A continuous peripheral nerve block is one of the strategies that can overcome the short duration of single-shot ISBPB by delivering a constant infusion of local anesthetics through a perineural catheter [12]. Clinical studies have demonstrated superior analgesia for up to 48 h after major shoulder surgery without increasing side effects [9]. However, continuous blocks with perineural catheter are labor intensive, may infringe on the surgical field, carry the risk of infection, myotoxicity, and phrenic nerve palsy, and can result in secondary failure related to catheter dislodgement [13]. Taken together, continuous ISBPB may be a useful strategy to prolong the analgesic duration; however, anesthesiologists need to weigh the risks and benefits of continuous ISBPB for shoulder surgery.

The use of perineural or IV additives, such as dexamethasone or dexmedetomidine, is a promising strategy for prolonging the duration of analgesia. However, the safety profiles of off-label perineural injections of most additives have not yet been confirmed; therefore, caution is required [14]. Alternatively, IV administration of several additives, including dexmedetomidine and dexamethasone, can provide prolonged analgesia while avoiding the theoretical risks inherent to perineural injection [7,8,15,16]. In our previous study comparing the effective dose of IV dexmedetomidine to prolong the analgesic duration of ISBPB, we found that IV dexmedetomidine (2.0 μg/kg) significantly increased the time to first pain at the surgical site following ISBPB. However, these extended analgesic effects were not observed at a dose of 0.5 and 1.0 μg/kg [7]. In a subsequent study [8], we demonstrated that IV dexamethasone (0.11 mg/kg) significantly prolonged the time to first rescue analgesic request (1.6-fold compared with the control group) following ropivacaine ISBPB analgesia. Moreover, the co-administration of IV dexamethasone (0.11 mg/kg) and IV dexmedetomidine (1.0 μg/kg) further increased this time (3.8-fold compared with IV dexamethasone alone). This finding is clinically important because 50% of the patients receiving the two drugs did not require rescue analgesics for up to 72 h postoperatively. Taken together, IV administration of dexmedetomidine and dexamethasone may provide prolonged analgesia, but the optimal dose has not yet been determined, and further studies are warranted to clarify this point. While conducting clinical studies using additives in Korea, it should be considered that the use of IV dexamethasone, IV dexmedetomidine, perineural dexamethasone, or perineural dexmedetomidine for the purpose of prolonging the duration of analgesia must be approved by the Korean Food and Drug Administration.

Liposomal bupivacaine was introduced to prolong the duration of single-shot ISBPB and has recently been approved for use in interscalene blocks [11]. Its mechanism of action is due to the structural characteristics of the capsule by multivesicular liposomal lipid bilayers, which leads to a slow release and maintains a constant plasma concentration; thus, prolonging its effects for up to 72 h after a single injection [17-19]. Therefore, it may be a reasonable alternative to other analgesic techniques. However, recent evidence has demonstrated that the addition of liposomal bupivacaine appears to have no extended analgesic and opioid-sparing effects when compared with bupivacaine alone [20] or with adjuvant dexamethasone used in ISBPB [21]. However, drawing a clear conclusion from the current standpoint is complex, and further studies are needed for clarification.

Extrafascial injection for ISBPB (Fig. 1) reduces the incidence of hemidiaphragmatic paresis and affects pulmonary function while providing analgesia similar to a conventional intrafascial injection [25]. Additionally, this may reduce the potential for neurologic injury inherent to ISBPB (see below section “risk of nerve damage”).

The supraclavicular brachial plexus block may be considered an effective and safe alternative to ISBPB for shoulder surgery, especially in patients with preexisting pulmonary impairment [26]. When performing supraclavicular brachial plexus block, we found that the incidence of hemidiaphragmatic paresis was effectively reduced when the local anesthetic was injected primarily in the corner pocket (20 ml) and secondarily inside the neural cluster (5 ml) during the right-sided supraclavicular brachial plexus block [27].

Injection around the superior trunk of the brachial plexus is an alternative technique that can reduce the risk of hemidiaphragmatic paresis [6,24,28]. The superior trunk is formed by fusion of the C5 and C6 nerve roots. Therefore, local anesthetic injection around the superior trunk should produce similar analgesia in the shoulder, because the major terminal nerves innervating the shoulder arise distal to the superior trunk (Fig. 2). Moreover, the injection site is farther away from the phrenic nerve, which theoretically reduces the risk of hemidiaphragmatic paresis. Based on this, we performed a non-inferiority clinical trial comparing ISBPB with a superior trunk block, in which the superior trunk block provided postoperative shoulder analgesia equivalent to ISBPB, as demonstrated by similar pain scores, duration of analgesia, and 24 h opioid consumption [6]. Our findings were confirmed by another previous study that prospectively compared ISBPB with superior trunk block [24]. They also observed less frequent hemidiaphragmatic paralysis in the superior trunk block group.

Another strategy to reduce the risk of hemidiaphragmatic paresis is to inject local anesthetic at the terminal nerves that innervate the shoulder more distal to the superior trunk [29]. A shoulder block is an alternative approach that blocks the suprascapular and axillary nerves [30]. These two nerves innervate majority of the shoulder with additional minor contributions from the subscapular and lateral pectoral nerves [31]. Theoretically, the shoulder block may spare pulmonary function and diaphragmatic movement. Compared with ISBPB, suprascapular and axillary nerve blocks reduced the incidence of hemidiaphragmatic paresis and pulmonary dysfunction, while providing similar postoperative analgesia [32-35].