based on a prospective cohort study.
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Over the course of 4 days, relative neonatal dosages of bupivacaine were less than 1%, remaining below the 10% threshold of concern, reported Hiba J. Mustafa, MD, of the University of Minnesota, Minneapolis, and colleagues.
Liposomal bupivacaine can achieve up to 4 days of postcesarean pain control, which is significantly longer than the 8 hours provided by standard bupivacaine, the investigators wrote in Obstetrics & Gynecology. But usage of the liposomal formulation has not been widespread, they noted, partly because of a lack of clinical studies evaluating breast milk transfer and neonatal safety.
To address this knowledge gap, Dr. Mustafa and colleagues enrolled 30 healthy pregnant women scheduled to undergo cesarean birth at full term. All patients were aged 18-40 years, with an American Society of Anesthesiologists physical status of I or II. Exclusion criteria included a number of maternal and neonatal health concerns, such as sensitivity to local anesthetics, metabolic disorders, fetal anomaly, fetal growth restriction, and others.
The day of surgery, before the procedure, maternal blood samples were collected and used for baseline measurements.
Each woman received a spinal anesthetic including 150 mcg of morphine, 15 mcg of intrathecal fentanyl, and 1.4-1.6 mL of 0.75% hyperbaric bupivacaine hydrochloride. Within 30 minutes after birth, a bilateral transversus abdominus plane block was performed using 266 mg of 1.3% liposomal bupivacaine and 52 mg of 0.25% bupivacaine hydrochloride.
Using the block as time point zero, maternal blood and breast milk samples were collected at hour 2, 6, 12, 24, 48, 72, and 96. Sparse sampling was employed, such that participants were randomly assigned in a 1:1 ratio to provide paired blood and milk samples at hour 2, 12, and 48; or hour 6, 24, 72, and 96. Bupivacaine was quantified in samples by liquid chromatography–tandem mass spectrometry.
Using these data, the investigators determined bupivacaine concentrations in plasma and milk, milk/plasma area under the curve (AUC) ratios, neonatal dosage, and relative neonatal dosage. In addition, adverse events in both mothers and neonates were recorded for 2 weeks post partum.
Mean bupivacaine concentrations peaked in breast milk at 6 hours, at 58 ng/mL. This peak was followed by a steady reduction to an “almost undetectable” level of 5.2 ng/mL at 96 hours. Maternal plasma levels peaked first at hour 6 (155.9 ng/mL), then again at hour 48 (225.8 ng/mL), followed by a steady decline until hour 96, when the level reached 80.6 ng/mL.
Relative mean concentrations of milk to plasma were 44%, 36%, 28%, and 18% at hour 2, 6, 12, and 24, respectively. AUC ratios were used to represent exposure across various time intervals. For instance, the AUC ratio for milk/plasma from hour 0 to hour 2 was 0.45. The AUC findings declined steadily until the final ratio, which spanned hour 0 to hour 96, at 0.15.
These AUC ratios allowed for calculation of neonatal dosage and relative neonatal dosage using an average daily milk intake of 150 mL/kg per day. For the longest range, spanning from hour 0 to hour 96, the neonatal dosage was 15,155.4 ng/kg, which translated to a relative neonatal dosage of 0.396%.
No mothers or neonates experienced adverse events.
“Bupivacaine was transferred into mother’s milk such that an exclusively breastfeeding neonate would ingest less than 1% (relative neonatal dosage) of the maternal dose,” the investigators wrote, noting that this falls safely below the acceptable threshold of 10%.
“Because bupivacaine is metabolized primarily in the liver, a neonate’s absorption will likely be even lower [than modeled] given the first-pass effect,” they added.
Based on these findings, Dr. Mustafa and colleagues concluded that “the level of bupivacaine ingested by the sucking neonate is acceptable and compatible with breastfeeding.”
Michael G. Ross MD, MPH, Distinguished Professor of Obstetrics and Gynecology and Public Health at Geffen School of Medicine at the University of California, Los Angeles, commented that, this study adds to the literature of drug excretion into breast milk. “For the vast majority of drugs with passive transfer from maternal plasma to breast milk, the effective dosages of exclusive breastfeeding neonates are approximately 5% of the maternal (oral) dose. In the present study, the authors demonstrated a relative neonatal dosage of less than 1%. This low value results from consequences of minimal maternal plasma absorption (in the present case from transversus abdominis injection), maternal volume of distribution, transfer into breast milk, and the volume of milk ingestion. These results should provide reassurance for the safety of breastfeeding term infants under the conditions of the study.
“There are a number of study concerns, including the inability to differentiate absorption of the spinal bupivacaine from the liposomal bupivacaine, the lack of paired maternal plasma and breast milk sample, and the lack of detail as to how much milk was expressed for each sample. Importantly, breast milk composition varies from foremilk to hindmilk. Thus, a single sample may not accurately reflect the composition ingested by the infant. The suggestion of two peaks in maternal plasma concentration was not demonstrated statistically and may be an artifact of the timing of spinal and liposomal injections, or the fact that different patients were studied at each time period.
“Most importantly, despite the demonstrated safety, the authors acknowledge conflicting results of clinical benefits of liposomal bupivacaine injection. As such, I recommend that postcesarean transversus abdominis blocks be performed only under institutional review board-approved study protocols,” said Dr. Ross, codirector of the Institute for Women’ and Children’s Health at the Lundquist Institute, Torrance, Calif.*
The study was funded by the Thrasher Research Fund. The investigators reported no conflicts of interest. Dr. Ross had no relevant financial disclosures.
SOURCE: Mustafa et al. Obstet Gynecol. 2020 Jun 6. doi: 10.1097/AOG.0000000000003886.
*This article was updated 6/16/2020.