A review of more than 5,000 patient records has concluded that women undergoing preterm cesarean delivery have a significantly higher risk for failed spinal anesthesia than women undergoing the procedure at full term. These findings, reported by a Duke University research team suggest that an increased spinal dose—or combined spinal–epidural technique—may be necessary to reliably ensure adequate anesthesia in this population.
Although studies have examined the relative spread of spinal block for surgical anesthesia in pregnant and nonpregnant women, only limited research has been conducted on spinal dosing for preterm and term parturients. “The purpose of the study was to investigate the hypothesis that preterm gestation may be associated with an increased incidence in failures of spinal anesthetics compared to those patients who were at term gestation,” said Lisa M. Einhorn, MD, a resident at Duke University Medical Center, in Durham, N.C.
To help answer that question, Dr. Einhorn and her colleagues searched their institution’s database for women who underwent cesarean delivery under spinal or combined spinal–epidural anesthesia between 2003 and 2012. Patients were included if their height was 152 to 183 cm, and they received standard doses of local anesthetic (0.75% hyperbaric bupivacaine ≥10.5 mg with fentanyl 15 mcg and morphine 0.1-0.2 mg).
“Our primary outcome was inadequate surgical anesthesia or failure after an initial spinal dose,” Dr. Einhorn said (abstract A1059). Failure was defined in several ways: the need to repeat the spinal technique to obtain adequate block height; conversion to general anesthesia secondary to pain; supplementation with IV analgesics; the use of nitrous oxide; or augmentation of the initial block with epidural lidocaine within 30 minutes if a combined spinal–epidural technique was used.
A total of 5,041 women were included in the analysis; 3,404 pregnancies were term and 1,637 were preterm. The groups were similar in demographics and hyperbaric bupivacaine dose; the most common bupivacaine dose was 12 mg, which 62% of patients received.
The investigators found that of 150 failed spinal anesthetics (3.2%), significantly more cases occurred in preterm women (4.5%) than term (2.2%; P<0.0001). “Those that are earliest preterm—less than 30 weeks—had the highest level of failure [6.8% vs. 3.6% for those with <37 weeks gestation],” Dr. Einhorn said.
A multivariable regression model revealed that gestational age was a significant predictor of anesthetic failure (adjusted odds ratio, 0.91; P<0.0001). No other factor—age, race, height, weight, bupivacaine dose—was statistically significant.
“Our thoughts behind this are that pregnant patients are known to need higher doses of local anesthetic to create the same block heights,” Dr. Einhorn said. “So it’s possible that these early preterm patients may not fully undergo the anatomic and physiologic changes that lead to increased spinal spread in pregnancy. Therefore, an increased spinal dose may be necessary to fully anesthetize these patients for surgical anesthesia for cesarean delivery,” she added. “We would recommend a combined spinal–epidural technique, which would allow for the augmentation of the initial spinal in these patients, if needed.”
Richard M. Smiley, MD, PhD, Virginia Apgar Professor of Anesthesiology and chief of obstetric anesthesia at Columbia University Medical Center, New York City, noted that these data are consistent with previous research. “It’s very similar to what you see if you look at postpartum bilateral tubal ligation, because once again, the anatomy has changed. But my clinical conclusion would not be to do a combined spinal–epidural,” Dr. Smiley said. “My preference would be to just give everyone 15 mg. It’s simpler and safe enough.”
An ancillary study by the same research team (abstract A1061) examined the incidence of high blocks and failed blocks in patients who received spinal block for cesarean delivery after an inadequate labor epidural. Using the same study cohort, the investigators divided the patients into two groups by whether they received or did not receive a top-up dose (epidural lidocaine ≥100 mg) before spinal administration.
There were 29 failed spinals (11%) and nine high spinals (3%). Interestingly, the incidence of failed spinals was significantly greater in patients who received an epidural top-up (24% vs. 4%;P<0.001).
This, the researchers hypothesized, may be partly due to the presence of a large volume of local anesthetic in the epidural space, which may be mistaken for cerebrospinal fluid. Multivariate regression analysis revealed that the top-up dose was a significant predictor of block failure (P=0.0005).