Liposomal Bupivacaine’s Effect on the Diaphragm: Reply

We thank Vetrugno et al.¹  for their relevant comments on our article.²  As Vetrugno et al. pointed out, the liposomal bupivacaine group started at a higher baseline diaphragmatic excursion than the bupivacaine-alone group. This was an unavoidable consequence of our randomization scheme. The difference between the groups, however, was not significant, and we do not believe it contributed to our primary outcome.

The goal of our study was not just to detect statistical differences, but to gain insights into the clinical significance of the observed changes. To do this, we used the clinical definitions by Renes et al.³  of diaphragmatic paralysis. We felt that it was important to use the same technique used in that study, rather than trying to cross compare different techniques.

Although we do agree that the addition of diaphragmatic thickening would have added to the article, we stand by our conclusions. We are not aware of a comparative study of diaphragmatic thickening and excursion with clinical findings, and our results correlated well with the traditional measure of pulmonary function we measured via spirometry, lending credence to its validity. We still appreciate their word of caution regarding its use as a sole measure of pulmonary function, because recent research shows that diaphragmatic excursion can be influenced by accessory muscle use.⁴ 

Indeed, we believe that the reduction we detected in diaphragm excursion in the liposomal bupivacaine group combined with the reductions found in forced expiratory volume and forced vital capacity provides valuable insight into the clinical impact of liposomal bupivacaine interscalene blocks on respiratory function. Hence, we are confident that the addition of liposomal bupivacaine to an interscalene block did not create a clinical difference in this population.

Support was provided solely from the Anesthesiology Department at the University of Minnesota (Minneapolis, Minnesota).

Dr. Hutchins was a speaker, consultant, and has received research funds from Pacira Pharmaceuticals (Parsippany-Troy Hills, New Jersey); he is a consultant and owns stock with Insitu Biologics (Oakdale, Minnesota); a consultant and speaker for Acel RX (Hayward, California); a consultant for Worrell (Minneapolis, Minnesota) and Johnson and Johnson (New Brunswick, New Jersey); a consultant for Atricure Inc (Minnetonka, Minnesota); and a speaker for Avanos (Alpharetta, Georgia). Dr. Berg was a consultant for Avanos and is a consultant for Pacira Pharmaceuticals. No companies were involved with any aspect of this manuscript. The other authors declare no competing interests.