[2] Hemi diaphragmatic paralysis (HDP) is a frequent complication of the brachial plexus block (BPB), caused by unintentional blockade of the phrenic nerve because of close proximity of brachial plexus and phrenic nerve that supplies diaphragm. Although HDP can reduce forced vital capacity and forced expiratory volume at 1 second but these reductions are generally well tolerated by most patients. HDP, however, can be a serious problem in some patients, including those with underlying lung disease or marginal pulmonary function. As brachial plexus and phrenic nerve move caudally they start to diverge from each other so a higher incidence of HDP is more likely to be associated with interscalene and supraclavicular block as compared to infraclavicular block.[4] The incidence of HDP after infraclavicular brachial plexus block is lower, likely due to the relatively long distance between the phrenic nerve and the block site.[3] Infraclavicular approach to BPB can be provided by traditional approach and the costoclavicular approach.[5] The costoclavicular brachial plexus block (CCB) is a relatively recently introduced (2018) infraclavicular approach that targets three cords located lateral to the axillary artery in the costoclavicular space. Anatomically costoclavicular approach is better than traditional block through lateral infraclavicular P a g e | 24 HDP AFTER COSTOCLAVICULAR BPB DR NAVAL R JETHALIYA approach. In lateral infraclavicular fossa, the cords are located deep to pectoral muscles and separated from each other by the axillary artery[6]. This requires large volume of LA and multiple needle punctures for a successful block, whereas in the costoclavicular fossa cords are located superficially and clustered together[10] so small dose of LA and single needle puncture can provide adequate anesthesia.[9] Moreover, the infraclavicular brachial plexus block alone has limited utility in patients undergoing shoulder surgery because of the poor coverage of the proximal branch, such as the suprascapular nerve. Costoclavicular space is located deep and posterior to middle third of clavicle with subclavius and clavicular head of pectoral muscle anteriorly. Costoclavicular space continues as supraclavicular fossa cranially and medial infraclavicular fossa caudally. The costoclavicular space is considered a retrograde channel to the supraclavicular area, enabling reliable anesthesia, including anesthesia to the suprascapular nerve during shoulder surgery. Because of these anatomical advantages, the CCB is emerging as a promising infraclavicular approach,[11] with several studies showing that the CCB can provide a successful and rapid onset of the blockade with a single injection of a relatively small volume of local anesthetic, with the effect similar to supraclavicular brachial plexus block. However, the costoclavicular space is located proximal to the lateral infraclavicular fossa, providing greater proximity to the phrenic nerve. The CCB may therefore increase the risk of HDP, with one study showing that local anesthetic injected into the costoclavicular space reached the interscalene region, at the level of the superior trunk.[12] Diaphragmatic dysfunction following brachial plexus blockade may be suspected based on clinical signs and symptoms of respiratory compromise. Traditional methods to confirm the diagnosis such as real-time fluoroscopy, pulmonary function testing, or even chest radiography involves radiation exposure, patient transportation, increased cost, are time consuming and can cause discomfort for the patient. Ultra-sonography (US) is an easy, reliable, real time, noninvasive and reproducible P a g e | 25 HDP AFTER COSTOCLAVICULAR BPB DR NAVAL R JETHALIYA alternative method to assess the diaphragm function that can be used at the bedside using standard ultrasound equipment which is readily available in the regional anesthesia block areas. Limited studies have primarily assessed the incidence of HDP following the CCB hence we planned a prospective observational cohort study to