Adjuvant drugs for SS blocks

  

Anesthesiologists have added many adjuvant drugs to LAs in an attempt to prolong block duration. These include but are not limited to: opioids, bicarbonate, neostigmine, clonidine and dexamethasone. With the exception of dexamethasone, results have been invariably disappointing. 1 For example, previous studies have shown clonidine produces a statistically significant prolongation of block duration, but the clinical relevance (approx. 15%) of the effect is questionable. 1 Dexamethasone has also been shown to prolong block duration, but the lack of a systemic control group makes interpretation of the studies difficult: it is unclear whether the prolonged block duration reflects a systemic effect of dexamethasone rather than a specific perineural effect. 2 Regardless, even if future studies show the prolonged block duration effect is not systemically mediated, a potentially problematic side effect demonstrated in previous studies is the increased and prolonged motor block. 3 This could be problematic where excessive and prolonged motor block might be poorly tolerated e.g. brachial plexus block and femoral block. That said, prolonged motor block is well tolerated in certain situations: in sensory only nerves (e.g. saphenous nerve) or nerves innervating areas of the body where motor block is of minimal consequence. For example, motor block is of minimal consequence for distal upper extremity blocks when the forearm is in a cast; likewise for distal thigh sciatic block when the ankle is also supported in a cast, and for the thoracolumbar spinal nerves (TAP block). 

Realistically, the only way to significantly prolong block duration is likely to be through perineural catheter placement. Compared to an all-or-nothing adjuvant enhanced long acting block, a perineural catheter has the added advantage of titratability: if excessive motor block or numbness compromises patient satisfaction or impairs mobilisation, the infusion is simply temporarily interrupted. 

The same problems will also be relevant for possible future ultra-long acting LA e.g. liposomal bupivacaine. Unless, a pure sensory only LA is developed, lack of titratability and motor block will limit the perineural route of administration.


References


1.         Axelsson K, Gupta A. Local anaesthetic adjuvants: neuraxial versus peripheral nerve block. Curr Opin Anaesthesiol 2009;22:649-54.

2.         Cummings KC, 3rd, Napierkowski DE, Parra-Sanchez I, et al. Effect of dexamethasone on the duration of interscalene nerve blocks with ropivacaine or bupivacaine. Br J Anaesth 2011;107:446-53.

3.         Vieira PA, Pulai I, Tsao GC, et al. Dexamethasone with bupivacaine increases duration of analgesia in ultrasound-guided interscalene brachial plexus blockade. Eur J Anaesthesiol 2010;27:285-8.