therefore no changes in postoperative pain over time has been possible to evaluate between or within the study groups.
The primary outcome in the present study was “experience of the worst pain since surgery”. The analysis was based on the mean value of the VAS ratings, analyzed as a continuously variable, among men and women. Also, we compared the prevalence of severe pain between the groups. According to the most frequently used categorization of pain from VAS rating, 0-3 is equivalent with mild pain, 4-6 with moderate pain and 7-10 with severe pain [12]. A multivariate analysis was conducted to standardize for other factors known to contribute to postoperative pain, namely body mass index (BMI), age and pre-existing pain. The purpose of the multivariate analysis was to adjust for co-variation, and was made with non-stratified data.
The secondary outcomes were prevalence of mild pain (defined as VAS 0-3), and also
“percentage of time with severe pain since surgery”. We considered the VAS as a continuous interval scale (see discussion, page 11).
Statistical methods
The collection of data to PainOut was registered in SPSS, and in this study SPSS version 22 (IBM Corporation, Armonk, New York, USA) have been used. For the variables: “worst pain since surgery”, surgery time and “percentage of time with severe pain” we used Students t-test as statistical method. For the categorical variables: prevalence of severe and mild pain, use of opioids and wish of more pain treatment we used Pearson Chi-Square test. For the multivariate analysis we used a linear regression analysis for the continuously variable, and a logistic regression analysis for the categorical variable. A p-value of 0.05 was considered to be statistical significant.
Ethical considerations
The Painout register was approved by the Uppsala/Örebro Region Ethics Committee (reference number: 2010/028). The study was conducted in accordance with the ethical standards of the Helsinki declaration (6th revision).
Results
The youngest patient in this study was 18 years old at the time of surgery, and the oldest 69 years old. The mean age among men was 2.1 years higher than among women. Among women, the prevalence of pre-existing pain was almost twice as high than among men.
The base-line characteristics are presented in table 1.
Table 1. Demographic data of the study groups, at the time of surgery.
Total Men Missing
values men, n
Women Missing values women, n Number of patients, n (%) 189 55 (29.1%) - 134 (70.9%) -
Age, yrs, mean ± SD 43.2 ± 12.5 44.7 ± 12.4 - 42.6 ± 12.5 -
Weight, kg, mean ± SD 117.7 ± 21.9 132.6 ± 24.0 - 111.5 ± 17.8 -
Height, cm, mean ± SD 169 ± 9.0 179 ± 6.7 15 166 ± 6.7 24
BMI, mean ± SD 40.4 ± 5.8 41.3 ± 6.3 15 40.0 ± 5.6 24
Patients with comorbidity,
n (%) 87 (57.6%) 27 (67.5%) 15 60 (54.1%) 23
Pre-existing pain ≥3 months prior to surgery, n (%)
28 (15.1%) 5 (9.3%) 1 23 (17.6%) 3
No statistical significant difference was seen for age, BMI, comorbidity or pre-existing pain prior to surgery.
The mean surgery time for women was 80.4 (±29.9) minutes and for men 79.0 (±24.3)
minutes. Preoperatively, 92.1 percent of the patients received opioids, 92.5 percent of the men and 90.9 percent of the women. In the recovery room 77.8 percent of the patients received opioids, 72.7 percent of the men and 79.9 percent of the women. Among women, 17.9 percent would have liked more pain treatment than they received. Among men, this number was 16.4 percent.
For the whole group, the mean rating of “worst pain since surgery” according to VAS was 5.6 (95% CI 5.2-6.0). The women in the present study rated their postoperative pain 1.0 step higher on the VAS compared to men (5.9, 95% CI 5.5-6.3 among women; 4.9, 95% CI 4.3-5.6 among men). Among women, there was a higher prevalence of severe pain (VAS 7-10) at any time since surgery, than among men. See table 2.
Table 2. Worst pain since surgery according to VAS (0-10), and prevalence of severe pain. Men Women P-value
Worst pain, mean ± SD 4.9 ± 2.7 5.9 ± 2.6 0.017 Prevalence of severe pain, n (%) 18 (32.7%) 65 (48.5%) 0.047 There were no missing values.
Of those patients who had pre-existing pain ≥ 3 months prior to surgery, 64.3 percent experienced severe pain postoperatively. The correlation of pre-existing pain and the prevalence of postoperative severe pain was higher among men than women. See table 3.
Table 3. Correlation between pre-existing pain and postoperative severe pain. Pre-existing pain ≥3 months
prior to surgery
No pre-existing pain ≥3 months prior to surgery
P-value
Women, n
Severe pain (women), n (%)
23 14 (60.9)
108
50 (46.3) 0.204 Men, n
Severe pain (men), n (%)
5 4 (80.0) 49 14 (28.6) 0.020 Total, n Severe pain, n (%) 28 18 (64.3) 157 64 (40.8) 0.021 There were 3 missing values for women and 1 for men.
Women experienced more time under severe pain after surgery than men (men 14.2%, 95% CI 10.2-19.3; women 22.8%, 95% CI 19.4-26.2; p=0.006).
Among men, the prevalence of mild pain (VAS 0-3) at any time since surgery was 34.5 percent, and among women this number was 18.7 percent (p = 0.019).
Adjusted analyses
In the linear regression analysis there was no statistical significant difference in “worst pain since surgery” between gender (adjusted beta coefficient -0.49, 95% CI -1.39 – 0.41 for
women compared to men, p = 0.282), and no statistical significant difference related to BMI (adjusted beta coefficient -0.06, 95% CI -0.13 – 0.01 per BMI unit, p = 0.103).
There was a statistical significant difference in “worst pain since surgery” related to the prevalence of pre-existing pain (adjusted beta coefficient 1.37, 95% CI 0.21 – 2.53 for the presence of pain compared to no pain, p = 0.021), and to the differences in age (adjusted beta coefficient -0.07, 95% CI -0.11 – -0.04 per year, p < 0.001).
In the logistic regression analysis there was no statistical significant difference in the
prevalence of severe pain between gender (adjusted OR 1.44, 95% CI 0.65 – 3.20 for women compared to men, p = 0.371), and no statistical significant difference related to BMI (adjusted OR 0.98, 95% CI 0.92 – 1.04 per BMI unit, p = 0.455).
There was a statistical significant difference in the prevalence of severe pain related to the prevalence of pre-existing pain (adjusted OR 2.82, 95% CI 1.00 – 7.93 for the presence of pain compared to no pain, p = 0.049), and to the differences in age (adjusted OR 0.96, 95% CI 0.93 – 0.98 per year, p = 0.002).
Discussion and conclusion
Key results
The primary outcome in the present study was the experience of ”worst pain since surgery”. Our results, before considering the multivariate analysis, did show a statistical significant difference in postoperative experience of pain. Women as a group rated their “worst pain since surgery” higher than men, and had a higher prevalence of severe pain.
In our study we found that the difference between men and women in pain rating according to VAS of “worst pain since surgery” was 1 step on the 11-step scale. Several previous studies [40-43] have stated that a difference of 7-13 mm on the 100 mm VAS is considered a minimum clinically significant difference. The mean VAS ratings in these previous studies were 51-61 mm, which is in the same level as the mean VAS ratings in the present study. One step on the 11-step scale is equal to 10 mm on the 100 mm scale, and therefore the results from our study can be considered to be clinically relevant.
The prevalence of mild pain, defined as VAS 0-3, was higher among men than women. This supports the results from the primary outcome showing that women to a higher extent experience severe pain after surgery.
In the adjusted analysis we standardized for the known risk factors for postoperative pain, namely younger age [5,22-27], preoperative or chronic pain [22-26,28,29] and BMI [5,24] in a multivariate analysis. We committed one analysis for the continuously variable “worst pain since surgery”, and another analysis for the categorical variable “prevalence of severe pain”. In the results from these adjusted analyses the difference between gender seen in the
univariate analyses was no longer statistical significant. However, the variables age and pre-existing pain did remain significantly correlated to postoperative pain, in both multivariate analyses for the continuously and the categorical variable. These results, together with the somewhat discrepant results from previous research [20,21], shows that it may be other factors (i.e. preoperative or chronic pain and age) that accounts for the difference in
experience of postoperative pain between men and women after LGBP surgery, rather than gender itself. The prevalence of preoperative pain was almost twice as high among women than among men in the present study. However, only 60.9 percent of the women with pro-operative pain did experience postpro-operative severe pain (although not statistically significant), while 80 percent of the men with pre-operative pain did experience postoperative severe pain.
Limitations
In the present study we compared mean values between men and women. In earlier studies there is no consensus regarding the analysis of mean versus median value of VAS. Some studies [13,15,21,30] use mean value for analyzing VAS and some studies [14,17,20] use median value.
The VAS is by definition an ordinal scale [44] , where a value to the right side of the
horizontal scale corresponds to more pain than values to the left side of the scale. By that, the values of the scale have an order. However, although 60 mm on the VAS is twice as much as 30 mm, it cannot be considered twice as much pain, since the steps of the scale are not equivalent in distance, and by that it is not possible to compute mean values from the results of an ordinal scale. For this reason VAS is often handled with as an interval level scale with equality between the intervals, making it possible to make calculations such as mean values [44] . The risk of handling VAS as an interval scale is that, when comparing VAS ratings between individuals or groups of individuals, one might easily consider the values to be equivalent in distance when they are actually not. For example, a rise of 1 step from 2 to 3 on the VAS is not necessarily an equally large change of pain as a rise from 8 to 9.
In the present study we considered the VAS as a continuous interval scale being normally distributed, and for that reason chose to analyze the mean value using unpaired t-test.
The cohort in our study consisted of 189 individuals. This study size was big enough to achieve statistical significant results. However, a larger study population would possibly result in a more narrow dispersion of the mean values, and by that increase the power of the study.
We compared the prevalence of receiving opioids pre- and postoperatively, and did not see a statistical significant difference between the two groups. In the Painout-register there is also data on what kind of opioids and the amount of opioids given to the patients. In a wider analysis one could make a calculation of the medications given pre-, intra- and
postoperatively, and compare between groups to analyze whether one group received higher doses of opioids than the other.
Of the 189 patients in the present study, data on BMI was missing for 39 patients. This could have affected the results from the multivariate analyses on BMI as an independent risk factor for postoperative pain. Our results did not show a statistical significant correlation between BMI and postoperative pain, complete data on BMI would lead to a more secure result regarding the correlation between BMI and postoperative pain.
In our study cohort there was no statistically significant difference in age, BMI or pre-existing pain between gender, although the men as a group was slightly older and had slightly higher BMI than the women, and the women had a higher prevalence of pre-existing pain. A larger study population would increase the robustness for these variables and thus increase the statistical power of the multivariate analyses.
Interpretation and generalisability
Most of the previous studies in the pain research area show gender related differences in the experience of postoperative pain, where women report more pain than men. Although there is quite a lot of research on gender related differences in postoperative pain, not much focus has been made on the possible causes of the differences. In the present study we adjusted the results for the known risk factors for postoperative pain in multivariate analyses, showing that the gender related differences were no longer significant when standardizing for age, BMI and pre-existing pain. Instead, age and pre-existing pain turned out to have a positive correlation with postoperative pain.
More research focusing on alternative causes to the differences seen in postoperative pain in men compared to women is needed.
The number of bariatric surgery procedures has had an increasing course, with 6800 procedures performed in Sweden during 2014, and 80 % being LGBP surgery. The vast majority of patients (75.6 % in 2014) undergoing bariatric surgery are women [34]. The primary results from the present study, without considering the multivariate analyses, leads to a suggestion that those institutions performing bariatric surgery should apply a pain treatment protocol specifically for women, with the aim of reducing postoperative pain and balancing the differences between gender. Also in other operating units at hospitals and other medical institutions where postoperative pain is being handled with, these results and the results from previous research can be applied for focusing on the imbalance of postoperative pain between men and women.
However, when taken into account the results from the multivariate analyses, previous research on gender related differences in postoperative pain should be regarded cautiously. Instead, more focus should be applied on postoperative pain regimens for younger patients and patients with chronic or pre-existing pain.
Conclusion
In the present study we tried to confirm the results from most previous research, indicating gender related differences in the experience of postoperative pain, where women report higher grade of pain and demands more pain treatment postoperatively.
We where able to show that women indeed do experience more pain than men after LGBP surgery. These results, however, seem to be caused not by gender itself, but instead by younger age and a higher occurrence of preoperative pain among women.
More research is needed to evaluate alternative causes to the gender related differences seen in postoperative pain.
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