Peripheral nerve blockade reduces pain scores and opioid consumption and increases patient satisfaction within the first 24 h after shoulder surgery. In the absence of neuraxial and peripheral nerve blocks, opioid consumption after shoulder surgery has been purported to rival the one recorded for thoracotomy. Interscalene brachial plexus blocks (ISBB) constitute the gold standard for analgesia after shoulder procedures. Compared with parenteral opioids, ISBB results in lower pain scores improved patient satisfaction \[4\]. ISBB is highly utilized in clinical practice as it provides anesthesia and analgesia to the shoulder and lateral aspects of the arm and the forearm, reducing opioid consumption and subsequent opioid-related adverse effects \[5\]. However, ISB is associated with numerous complications and adverse effects3 such as phrenic nerve palsy (100%), recurrent laryngeal nerve block (3 - 21%), stellate ganglion block (5 - 75%) (Horner's syndrome), spinal (0.4 - 4%) and epidural anesthesia (2.2%), and convulsions (0.2 - 3%) at standard volumes of 20 - 30 ml. Phrenic nerve block is associated with significant reductions in ventilatory function including a 21 - 34% decrease in forced vital capacity (FVC), 17 - 37% decrease in forced expiratory volume, and 15.4% decrease in peak expiratory flow rate. The most common adverse effect after ISB remains the occurrence of ipsilateral phrenic nerve block. Although well tolerated by healthy subjects, HDP becomes a prohibitive risk for patients with pulmonary pathology, who may be unable to withstand the 30% reduction in FVC. Viable diaphragm-sparing alternatives to ISB should achieve three separate and distinct goals: adequate surgical anesthesia (without general anesthesia), adequate postoperative analgesia, and low incidence of HDP. Alternative nerve blocks are performed in shoulder surgery to prevent HDP. Therefore, different approaches such as suprascapular nerve block, costoclavicular block, shoulder block, supraclavicular block, combined infraclavicular and suprascapular block, which are defined as diaphragm sparing blocks, are recommended.

Our study aimed to investigate the minimum effective local anesthetic volume in postoperative analgesia of the anterior suprascapular nerve block (ASSB) and the maximum local anesthetic volume that can be diaphragm-sparing. Our secondary aim is to reveal an important parameter in the diaphragm sparing nerve blocks. The volume of a local anesthetic to be administered should be expected to have a common volume that is the most effective and protects the diaphragm. We envisioned that this should be defined as the minimum target-specific volume (MTsV).

Adults (over 18 years of age) with the American Society of Anesthesiologists Physical Condition classification I to III who are scheduled to undergo unilateral ambulatory arthroscopic shoulder surgery under general anesthesia will be included in the study. Written informed consent will be obtained from all study subjects before participation in this study. Surgical procedures are planned to include shoulder arthroscopy, rotator cuff repair, acromioplasty, Bankart repair, and superior labrum anterior-posterior repair. Patients with a history of serious broncho-pulmonary disease, known phrenic nerve pathology, existing neurological deficits or neuropathy involving the brachial plexus on the surgical side, contraindications to nerve blocks (infection, bleeding diathesis, allergy to local anesthetics), or history will be excluded from the study. A full pre-operative evaluation will be performed with the necessary laboratories the day before the surgery, and written informed consent will be obtained.

The 0.5% initial injected volume of bupivacaine was 10 mL, providing a complete sensory block of anterior suprascapular nerve block based on our clinical experience. According to the Dixon and Massey \[10\] up-down method, the 0.5% bupivacaine volume used for a given patient was determined by the result of the previous block, as defined above as block success. Once effective ASSB is achieved, the volume of bupivacaine 0.5% to the next patient will be reduced by 1 mL. Conversely, in case of block failure, the volume of 0.5% bupivacaine will be increased by 1 mL in the next patient.

An independent observer will assess ipsilateral hemidiaphragmatic motion by real-time M-mode ultrasonography using a 1 to 5 MHz broadband phased array US probe (Philips CX50). Patients were examined in an upright sitting position and scanned from a low intercostal or subcostal approach using the liver or spleen as the acoustic window. The range of diaphragm movement from the resting expiratory position to deep inspiration (sigh test) and the range of diaphragm movement from the resting expiratory position when quickly inhaling through the nose (sniffing test) will be recorded. More than 75% hemidiaphragmatic movement reduction, no movement, or paradoxical movement will be considered complete paresis. Between 25% and 75%, hemidiaphragmatic movement decrease was considered partial paresis, and less than 25% hemidiaphragmatic movement decrease paresis was considered absent. The movement of the diaphragm will be measured in centimeters. The normal caudal movement was positive (+), paradoxical head movement was negative (-). Each test will be performed three times, and the values will be averaged. All measurements will be made before (beginning) and 30 minutes after the block.

Fifty minutes after the block performance, general anesthesia will be given using a standard protocol of intravenously administered propofol 2 mg/kg, Fentanyl 1 ug/kg, and rocuronium 0.6 mg/kg. Pain scores of the patients will be recorded for 48 hours postoperatively.