This study was planned as a prospective, single-blind randomized study, and commenced after the Institutional Ethical Committee approval, and registration at Central Trials Registry. It was conducted in the general surgery operation theatre with postoperative follow-up of up to 24 h in the post-anesthesia care unit as well as in the postoperative wards.

This study comprised 60 patients, of ASA grades I–III, between the age group 18 and 75 years, planned for elective laparoscopic cholecystectomy. Patients with bleeding disorders, on anticoagulants, pre-existing cardiovascular disease, hypertension, hepatic or renal failure, or advanced psychiatric illness were not considered for the study; however, patients with diabetes on anti-diabetic treatment, with mild jaundice, were included after optimization of blood sugar levels.

Pre-anesthetic evaluation was conducted on the day preceding surgery. Baseline vitals of the patients including the heart rate, non-invasive blood pressure, and SpO2 were recorded and baseline blood investigations were reviewed. Patients were informed regarding the numerical rating scale (NRS) with 0 being the absence of pain, and 10 being the worst pain imaginable, and how to quantify pain intensity between these two values.

Preoperatively, informed written consent was obtained from patients. After entering the operation theater, intravenous access was established, and patients were premedicated with ondansetron 0.15mg/kg, glycopyrrolate 0.004mg/kg, and fentanyl 2mcg/kg intravenously. All monitors including electrocardiogram, non-invasive blood pressure, pulse oximetry, and capnometry were applied. After premedication, preoxygenation was done with 100% oxygen and the patient was induced using propofol 2mg/kg and succinylcholine 2mg/kg. After the airway was secured, anesthesia was maintained using oxygen and sevoflurane 0.8–1.5% and atracurium 0.5mg/kg intravenously. Intraoperative local infiltration at the port site or intraperitoneal instillation was not done.

The CONSORT flow diagram of the study is shown in Fig. 1. Patients were randomly allocated into two groups, Group O (n=30) and Group E (n=30) with the help of computer-generated software. Group O (OSTAP group) was administered oblique subcostal transversus abdominis plane block, using 10ml of 0.375% bupivacaine 10 and 10ml of 1.5% lignoadrenaline. Group E (ESPB group) was administered erector spinae plane block at the T9 level using the same drug concentration.

CONSORT flow diagram of the study

All blocks were performed under complete aseptic precaution (under ultrasonographic guidance) using a high-frequency linear ultrasound probe. OSTAP block was performed in supine position. The transducer was placed immediately below the costal margin in the oblique plane and the rectus abdominis, transverse abdominis, internal oblique, and external oblique muscles were identified. A 21-gauge 8-cm needle was introduced using an in-plane approach, 2–3 cm lateral to the transducer from medial to the lateral direction. One to 2 mL of solution was injected between the rectus abdominis muscle and the transversus abdominis muscle. After confirming the correct placement of the needle, the rest of the anesthetic substance was injected along the subcostal line in the transversus abdominis plane. The block was performed bilaterally. ESP block was performed in lateral position and the transducer was placed in a longitudinal parasagittal orientation 3-cm lateral to the T9 spinous process. The erector spinae muscle was identified superficial to the tip of the T9 transverse process. An 18-gauge 8-cm needle was inserted using an in-plane superior to inferior approach to place the tip into the fascial plane on the deep (anterior) aspect of the erector spinae muscle. The location of the needle tip was confirmed by visible fluid spread lifting the erector spinae muscle off the bony shadow of the transverse process. The procedure was repeated on the other side.

After completing the block procedure, the neuromuscular block was reversed with neostigmine 0.05 mg/kg and glycopyrrolate 0.008mg/kg intravenously. Extubation was performed after the patient regained consciousness, and the patient was shifted to the postoperative room.

The numeric rating scale (NRS) pain score was recorded from the 20th minute in the recovery room followed-up by 1, 3, 6, 12, and 24 h postoperatively. Intravenous tramadol 50–100mg as rescue analgesia was administered in patients with a NRS score of 3 and over, while intramuscular diclofenac was also administered for patients with NRS score of 7 and above in the postoperative period along with tramadol. In 24 h, the time of administration of first rescue analgesic given was noted. Postoperative vitals were documented.

Patients were also observed for complications such as local hematoma, subcutaneous emphysema, pneumothorax, signs of local anesthetic toxicity, signs of visceral or peritoneal injury, or motor weakness.

The sample size was calculated based on the previous study (Altiparmak et al. 2019) which revealed that at least 30 patients were needed in each group for the detection of 25% variation in NRS score at the 120th minute postoperatively with a power of 0.1 and a significance level 95% (α=0.05, β = 0.9).

The statistical analysis was done using Microsoft Excel. To find the significant difference between the bivariate samples in independent groups, the unpaired sample t test was used while chi-square test was used to find the significance in categorical data. In all the above statistical tools, the probability value P value <0.05 was considered significant.