Advances in regional anaesthesia and pain management

Vincent Chan FR.CPC 0 0 From the Department of Anaesthesia, The Toronto Hospital, General Division , 200 Elizabeth Street, Toronto , Ontario M5G 2C4 E G I O N A L anaesthesia performed for R ment is extremely safe1 and has enjoyed a surgery and for postoperative pain managerenaissance in recent years. A new local anaesthetic agent has been developed and recently launched in Canada. Block techniques are being refined or newly developed to localise nerve targets more accurately. New clinical knowledge is continuously generated, improving both block efficacy and patient safety. More than ever before, regional anaesthesia is part of a multi-modal approach to perioperative pain management that expedites patient recovery and discharge. This lecture will focus selectively on some of the important new developments of the past five years that have significant clinical relevance in the adult non-obstetrical patient population. - Ropivacaine Ropivacaine (Naropin | Astra Canada), a new aminoamide local anaesthetic, made its debut in Canada in 1997. It is the first local anaesthetic prepared isomerically pure in the S-enantiomer form, making it less toxic than bupivacaine in the racemic preparation (containing both R- and S- enantiomers). Toxicity Both drugs have similar physical and chemical profiles, but their cardiovascular and central nervous system (CNS) effects differ. Ropivacaine has less potential to depress myocardial contractility and conduction and to trigger ventricular arrhythmia. The fatal cardiotoxic dose ratio in sheep is 1:2:9 for bupivacaine:ropivacacine:lidocaine. A CNS toxicity study showed that human subjects can tolerate a slow infusion of 30% more ropivacaine than bupivacaine before symptoms occur (mean: 124 mg vs99 mg). 2 A case of seizure and hypotension has been reported after inadvertent intraarterial injection of 150 mg ropivacaine, but without development of arrhythmia or impaired recovery,s The toxic blood concentration for ropivacaine has not yet been documented in humans. Advances in regional anaesthesia and pain management Anaesthesia Ropivacaine has been used for anaesthesia for a variety of surgical procedures4 and the maximum recommended dose is 2.5-3 mg-kg-1 for peripheral nerve, brachial plexus and epidural blocks. When used in brachial plexus block, ropivacaine 0.5% and bupivacaine provide equivalent motor and sensory blockade, with comparable onset time, long duration and analgesic efficacy. However, higher concentrations (0.75%-1%) and doses of ropivacaine are required to achieve the same degree and duration of motor block in epidural anaesthesia when compared with bupivacaine, s It is important to note that the safety margin for ropivacaine narrows if the dose is increased substantially to augment motor block intensity. The addition of epinephrine to ropivacaine is not warranted because it does not alter block intensity or duration. Ropivacaine has not been approved for spinal anaesthesia, thus experience with this application is limited. 6 Analgesia The greater sensory-motor separation property o f epidural ropivacaine makes it an attractive drug for postoperative pain management. So far, studies o f epidural ropivacaine for postoperative analgesia have only examined the analgesic efficacy o f continuous ropivacaine infusion alone in concentrations o f 0.1, 0.2 and 0.3%. 7 Analgesic efficacy proved dose-dependent, but even with 0.3% at 10 ml-hr-1, systemic opioid supplementation was necessary to achieve satisfactory analgesia. Systemic accumulation over a 24-hr period of infusion at 10-30 mg.hr -1 produced no systemic toxicity and low plasma concentrations, s However, motor block developed over time even with lower concentrations. 9 Thus, epidural infusion ofropivacaine alone is not a useful postoperative management technique. Alternatively, 0.1% combined with epidural opioid might produce adequate analgesia with lower risk of motor block but study results are pending. At present, the 0.2% solution is commercially available as a Polybag| epidural infusion kit with 100-ml and 200-ml doses. Intravenous regional anaesthesia New techniques of/VRA for upper extremity blocks are applied to improve patient safety and post-block analgesia. A recent survey o f North American anaesthetists revealed that 66% of the respondents encountered minor local anaesthetic-related side effects in patients undergoing IVRA, 14% o f the anaesthetists reported seizures and 1.2%, cardiac arrest. I~ Systemic toxic complications can occur as a result of considerable leakage or release o f local anaesthetic when the tourniquet is inflated or deflated respectively. Drug leakage past a properly inflated tourniquet (15% from the upper limb and 29% from the lower limb) has been reported, ll Lidocaine 0.5% (3 mg.kg-I, average 200 mg) is most commonly used for IVRA. Since systemic toxicity is dose-dependent, new approaches aim to reduce the dose of lidocaine and supplement with opioid, nondepolarising muscle relaxant or nonsteroidal anti-inflammatory drugs (NSAID); this is an example of balanced analgesia. Morphine (6 mg), meperidine (100 mg) and fentanyl (100-200 lag) have been used with local anaesthetics with varying success. Meperidine distinguishes itself from other opioids in that, when used alone, it produces IVRAI2 but the optimal supplemental dose to local anaesthetic has not been established. Low-dose nondepolarising muscle relaxants (0.5 mg pancuronium; 2 mg atracurium; and 0.6 mg mivacurium) have been used successfully to improve the onset time and intensity o f motor block with IVRA. The combination o f low dose 1.5 mg.kg-~ lidocaine with 1 tag.kg-1 fentanyl and 0.5 mg pancuronium produces an aaaaesthetic effect similar to that o f 3 mg.kgq lidocaine at 20 min. ls,I4 Only minor side effects o f dizziness and diplopia are observed at the time of tourniquet deflation. It must be emphasised that muscle relaxant administration must be carefully monitored, because the systemic effect of higher doses is potentially lethal. The addition of NSAIDs to IVRA can substantially prolong postoperative analgesia, xs Injectable ketorolac (60 mg) added to 190 mg lidocaine can prolong analgesia to 10 hr (mean) and substantially reduce the need for analgesic supplementation. The mechanism o f action presumably is an anti-inflammatory response at peripheral nerve endings, not a systemic effect. A similar response is seen following direct wound infiltration with ketorolac) 6 Although side effects have not been reported following 60 mg ketorolac, a reduction in dose (e.g., to 30 mg) might be worth exploring. Brachial plexus block Recently, several new approaches to brachial plexus block have been described. The mid-humeral approach blocks the brachial plexus below the axilla through a single needle puncture in the upper third o f the arm by anaesthetising, individually, each o f the four terminal branches (median, ulnar, radial and musculocutaneous nerves) with 10 ml local (...truncated)