“Don't Throw the Baby Out With the Bath Water: ” A Reply to Dr. Onwochei et al

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To the Editor:
We thank Drs Onwochei, West, and Pawa1 for their interest in our study.2 Pain is a subjective phenomenon3 and may have additive or multiplicative effects from the underlying disease or condition that brought patients to surgery in the first place.4 We adjusted for subjectivity in pain ratings by using each patient as his/her own control and looking at differences in reported pain from baseline. Despite the multimodal analgesic regimen given to all patients in our study, pain levels were reduced postoperatively, meaning that liposome bupivacaine does dampen pain intensity. Small or large, yet statistically significant, reductions may be appreciated by some or summarily dismissed by others.
We disagree that a continuous catheter group would have been a more relevant active comparator. Because of their well-known limitations, catheter techniques are not the most commonly used modality to manage postoperative pain after major shoulder surgery in North America. For instance, studies that assessed the value of adjuvants (eg, dexamethasone, clonidine, dexmedetomidine) in nerve blocks have used single-injection comparators.5–7 Catheters may introduce confounding covariates that could complicate their comparison with single-injection interventions.8
Lack of motor block and preserved functionality of the arm are not indicative lack of efficacy. An injection of 10 mL of 1.33% liposome bupivacaine yields 133 mg of free base bupivacaine (150 mg equivalent of bupivacaine HCl) that is released over a period of 72+ hours. While active drug is not released in a linear fashion, the absolute mass of drug available for nerve block at the injection site is small and unlikely to result in appreciable motor block in the interscalene space. Liposome bupivacaine provides a block that can vary throughout 72+ hours from overt sensory block to mild sensory block to sympathetic block, all of which may confer analgesia.
We certainly do agree that there is more work to be done on defining optimum dose of liposome bupivacaine, volume, and mixture for specific nerve block indications; however, this is the first time that postoperative analgesia was extended through a novel delivery mechanism whose pharmacokinetics and mechanism of action are well understood. So, “don't throw the baby out with the bath water.

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