The effects of intrathecal and systemic adjuvants on subarachnoid block

Various intrathecal and systemic adjuvants to local anaesthetics have been found to improve the quality and extend the duration of spinal block. Intrathecal opioids are the most frequently used; the lipophilic fentanyl and sufentanil enhance and moderately prolong the sensory block, whereas the hydrophilic morphine significantly prolongs spinal analgesia. Nausea/vomiting, pruritus, urinary retention and respiratory depression are possible side effects. Adrenergic agonists, such as adrenaline and phenylephrine may prolong the block due to vasoconstriction, while clonidine and dexmedetomidine accelerate the onset and prolong the duration of block and analgesia. Hypotension, sedation and respiratory depression have been reported with clonidine. Other intrathecal adjuvants, such as midazolam, ketamine and neostigmine may also improve the quality of block and prolong analgesia, but are not popular because of their adverse effects. Intrathecal magnesium sulphate mainly potentiates the analgesic action of intrathecal opioids, without significant side effects. A positive impact on spinal analgesia has also been suggested ‑ from animal studies ‑ for intrathecal calcium channel blockers, while the analgesic efficacy of intrathecal nonsteroidal anti-inflammatory drugs remains questionable. Several drugs may also affect the spinal block characteristics after systemic administration. Opioids enhance, alpha-2 agonists and ketamine prolong the block, magnesium sulphate reduces postoperative analgesic consumption and nimodipine may delay the regression of sensory block. Nitrous oxide inhalation has also been found to enhance the level of sensory spinal block. Even though opioids are the most popular adjuvants to spinal local anaesthetics, a variety of drugs given intrathecally or systemically, can accelerate, improve and extend the spinal block.