Bilateral ultrasound-guided erector spinae plane block versus wound infiltration for postoperative analgesia in lumbar spinal fusion surgery: a randomized controlled trial

Study design and participants

This randomized trial was conducted in a tertiary hospital in Beijing, China. The study protocol was approved by the local Biomedical Research Ethics Committee (2021–414) and was registered prior to patient enrollment at the Chinese Clinical Trial Registry (http://www.chictr.org.cn, ChiCTR2100053008; 8 November 2021). Written informed consent was obtained from each participant.

We enrolled patients who were (1) aged 18–70 years, (2) diagnosed with degenerative lumbar spine disease not responsive to non-surgical treatments including medication, physical therapy, and epidural steroid injection for at least 6 months, and (3) scheduled for elective open posterior lumbar interbody fusion (PLIF) surgery. We excluded those who underwent corrective surgery for degenerative scoliosis and kyphosis, revision lumbar surgery, or surgery involving the thoracic vertebra. Other exclusion criteria included the following: (1) severe renal (serum creatinine > 442 μmol/L or requiring renal replacement therapy) or liver (Child–Pugh grade C) insufficiency, or ASA class IV or higher; (2) chronic opioid dependence or use of painkillers for more than 3 months; (3) inability to communicate due to severe dementia, language barrier, or end-stage disease; (4) comorbidity of the central and/or peripheral nervous system; or (5) allergy to local anesthetics.

Randomization and blinding

Random numbers were generated by a statistician using the SAS statistical package version 9.3 (SAS Institute, Cary, NC, USA) in a 1:1 ratio with a block size of 4. The generated random numbers were sealed in sequentially numbered opaque envelopes and kept by a study coordinator. On the day of surgery, the envelopes were opened in the operating room after general anesthesia according to the recruitment sequence. Patients were randomized to receive either ESP block which was performed by experienced anesthesiologists (ZZ and XL) before surgery, or wound infiltration which was performed by attending surgeons before the end of surgery. Anesthesiologists and surgeons were aware of the study intervention. However, all patients, investigators in charge of postoperative following-ups (ZRL and YL), and other healthcare team members were masked to group assignments.

Anesthesia, surgery, and perioperative care

No pre-anesthesia medication was administered. Intraoperative monitoring was per ASA guidelines and usually included intraarterial blood pressure monitoring. General anesthesia was induced with midazolam, sufentanil, propofol and/or etomidate, and rocuronium, and maintained with propofol infusion, remifentanil infusion and/or sufentanil injection, and muscle relaxants (rocuronium or cisatracurium), with or without sevoflurane inhalation. Anesthesia depth was targeted to a bispectral index between 40 and 60. Mechanical ventilation was established with an oxygen–air mixture. Fluid therapy was provided according to routine practice. Vasoactive drugs were administered when necessary to maintain mean arterial pressure and heart rate within 20% from baseline. Glucocorticoids were administered at the discretion of surgeons.

Patients were placed in prone position under general anesthesia. For patients assigned to ESP block, a curvilinear ultrasound probe (3–5 MHz, GE Healthcare, Boston, Massachusetts, USA) was used to identify the target spinous process by paramedian sagittal scan starting from the sacral regions; the probe was then moved 2 to 3 cm lateral to the midline to identify the tip of the corresponding transverse process. The target spinous process was determined according to the level of lumbar surgery, i.e., intermediate one for surgical level(s) of odd number or the upper or second one for surgical levels of even number. An 8-cm 21-gauge block needle (Stimuplex D, B.Braun, Melsungen, Germany) was inserted in an out-of-plane mode until it contacted the transverse process. After verifying the position of the needle tip with 2 mL normal saline, 20 mL 0.375% ropivacaine was injected (Fig. 1). The procedure was repeated on the contralateral side. For patients assigned to wound infiltration, 40 mL of 0.375% ropivacaine was infiltrated along each side of the wound edges after closure.

Fig. 1figure 1

Sonographic anatomy of the erector spinae plane (A, longitudinal; C, transverse) and erector spinae plane block (B, out-of-plane approach longitudinally; D, in-plane approach transversely). The area within the white line indicates local anesthetic (LA) spreading deep to the erector spinae muscle. ESM, erector spinae muscle; TP, transverse process; MP, mammillary process; SP, spinous process

For the open PLIF procedure, a midline incision was made, paravertebral muscle was dissected to expose the spinous process and laminae, and pedicle screws were inserted. After a standard laminotomy and discectomy, the autologous bone was implanted into the disk space, and an appropriate size cage packed with bone autograft was inserted. Bilateral pedicle screws were then connected with elongated screw rods and fixed with nuts. A drainage tube was placed in the wound, and the incision was closed.

At 30 min before the expected end of the surgery, 50 mg flurbiprofen axetil was administered (in patients without contraindications) for supplemental analgesia and 5 mg tropisetron was administered for prophylaxis of nausea and vomiting. At the end of the surgery, 2 mg neostigmine and 1 mg atropine were used to antagonize neuromuscular blockade. Patients were extubated in the operating room, monitored in the post-anesthesia care unit for at least 30 min, and transferred to the general wards when the modified Aldrete Score reached 10; otherwise, they were transferred to the intensive care unit.

All patients were provided with a patient-controlled analgesia (PCA) pump for postoperative analgesia. The pump was established with 100 ml of 1.25 μg/ml sufentanil and programmed to deliver 2 mL boluses at an 8-min lockout interval without a background infusion. As a routine practice, 50 mg flurbiprofen axetil was given intravenously twice daily whenever possible. Other analgesics were prescribed when necessary. The target was to maintain the Numerical Rating Scale for pain intensity (an 11-point scale where 0 = no pain and 10 = the worst imaginable pain) at a rest of less than 4.

Data collection

Baseline data included demographics, comorbidities, and American Society of Anesthesiologists classification. Preoperative evaluations were performed. Among these, anxiety and depression were assessed with the Hospital Anxiety and Depression Scale (score ranges from 0 to 21 for either anxiety or depression, with a higher score indicating more severe anxiety or depression status) [15]; pain intensity was assessed with the Numeric Rating Scale; sleep quality was assessed with the Pittsburgh Sleep Quality Index (score ranges from 0 to 21, with a higher score indicating worse sleep quality) [16]; low back pain related disability was assessed with the Oswestry Disability Index (percentage ranges from 0 to 100%, with a higher percentage indicating more severe spinal dysfunction) [17].

Intraoperative data were collected and included the durations of anesthesia and surgery, medications and fluid infusion during anesthesia, estimated blood loss, transfusion of blood products, and number of fused levels.

Postoperative follow-ups were performed by investigators who were not involved in anesthesia and surgery and were blinded to the study group assignment. Our primary outcome was cumulative opioid consumption within the first 24 h after surgery and calculated as sufentanil equivalent [11, 18]. Among secondary outcomes, pain intensity both at rest and with movement was assessed with the Numeric Rating Scale at 2, 6, 12, 24, and 48 h after surgery; sleep quality during the night of surgery was assessed with the Richards-Campbell Sleep Questionnaire (overall score ranges from 0 to 100, with a higher score indicating better sleep quality) [19]; quality of recovery at 24 h was assessed with the Quality of Recovery-15 scale (overall score ranges from 0 to 150, with a higher score indicating better postoperative recovery) [20]; postoperative complications were generally defined as new-onset medical conditions that were harmful to patients’ recovery and required therapeutic intervention, i.e., grade II or higher on the Clavien–Dindo classification [21]. Other outcomes including opioid consumption between 24 and 48 h, required bolus via patient-controlled analgesia pump, supplemental analgesics, and Oswestry Disability Index at 30 days were also collected.

Adverse events were monitored from the beginning of anesthesia until 24 h after surgery. Specifically, hypotension was defined as systolic blood pressure < 90 mmHg or a decrease of > 30% from baseline; hypertension as systolic blood pressure > 180 mm Hg or an increase of > 30% from baseline; bradycardia as heart rate < 50 beats/min or a decrease of > 30% from baseline; tachycardia as heart rate > 100 beats/min or an increase of > 30% from baseline; respiratory depression as spontaneous breathing rate < 8 breaths/min; desaturation as pulse saturation < 90% in room air and required supplemental oxygen; and nausea and vomiting as any retching, vomiting, or requirement for antiemetics [22].

Statistical analysisSample size estimation

In our pilot study, the cumulative sufentanil consumption within the first 24 h after spinal fusion surgery was (mean ± SD) 16.2 ± 6.7 μg in 5 patients with wound infiltration and 10.0 ± 4.7 μg in 5 patients with ESP block. We expected the same difference and a standard deviation (SD) of 6.7. With the significance level set as α = 0.05 and power set as 1–β = 90% on a two-sided t test, the sample size required to detect difference was 52 patients (26 in each group). Taking into account the dropout rate, we planned to enroll 60 patients. Sample size calculation was performed with the PASS 15.0 software (Stata Corp. LP, College Station, Texas, USA).

Data analysis

Outcome analysis was performed in the intention-to-treat population. Continuous data were evaluated for normality using the Shapiro–Wilk test and Q–Q plots. Variables with normal distribution were expressed as mean ± SD and compared with the independent t test; otherwise, they were expressed as median (interquartile range) and compared with Mann–Whitney U test. Differences between the two medians (and 95% CIs) were calculated with Hodges–Lehmann estimators. Repeatedly measured variables (scores of pain intensity, cumulative sufentanil consumption, and cumulative number of demanded PCA bolus) were compared with the generalized estimating equation method; the significance criterion for each comparison was P < 0.01 (0.05/5) after Bonferroni correction. For categorical variables, data were expressed as n (%), and inter-group differences were analyzed using the chi-squared or Fisher's exact tests. Relative risks (and 95% CIs) were provided. For the 2 × 2 table containing at least one zero cell, we used “modified Haldane-Anscombe” correction by adding 0.5 to all cells to calculate the relative risk [23]. Considering the possible relationship between pain and hypertension [24], adjustment via multiple linear regression model was performed in our primary outcome analysis. Missing data were not replaced. Generally, a two-sided P < 0.05 was considered statistically significant unless otherwise indicated. Statistical analysis was performed with SPSS 25.0 (IBM, New York, USA).

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