An institutional-based prospective cohort study was employed from February 28/2020 – may28, 2020 at Wolaita Sodo University, Teaching and Referral Hospital. The source of population were all adult patients who underwent abdominal surgery, and the study participants were selected elective adult patients who underwent lower abdominal surgeries during the study period. Those patients on long-term opiate users, allergy to the drug, with chronic pain diagnosis, emergency surgical patients, pregnant mothers, BMI >30 km/kg and patients with known epileptics were excluded from the study. Whereas those Men and women patients age greater than 18 years and ASA class I–II patients, who underwent lower abdominal surgery were included in the study.
The primary endpoint of this study was to assess and measure postoperative pain severity, time to first analgesic request and total analgesic consumption for 24hours. The sample size was determined based on the largest sample size and calculated by using a priori power analysis (G Power version 3.01) based on the results of a similar study performed by Roopa Sachidananda et al(32). Prior power analysis for two independent t-tests with 2 groups was conducted by taking the time for the first analgesia request from the previous study done, pain scores of the mean (SD) (μ1= 386.17±233.84 min and μ2=192.50±134.77 min).
To determine the sample size an alpha of 0.05, a power of 0.95% and effect size of 1 were substituted into the software, and the calculated sample size is 54. Sample size was doubled for the purpose of considering subgroup analysis for postoperative analgesic effectiveness effect and became 108. To ensure a minimum of 108 patients, an additional 12 patients (10 %) were added to account for potential protocol violations and missing data, so the total sample size is 120 patients with 60 in each group.
Based on monthly institutional average reports, approximately 80 patients underwent elective lower abdominal surgery. Depending upon the average number of previous records and duration of the study period, which was planned for three months, the total population size at the operating theatre expected to be 240 patients. The patients were selected every Kth unit based on log book data from the study area, and 240 patients underwent lower abdominal surgery. The sampling interval K was calculated using formula=N/n= 2. A total of 120 participants were enrolled with a chance of 50%. The first random case was chosen by a lottery method, and then every 2nd consecutive patient on the day of data collection was included. All patients who were scheduled for elective lower abdominal surgery and who satisfied the inclusion criteria and volunteered to take part in the study were instructed, on how to self-report pain severity by using the 11-point NRS score (0 to 10), on the day of operation by trained anesthetist. Following this written, informed consent was obtained from all patients. At the end of the surgery and after the patients’ vital sign stability was confirmed , based on the preference of the Anesthetist, after drug dose adjustment was performed and the infiltration site was cleaned with alcohol, the responsible anesthetist could perform infiltration at the wound site with tramadol 2 mg/kg mixed in bupivacaine 0.25% 0.7 mg/kg (diluted to 20 mL with 0.9% normal saline) or with bupivacaine 0.25% 0.7 mg/kg (diluted to 20 mL with 0.9% normal saline) for postoperative pain management. Starting from the time of wound infiltration, 24 hours postoperatively in the recovery room and ward, the presence and severity of pain (NRS for pain), time for the first analgesic request and adverse side effects were passively followed and assessed by using systematically structured questionnaires by two trained data collectors. And, also another one trained senior anesthetist was assigned to assist and supervise data collectors. The NRS score and other variables were documented at six time points: at 0 hr, 2 hr, 6 hr, 12 hr, 18 hr, and 24 hrs. Data collection was carried out by structured and pretested questionnaires consisting of general patient characteristics information. Then, the collected data were checked for completeness, accuracy, and clarity by the principal investigator and research assistant. Data clean up and crosschecking were performed before analysis, and incomplete data were not entered into the database for analysis.
Data Processing and Analysis
After the data obtained from the data collector and manually checked and cleaned for errors and coded, it was entered into Epi-data software. After data entry was completed, the data were exported into SPSS version 23 statistical package software, and statistical analyses were performed. The Shapiro-Wilk test and histogram were used to test for normal distributions. Frequency and percentage were used to describe the categorical variables. Comparison of asymmetric variables between study groups to compare median pain score and total analgesia consumption between groups and significant differences between groups were described by using the Mann-Whitney test and chi-square test. Kaplan-Meier survival analysis was used to compare the time to first analgesic request using the log-rank test and curve graph to compare the proportion of patients not requesting analgesia over 24 hours between the two groups. Box and whisker plots were used to show median pain score differences between groups. Statistical data were reported as the mean ± SD for normally distributed data, median ± IQR for non-normally distributed data and categorical data were reported as numbers and frequencies (percentages), and statistical significance was stated at a p-value < 0.05 with a power of 95%.
Local wound infiltration: is the technique of administering local anesthetic directly into the surgical incision site at the end of the procedure.
Acute postoperative pain: is the pain experienced immediately after an operation, usually lasting for days or sometimes weeks.
Chronic pain: is normally considered to be pain that persists or keeps coming back for more than three months or for longer than the expected healing time.
Chronic post-surgical pain; pain that develops after a surgical operation that lasts for at least three months.
Postoperative pain: the presence of pain in the postoperative period as a patient complaining of pain and any pain score other than zero within 24 hrs.
Numerical Rating Scale: is a valid pain intensity assessment tool that involves asking a patient to rate his or her pain from 0-10 (11-point scale) with the understanding that 0 is equal to no pain and 10 equals the worst possible pain(9).
Total analgesic consumption: is the total amount of analgesic drugs in milligrams used in the first 24 hrs.
Lower abdominal surgery: any operation that involves opening the abdominal cavity below the umbilicus
Time to first analgesic request: is the time in minutes measured from the end of the procedure to time when the patient requests analgesics.
Duration of surgery: time in minutes from skin incision to skin closure.
Duration of anesthesia: a time in minutes it takes from loss of consciousness to awake fully from anesthesia
Postoperative analgesic Effectiveness: Postoperatively after analgesic agent was used and, if the pain intensity score was NRS ≤ 3 during rest and on coughing for over 24 hours.
Censored (0): A patient did not request analgesia until the end of the study period.
Die censored (1): A patient requesting analgesia until the end of the study period.
ASA status: is a surgical risk stratification validated by the American Society of Anesthesiologist.
ASA I: a healthy patient with no organic/physiological/psychotic problems.
ASA II: controlled medical conditions with mild systemic effects and no limitation of functional ability.