To the Editor:
We read with great interest the article by Berg et al. about the evaluation of diaphragmatic excursion after interscalene nerve block with liposomal bupivacaine versus plain bupivacaine.
In their study, the authors investigated as a primary outcome the effect of liposomal bupivacaine combined with plain bupivacaine versus plain bupivacaine alone on diaphragmatic excursion during sigh (deep) and sniff (shallow) breathing in patients undergoing total shoulder arthroscopy in the immediate postoperative period and after 24 h.
The study did not detect a difference between the two groups in absolute values of diaphragm excursion during a sigh breath at 24 h (P = 0.112), despite a greater than 1 cm difference observed. In addition, a significant difference between groups was noted in the percentage change with respect to preblock levels: –24% in the liposomal bupivacaine with bupivacaine group compared with +9% in the bupivacaine group.
The authors concluded that the addition of liposomal bupivacaine to bupivacaine, in an interscalene block, results in statistically significant reductions in diaphragm excursion and pulmonary function testing 24 h after block placement compared with bupivacaine alone. The accompanying editorial underlined the need for caution in the use of liposomal bupivacaine for interscalene nerve block in patients with any pulmonary compromise.
We have two comments on this study. First, we observed that the two groups were already rather different in their baseline values, as reported in their table 2. This was particularly true for the diaphragmatic excursion (higher in the liposomal bupivacaine with bupivacaine group), the percent change of which at 24 h was the primary outcome of the study. The authors did not discuss this issue, but, indeed, this baseline difference might have influenced the finding of a larger percent change in diaphragmatic excursion at 24 h compared with baseline.
Second, the authors used the percentage change in diaphragmatic excursion as a measure of the level of diaphragmatic paralysis, as defined by Renes et al., who proposed that a 0 to 25% reduction from baseline could be considered as no paralysis, 25 to 75% reduced could be considered as partial paralysis, and a greater than 75% reduction could be considered as complete paralysis. However, as recently reported in this journal, 24 h after interscalene nerve block, the level of diaphragmatic paralysis measured with diaphragmatic excursion may be compensated by parasternal muscle activity.4 Therefore, we believe that changes in diaphragmatic excursion alone cannot be considered as an adequate measure of diaphragmatic function.
For this reason, we propose that diaphragmatic thickening fraction can be a more appropriate tool for investigating the action of medications on the diaphragm. The thickening fraction is calculated as the maximal diaphragm thickness (assessed using linear probe) during inspiration (Tdi, pi) minus the diaphragm thickness at end-expiration (Tdi, ee) divided by the Tdi, ee and multiplied by 100. Compared with diaphragmatic excursion, thickening fraction is a more sensitive and qualitatively accurate parameter and provides a more comprehensive measure of diaphragm contraction.
Excursion of the left hemidiaphragm is more difficult to assess than the right one and this is due to interference from gastric contents and the less favorable spleen’s window. Berg et al. performed 13 left blocks, but they did not comment on difficulties experienced in the evaluation of the excursion of the left hemidiaphragm.. In any case, use of the thickening fraction to measure and evaluate diaphragm activity would overcome this problem and should be used in the research field.
Of course, we understand that standard perioperative ultrasound practice does not normally include calculating the thickening fraction, being a more difficult measure to obtain and is mostly used to assess weaning from mechanical ventilation in the intensive care unit. As a matter of fact, there is a bit of a schism between point-of-care ultrasound use in the operating room and intensive care settings. However, the skills required to measure thickening fraction are relatively easy to acquire under expert supervision.
In conclusion, considering the aforementioned limitations, we suggest caution in assessing the risk of adverse events in terms of diaphragmatic and pulmonary function, including dyspnea and oxygen need, with the use diaphragmatic excursion only.