Different ropivacaine concentrations in transversus abdominis plane (TAP) block for post-cesarean analgesia yield unpredictably varying analgesic benefits and wide confidence intervals in morbidly obese women, according to a randomized clinical trial. The results have served to add fuel to the fire over the controversy of the block’s efficacy.
“At our institution, morbidly obese parturients do not receive neuraxial morphine for post-cesarean analgesia because of concerns about respiratory depression,” said Joseph B. Bavaro, MD, assistant professor of anesthesiology at Northwestern University’s Feinberg School of Medicine, in Chicago. Such restrictions highlight the need for a multimodal analgesic approach in these patients, one that sometimes includes TAP blocks. Nevertheless, the optimal technique for administering post-cesarean TAP blockade has yet to be defined.
The investigators compared the effect of varying ropivacaine concentrations during TAP blockade on 24-hour opioid consumption after cesarean delivery in 120 obese women (body mass index [BMI], >40 kg/m2) who had undergone scheduled cesarean delivery. Each participant received spinal or combined spinal–epidural anesthesia (12 mg of intrathecal bupivacaine, 15 mcg of fentanyl). Patients were randomly assigned to receive the postoperative, ultrasound-guided, bilateral TAP block with 15 mL per side of either saline or three different ropivacaine concentrations: 0.2%, 0.5% or 0.75%.
The women also received postoperative hydromorphone via patient-controlled analgesia, as well as scheduled IV ketorolac (30 mg every six hours) for 24 hours, followed by oral hydrocodone-acetaminophen and ibuprofen. The study’s primary outcome was 24-hour morphine-equivalent opioid consumption. Secondary outcomes included total opioid consumption, nonsteroidal anti-inflammatory drug consumption, and visual analog scale pain scores at two, six, 24 and 72 hours after the TAP block.
“What we found was somewhat puzzling,” Dr. Bavaro reported at the 2016 annual meeting of the American Society of Anesthesiologists (abstract A2078). Indeed, of the 115 women for whom data were available, median 24-hour morphine-equivalent consumption was lower in patients who received the TAP block with 0.2% of 0.5% ropivacaine than their counterparts who received saline, but there was no significant difference in such consumption between the saline and 0.75% ropivacaine groups.
“This was a surprising outcome,” Dr. Bavaro said. “And after looking at the data, as well as some of the other literature on morbidly obese patients not undergoing obstetric surgeries, we concluded that TAP blocks are unreliable in the morbidly obese.
“What’s more, the large confidence intervals in our study make me think that there are some patients who are not going to get any benefit from receiving a TAP block because their pain relief is not going to be that great.” Given that there were no clear differences in patient characteristics between groups—or a bias toward a particular provider performing the blockade—these wide confidence intervals support the lack of reliability of TAP analgesia in this population.
“I think the other thing I take from this is that automatically doing a TAP block on all of these patients doesn’t make much sense,” Dr. Bavaro continued. “I think it may be good as a rescue analgesia modality, but I don’t think the data support using it routinely. And I don’t think we can definitively say which concentration is optimal, either.”
Others Weigh In
“I’m wondering if other people have looked at the literature in nonobese patients,” said session moderator Katherine W. Arendt, MD, associate professor of anesthesiology at Mayo Clinic, in Rochester, Minn. “Has it been shown to lack reliability in this population as well?”
“The results are mixed,” Dr. Bavaro replied. “Some studies have shown benefits while others have not.”
“Based on meta-analyses of saline versus TAP block for C-sections, it adds no benefit if you use intrathecal morphine, but adds benefit if you don’t use intrathecal morphine,” added Cynthia Wong, MD, professor and chair of anesthesia at the University of Iowa Carver College of Medicine, in Iowa City. “But the meta-analyses included mixed patients, presumably both lean and obese. I don’t think there’s ever been a dose–response study looking at the response to TAP block between lean and obese patients.”
“I actually think that most of the studies done to date with TAP block did not include obese patients—most of them had exclusions above BMIs of 35 [kg/m2],” added Ashraf S. Habib, MD, professor of anesthesiology at Duke University Medical Center, in Durham, N.C. “So I would think that there haven’t been any studies to date of TAP block on obese C-section patients.”
With all the questions surrounding their efficacy, practitioners need to ask themselves what ultimate benefit they may get from administering TAP blocks, said Mark C. Norris, MD, clinical professor of anesthesiology at Boston University. “We looked at ilioinguinal and iliohypogastric nerve blocks years ago, and it was the same issue,” Dr. Norris said. “Perhaps patients have some measurable pain relief at two hours. But at four or six hours when the block wears off, everyone is in the same boat again. So is it really the answer to our 24-hour problem?”