Roughly 40% of women receiving intrathecal opioids for cesarean delivery experience hypercapnia, according to a study presented at the 2016 annual meeting of the Society for Obstetric Anesthesia and Perinatology. The researchers said the condition is underdetected in the obstetric population, as per the current guidelines of the American Society of Anesthesiologists (ASA).
“Women with a higher baseline transcutaneous carbon dioxide (TcCO2) may need to be more closely monitored postpartum,” said lead author Jeanette Bauchat, MD, section chief of obstetric anesthesiology at Northwestern University Feinberg School of Medicine, in Chicago.
While guidelines for prevention, detection and management of respiratory depression associated with neuraxial opioid administration exist, they are somewhat weak (Anesthesiology 2016;124:535-552). “The guidelines recommend intermittent respiratory rate and sedation monitoring and pulse oximetry when appropriate, but we don’t really have a good definition for respiratory depression, so it’s hard to determine the true incidence of respiratory depression,” Dr. Bauchat said.
Respiratory depression can be defined as low respiratory rate, desaturation or hypercapnia, and within each category, there are different thresholds (Can J Anaesth 2003;50:679-688). The ASA defines hypercapnia as a blood gas CO2level greater than 50 mm Hg.
“Historically, respiratory depression in our patient population following intrathecal morphine has ranged from 0.1% to 1%, using either intermittent or continuous respiratory rate and pulse oximetry,” Dr. Bauchat said. In 2013, a study from the United Kingdom suggested that the incidence of opioid respiratory depression detected by Topological Oscillation Search with Kinematic Analysis (TOSCA; Radiometer America) is a lot higher, between 7% and 18%, than previously reported by other monitoring methods (Int J Obstet Anesth2013;22:217-222).
In the new study, researchers recruited healthy, term women who were scheduled for cesarean delivery with spinal anesthesia and intrathecal morphine. To be included, women had to have a body mass index less than 40 kg/m2.
Baseline sleep apnea was assessed with the STOP-BANG Sleep Apnea Questionnaire, which has been validated in nonpregnant women (Br J Anaesth2012;108:768-775). Spinal anesthetic medications included 12 mg bupivacaine plus 15 mcg fentanyl, and 150 mcg morphine. Postoperative analgesia included nonsteroidal anti-inflammatory drugs every six hours, as well as 325 mg acetaminophen and 10 mg hydrocodone, as needed.
The researchers measured TcCO2 using the TOSCA monitor. “The device has an accuracy of TcC02 within 0 to 6 mm Hg compared to the gold standard, arterial blood gas measurements,” Dr. Bauchat said. “This is within 0% to 15%, which is considered acceptable for a noninvasive model compared to the standard. This monitor uses a Stow-Severinghaus–type electrode that measures pH change and then converts it to a CO2 reading. TcCO2 monitoring was started in the post-anesthesia care unit and lasted for 24 hours.”
Of the 108 women who completed the study, the average body mass index was 29 kg/m2 and 12% had high-risk obstructive sleep apnea (OSA). At baseline, the median TcCO2 was 35 mm Hg; the peak was 47 mm Hg. Almost 100% of women had an increase in TcCO2 from baseline, and the median increase was 13 mm Hg. Thirty-seven percent of women reached the hypercapnia threshold (>50 mm Hg) in the first 24 hours, with 40% achieving it within the first three hours.
“There was no correlation between the time to first achieving the hypercapnia threshold of 50 mm Hg and the highest TcCO2 achieved for the duration of time spent above 50 mm Hg, essentially saying that these were episodic in nature,” Dr. Bauchat said.
On the Richmond Agitation-Sedation Scale, 97% of the women were calm and alert. High-risk OSA and respiratory rates were not associated with hypercapnia. Three-fourths of the women required supplemental opioids, and there was no correlation between cumulative opioid intake and peak TcCO2 levels.
In a logistic regression model, baseline TcCO2 level was the highest predictor of hypercapnia, with a cutoff of 38 mm Hg. If women had a baseline TcCO2 level above 38 mm Hg and received supplemental opioids, they were at a higher risk.
“A higher TcCO2 level of 38 [mm Hg] is predictive of reaching a hypercapnic threshold of 50 [mm Hg], and by inference, it may actually predict respiratory depression and potentially adverse outcomes,” Dr. Bauchat said. “Clearly, I think our current ASA guidelines don’t distinguish which women are at risk of having respiratory depression.”
In a question-and-answer session, Richard M. Smiley, MD, PhD, chief of obstetric anesthesiology at NewYork-Presbyterian Hospital/Columbia University Medical Center, in New York City, said more research is needed. “My question is how sure are you that any of this has anything to do with spinal morphine and not the post-op period or being pregnant? I’m not so convinced that it has to do with neuraxial morphine,” he said. “It probably does, but it is not entirely clear.”
Dr. Bauchat said she could not find anything in the literature about the normal recovery to pre-pregnancy CO2 levels. “Clearly that needs to be studied,” Dr. Bauchat said. “Should we abandon intrathecal morphine? Should we just use PCA [patient-controlled analgesic] opiates? Will there be less women who have hypercapnia or respiratory depression [if we do]? I think that is something we need to look at in our patient population, but the literature is all over the place as to which is better.”