The duration of a nerve block depends mainly on the type of local anesthetics used and the amount (volume and concentration, actually total ‘mass’ of local anesthetic) injected around the target nerve. Block duration can be prolonged with the use of a vasoconstrictor such as epinephrine, which decreases the diffusion of the anesthetic away from the nerve or the use of other adjuvants, each with a different mechanism of action and final effect (dexamethasone, clonidine, dexmedetomidine, magnesium).
When deciding, which local anesthetic to use, what concentration, which adjuvant and whether we should place a catheter for continuous infusion, there is no actual dogma to challenge. We just need to know what is our aim. When the aim is to provide a long lasting analgesic effect, long lasting local anesthetics may be used and/or adjuvants may be added and/or a perineural catheter may be placed. Drawbacks of this decision are: the delayed ambulation and risk of falls in case lower limb blocks are performed, the delayed neurological and motor examination, the requirement for specialized equipment and additional skills when catheters are used and the need for prolonged patient monitoring and management from dedicated acute pain teams. In case short lasting effects are required, so that motor and sensory function return as quickly as possible, to achieve early ambulation and neurological examination, short acting local anesthetics may be used and the known drawbacks are: the risk of rebound pain upon resolution of nerve blockade, the need for higher opioid doses, delayed rehabilitation due to opioid side effects and the pain itself and prolonged hospital stay. Additionally, if eventually acute pain is not managed adequately, the risk of patient dissatisfaction and the incidence of cardiovascular and pulmonary complications and persistent postsurgical (chronic) pain are increased.
So, the solution is to balance the risks and benefits of each technique, choose the proper one for each individual and of course not to rely just to the peripheral nerve blockade for postoperative pain management but on to a multimodal regimen including a block but also other agents administered from different routes. In this case drawbacks of short acting blocks will be successfully managed. In case long lasting blocks are used, proper multi-intervention fall-prevention strategies may be applied, so that patients are safely mobilized early and dose adjustments both in continuous and single shot techniques or proper adjuvants (that tend to prolong analgesia more than they prolong motor block, according to studies can counteract the relevant drawbacks effectively.
References 1. Salinas FV, Joseph RS. Peripheral nerve blocks for ambulatory surgery. Anesthesiol Clin 2014;32:341–55
2. Dawson S, Loewenstein SN. Severe rebound pain after peripheral nerve block for ambulatory extremity surgery is an underappreciated problem. Comment on Br J Anaesth 2021; 126: 862–71. Br J Anaesth 2021;126:e204-e205.
3. Eng HC, Ghosh SM, Chin KJ. Practical use of local anesthetics in regional anesthesia. Curr Opin Anaesthesiol 2014;27:382–7
4. Barry GS, Bailey JG, Sardinha J, Brousseau P, Uppal V. Factors associated with rebound pain after peripheral nerve block for ambulatory surgery. Br J Anaesth 2021;126:862–871.
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