Although rare, pulmonary aspiration of gastric contents remains one of the most feared complications in obstetric anesthesia. A research team in Canada has addressed this occurrence by creating a predictive model that uses bedside gastric ultrasound, which accurately estimates gastric volume based on patient demographics and the cross-sectional area of the antrum.
“Given the popularity of bedside ultrasound, we thought we might be able to use the technology to assess risk in terms of patients’ gastric content,” said Cristian Arzola, MD, associate professor of anesthesia at the University of Toronto. “So the purpose of this study was to correlate the cross-sectional area of the antrum with a certain amount of fluid ingested by the patient. The goal was to eventually develop a predictive model to estimate gastric volume.”
Dr. Arzola and his colleagues enrolled 60 nonlaboring pregnant women at term into the randomized, single-blind study. The investigators first performed a baseline series of sonographic measurements of the cross-sectional area of the antrum after the women had fasted for eight hours; patients were both supine and in the right lateral decubitus position.
Patients were classified using a three-point grading system:
- grade 0 = no fluid
- grade 1 = fluid seen in right lateral decubitus only
- grade 2 = fluid seen in both positions
The study’s participants were then randomly assigned to ingest one of six predetermined volumes of apple juice (0, 50, 100, 200, 300 or 400 mL), at which point another ultrasound was performed.
The anesthesiologist performing the ultrasound examinations was blinded to the volume allocation, and used a standardized scanning protocol of the gastric antrum with a 2- to 5-MHz curvilinear array transducer in a sagittal–to–right-parasagittal plane on the epigastric area. All patients were in a 45-degree semirecumbent position.
“If a patient is laying flat and has a small amount of gastric fluid, it’s going to spread out,” Dr. Arzola said. “But if you have them in right lateral decubitus and at 45 degrees, you’re able to increase sensitivity and detect any fluid or gastric content in the antrum, and that’s what we found.”
As Dr. Arzola reported at the 2016 annual meeting of the Canadian Anesthesiologists’ Society (abstract 152914), preliminary results show that the cross-sectional area of the antrum in the right lateral decubitus position correlated well with volumes ingested (Pearson correlation coefficient, 0.7). The researchers then tested a series of mathematical models that combine cross-sectional area of the antrum in the right lateral decubitus position with patient demographics such as age, gestational age, height and body mass index.
Dr. Arzola noted that although there are models to estimate gastric volume in adults, they are lacking in pregnant women. “We were motivated to do this study because we already have another model to estimate gastric volume in adults, which was confirmed in a previous trial using gastroscopy [Anesth Analg2011;113:93-97],” he said. “However, it’s quite difficult to do gastroscopy in pregnant patients; you’re not going to get the sample size.
“Nevertheless, when we compared the current model with our previous one, it correlated pretty well,” he added. “So even though we did not use gastroscopy in the current study, it seems that the model performs similar to the previous one, with a concordance of 0.9.”
Despite these promising results, Joanne M. Douglas, MD, was apprehensive about the effect of patient position on the well-being of the fetus. “One of the concerns I have is that you were placing patients supine,” said Dr. Douglas, who is a clinical professor emeritus of anesthesiology, and pharmacology and therapeutics at the University of British Columbia, in Vancouver.
“Did you exclude anyone who might have had symptoms of aortocaval compression syndrome? Did you monitor the fetus to make sure there were no changes of fetal heart with the change in position? Because I can imagine that even though you’re very skilled, I’m sure it still took a while to position the women in such a way and get the probe in the right spot. It would not be a one-minute exercise,” she pointed out.
“All of our patients were inpatients in the high-risk unit,” Dr. Arzola replied, “and I have the impression that we didn’t spend much time doing the actual scanning. We didn’t have any exclusions because of symptoms, but we didn’t have any problems with respect to position.”
Christian Loubert, MD, asked how the predictive tool might play into enhanced recovery after surgery (ERAS) protocols. “In our institution, they’ve moved to an ERAS protocol for all patients,” he said, a clinical assistant professor of anesthesiology at the University of Montreal. “So our elective C-section patients are allowed to drink 500 mL of clear fluid the morning of surgery. Using your model, do you think this is dangerous for our obstetric patients?”
“We studied that issue last year,” Dr. Arzola replied. “We scanned 100 patients before elective C-sections, and found that all of them behaved the same way as did nonpregnant adults. So 50% of the patients have no fluid whatsoever in the stomach, and the other 50% will have a small amount, which we believe is up to 109 mL in the gastric antrum.
“So we confirmed that pregnant patients do not have a problem with gastric emptying, provided that they’re not in labor and aren’t receiving any opioids,” Dr. Arzola added. “So we believe in our group that it’s safe to allow them to drink water up to two hours prior to surgery as well.”