SPENG Block Versus Superior Trunk Block for Shoulder Surgery: Effects on Diaphragmatic Function, Mechanical Power, Nociception, and Analgesia: A Randomized Controlled Trial

Rationale

After superior trunk block (STB), hemidiaphragmatic paralysis (HDP) has been reported in 76.3% of patients (Kang et al., Anesthesiology 2019) and in 70.8% of patients with 20 mL of local anesthetic (Lee et al., Pain Physician 2021). Even after ultrasound-guided supraclavicular block, the reported HDP incidence is 59.5% (Ferré et al., 2019). These data suggest that all approaches at the level of the brachial plexus carry a substantial risk of phrenic nerve involvement. The SPENG block (Kilicaslan et al., Pain Med 2026) targets the articular branches of the suprascapular and axillary nerves, which provide approximately 70% of the sensory innervation of the shoulder joint, at a site posterior to the scapula. M-mode ultrasound assessment of diaphragmatic excursion is a non-invasive, repeatable, and objective bedside method for detecting block-related HDP.

Randomization and Blinding Procedures

The allocation sequence will be generated by computer (www.random.org) and concealed in sequentially numbered, opaque, sealed envelopes. Each envelope will be opened immediately before the block by the anesthesiologist performing it, who will not be involved in data collection. To maintain blinding, identical sterile dressings will be applied to both the supraclavicular region and the posterior shoulder in all patients. All block procedures will be performed with real-time needle-tip visualization, in 5 mL increments with aspiration before each increment. Lipid emulsion (20%) and resuscitation equipment will be available.

Standardized Perioperative Management

Standard monitoring will be applied in all patients. IV midazolam 1-2 mg may be given after baseline diaphragm measurement. Anesthesia will be induced with propofol 2-2.5 mg/kg, fentanyl 1 µg/kg, and rocuronium 0.6 mg/kg, and maintained with sevoflurane titrated to a qCON of 40-60. Fentanyl 0.5 µg/kg will be administered if qNOX remains above 60 for more than 1 minute. All patients will receive IV paracetamol 1 g and ondansetron 4 mg 30 minutes before the end of surgery. Ventilation will be standardized in volume-controlled mode (tidal volume 6-8 mL/kg ideal body weight, PEEP 5 cmH₂O, FiO₂ 50%, respiratory rate adjusted to EtCO₂ 35-40 mmHg). Plateau pressure will be measured with a 0.5-second inspiratory pause, and mechanical power will be calculated with the simplified Gattinoni formula: MP (J/min) = 0.098 × RR × VT (L) × [Ppeak - ½ × (Pplat - PEEP)]. Postoperative analgesia will consist of IV paracetamol 1 g every 6 hours and IV dexketoprofen 50 mg every 12 hours unless contraindicated, with IV tramadol 1 mg/kg (maximum 400 mg/24 h) as rescue analgesia.

Diaphragm Ultrasound Technique

The ipsilateral hemidiaphragm will be imaged in the supine position with a 2-5 MHz convex probe using a subcostal approach along the anterior axillary/midclavicular line (liver window on the right, spleen window on the left). Excursion will be measured three times in M-mode during each maneuver, and the mean value will be recorded. The postoperative measurement will be performed when the patient is fully awake and cooperative (Modified Aldrete score ≥9), using the same position, probe placement, and maneuvers, by the same blinded assessor.

Sample Size and Statistical Analysis

Sample size was calculated with G*Power 3.1.9.7 based on the primary outcome. Assuming an HDP incidence of 70% with STB and a conservatively estimated 30% with SPENG, 31 patients per group are required (two-sided α = 0.05, power = 90%). Allowing for an anticipated 15% loss, 36 patients will be enrolled per group. This sample size is also sufficient to detect a 1.5-point (SD 2.0) difference in VAS scores with 80% power. Data will be analyzed with SPSS v26.0. Relative risk with 95% confidence interval will be reported for the primary outcome. Repeated measures ANOVA or linear mixed-effects models will be used for time-dependent variables, and Kaplan-Meier analysis with the log-rank test for time to first rescue analgesic. Analyses will follow a modified intention-to-treat approach, and p < 0.05 will be considered statistically significant.