To the Editor:
We are writing to raise some concerns about the study conducted by Tan et al. Their article “Quality of Labor Analgesia with Dural Puncture Epidural versus Standard Epidural Technique in Obese Parturients: A Double-blind Randomized Controlled Study” demonstrated that dural puncture epidural did not provide additional benefits in improving labor analgesia in obese parturients.
In their study, labor analgesia was maintained with programed intermittent epidural boluses of 6 ml of 0.1% ropivacaine with 2 µg/mL fentanyl every 45 min. With such small bolus volumes, the analgesic improvement resulting from drug translocation through the dural conduit with the dural puncture epidural technique may be obscured. Moreover, the delivery rate of programed bolus dose administration was not reported in this study. Generally, high-rate epidural boluses increase injectate pressure and might facilitate drug translocation. We speculated that the inadequate bolus volume and/or the low delivery rate failed to generate sufficient pressure gradient and made the drug hard to “press” from the epidural space into the subarachnoid space.
We noticed another study conducted in the same institution using the same programed intermittent epidural bolus protocol (6 ml every 45 min) that failed to find improved outcomes compared to conventional epidural continuous infusion (8 mL/h) although the attempts of patient-controlled epidural analgesia were significantly higher with programed intermittent boluses compared to that with continuous infusion. The increasing analgesia demands for pain control of patient-controlled epidural analgesia might be a surrogate for inadequate pain relief. Since there are many variables in the programed intermittent epidural bolus settings, Wong et al. believe that the programed intermittent bolus volume and interval time might influence the quality of analgesia during the maintenance of epidural labor analgesia. In our randomized controlled study, we used relatively larger programed intermittent volumes and longer intervals (8 ml every 60 min) compared to the current study and found improved analgesia quality and drug-sparing effect with dural puncture epidural compared to standard epidural technique.
For the mechanism, Tan et al. explained that the small drug mass might fail to generate the required pressure to drive ropivacaine molecules across dural hole. However, in our study with the same concentration of ropivacaine, the results were not consistent, possibly indicating that the major reason for the failure of primary outcome of the current study might be the inadequate volume of the programed intermittent bolus rather than the drug mass.