In recent years, bedside ultrasonography has improved the quality and safety of common technical procedures in anesthesia practice (e.g., central line insertion, spinal ultrasound assessment to guide neuraxial anesthesia, and ultrasound-guided peripheral nerve blocks). In addition, bedside ultrasonography is used for point-of-care diagnosis of various clinical conditions with important implications to the perioperative decision-making process. There is increasing evidence of the benefits of this change in practice.1 Point-of-care ultrasonography, performed and interpreted by the clinician at the bedside, allows for correlation of real-time findings with the patient’s history, as well as for real-time reassessments as the patient’s condition changes.2
Pulmonary aspiration of gastric contents in pregnant women undergoing general anesthesia remains one of the most feared complications of obstetric anesthesia.3 An empty stomach before induction of anesthesia is desirable, regardless of the anesthetic technique. Currently, we follow guidelines that recommend similar preoperative fasting intervals for elective cesarean delivery and elective surgery in nonpregnant adults.4–6
Although there is substantial evidence that gastric emptying time is similar in nonlaboring term pregnant women and nonpregnant subjects,7–9 this evidence has been produced in a limited number of controlled observations and does not account for real-life conditions in daily anesthesia practice. Gastric content and volume assessment has recently emerged as a feasible point-of-care ultrasonographic application that can help determine aspiration risk in the perioperative setting.10 Real-time assessment may offer an opportunity to improve patient safety and decision making. Early studies in pregnant women were able to consistently identify solid contents in the stomach; however, an empty stomach could not be visualized in many of the subjects unless ingestion of water streaming into the stomach could be seen in real time.11 Current technology provides better imaging quality, which allows improvements in the ultrasonographic examination.12,13 The purpose of this study was to describe the qualitative and quantitative sonographic patterns of the gastric antrum in a cohort of term pregnant patients presenting for elective cesarean deliveries.
After approval by the research ethics board (13-0162-E) at Mount Sinai Hospital (University of Toronto affiliated teaching hospital, Toronto, ON, Canada), we conducted an observational cohort study and registered at ClinicalTrials.gov (NCT01980121). We followed the Strengthening the Reporting of Observational Studies in Epidemiology statement and checklist14 in conducting and reporting our investigation.
We studied nonlaboring term pregnant women scheduled for cesarean delivery under neuraxial anesthesia. Written informed consent was obtained from all enrolled subjects. The inclusion criteria consisted of nonlaboring pregnant women ≥36 weeks gestational age, scheduled cesarean delivery, ≥18 years of age, ASA physical status I to III, weight 50 to 120 kg, height ≥150 cm, and ability to understand the rationale of the study assessments. Exclusion criteria were multiple gestations; abnormal anatomy of the upper gastrointestinal tract; and previous surgical procedures on the esophagus, stomach, or upper abdomen. Conditions such as gastroesophageal reflux disease and diabetes mellitus were not exclusion criteria. Patients were asked to follow institutional fasting instructions (minimum of 6 hours after a light meal such as toast and a clear liquid; 8 hours after a meal that includes meat, fried, or fatty foods; and 2 hours for clear liquids).4,5 At our institution, no routine pharmacologic aspiration prophylaxis is used before elective cesarean delivery unless specifically requested by the attending anesthesiologist.
Ultrasonographic examinations were performed within 1 hour before the scheduled time of the cesarean delivery by 2 anesthesiologists with a previous experience of 3 years performing ultrasonographic gastric assessment in the pregnant and nonpregnant population.12,15 A standardized scanning technique was used with a portable sonographic system equipped with a 5- to 2-MHz curved-array transducer (M-Turbo® ultrasound system, SonoSite Canada, Inc., Markham, ON, Canada). Subjects were first placed in the supine position and then in the right lateral decubitus (RLD) position, always in a 45° semirecumbent position. In both of these positions, fluid or semifluid content gravitates preferentially to the antrum, and air or gas is displaced proximally toward the body or fundus, thus facilitating antral sonography.16 The examination focused on the antrum, which is the portion of the stomach most amenable to ultrasonographic imaging because of its consistent shape, location, and least amount of air content. We considered both positions as part of the same continuous diagnostic process and not 2 different diagnostic modalities. The gastric antrum was imaged in a sagittal plane in the epigastrium, along the edge of the left lobe of the liver, and at the level of the aorta (Fig. 1). A detailed description of the technique and sonographic characteristics of gastric antrum content has been previously reported.17
The ultrasonographic assessment consisted of a qualitative and quantitative evaluation of the gastric antrum. The qualitative assessment aimed at the nature of gastric content (empty, fluid, or solid content). Based exclusively on this qualitative assessment of the antrum, and once the presence of solid content was ruled out, patients were classified into 3 grades. Grade 0 antrum was defined as the absence of fluid content in both supine and RLD positions, suggesting a “completely empty” state.18 If fluid content was observed only in the RLD position, but not in the supine position, it was classified as grade 1 antrum, which has been shown to correlate with low-volume states compatible with baseline gastric secretions in fasted patients.19 In contrast, if fluid was observed in both supine and RLD, the antrum was classified as grade 2. In nonpregnant patients, a grade 2 antrum suggests higher than baseline gastric secretion and is observed uncommonly in fasted patients.18,19
The quantitative assessment of the gastric volume was performed by measuring the cross-sectional area (CSA) of the gastric antrum (antral CSA) between peristaltic contractions, using the free-tracing caliper of the ultrasound unit. This free-tracing method is equivalent to the 2-diameter method of area measurement, and it is simpler and highly reproducible (high intrarater and interrater reliability).20 This validated method allows rapid point-of-care assessment at the bedside, which is relevant for clinical practice. Although only 1 measurement was obtained in the supine position, 3 measurements from 3 consecutive images in the RLD position were averaged for gastric volume estimation, as per standard practice in gastric sonography.19,21
The primary outcome was the incidence of antrum grade 2 in our population sample of fasting pregnant women at term. Secondary outcomes included (a) the incidence of antrum grade 0 and 1; (b) the antral CSA measurements in both examination positions; (c) the relationship between antral grades and antral CSA; (d) the association between antral CSA and patient characteristics; (e) an assessment of reliability of antral size measurement determined by both intraobserver variability (the ratio of the difference between the values obtained by each observer, expressed as absolute value, divided by the mean value) and reproducibility of measurements in each subject (the mean [SD] difference between 3 replicate measurements in the RLD position); (f) an estimate of gastric volume based on the antral CSA in the RLD (CSARLD), using an existing model previously validated in nonpregnant adults as follows: volume (mL) = 27.0 + 14.6 × CSARLD (cm2) − 1.28 × age (years).19 In addition, we compared the antral CSA and the estimated gastric volumes with historical data from a previously published study in fasting nonpregnant adult subjects.18
Descriptive statistics were calculated for continuous data using mean and SD if variables presented a normal distribution, otherwise median, interquartile range (IQR), and percentiles 25, 75, and 95 were reported. Percentages were calculated for discrete variables. We calculated the 2-sided confidence intervals (CIs) for the incidence of antrum grade 2 as a single proportion using the method described by Robert Newcombe and the Wilson procedure with a correction for continuity.22 Otherwise, the raw counts were presented for antrum grade 0 and 1. The Shapiro-Wilk test was used to test for normal distribution of continuous variables (P > 0.05). The upper and lower limits of the conservative 95% CIs for antral CSA and estimated gastric volumes were calculated using the binomial-based method while considering the distribution of the variables. Variables that were not normally distributed or ordinal data (e.g., gastric antrum grades) were analyzed using nonparametric statistics (Mann-Whitney U test; Wilcoxon signed-rank test for matched pairs for nonindependent variables such as pairs of CSA measured in supine and RLD positions in the same subject; and Kruskal-Wallis equality-of-populations rank test among grades). Furthermore, antral CSAs were compared in both positions and grades using Bland-Altman analysis, which allows placement of the magnitudes of the differences between the 2 measurements in a clinical context.23 Associations between antral CSA and subject characteristics were tested through Spearman rank correlations. In addition, we performed a multivariable analysis to evaluate the antral CSA (dependent variable) and subject characteristics (independent variables), while assessing for colinearity and interaction among independent variables. A stepwise selection was used for developing a multiple linear regression model considering the coefficients of determination (R2). The distribution of the data on antral size and gastric volume for the purpose of comparison with the historical cohort was visually inspected with distributional diagnostic plots (quantile–quantile and density) and tested with the Kolmogorov-Smirnov test.
The sample size calculation was based on a 3.5% incidence of grade 2 antrum in fasted nonpregnant surgical patients.18 We powered our study to find a 7% incidence of grade 2 antrum with an acceptable error of ±0.05 from the expected proportion, with a 2-tailed test and 5% type I error. A sample size of 100 subjects was required, and we planned to recruit 30% more (130 patients) because of possible losses and protocol violations. The statistical analyses were performed using STATA/IC Statistical Software for Macintosh, Release 13.1 (StataCorp, College Station, TX).
The study recruitment was conducted from October 2013 to May 2014. One hundred twenty-nine women were assessed for eligibility; 18 declined to participate and 6 were excluded (1 twin pregnancy, 2 cases were rescheduled for a later date, and 3 cases because of investigators not being readily available). Finally, 105 women consented to the study and underwent the ultrasonographic examination. The analysis was carried out in 103 women. Two women were excluded from the analysis because of the lack of full data collection in both examination positions (only 1 measurement was obtained in RLD position in both cases and it was not feasible to visualize the antrum in the supine position in 1 case). Five women had a diagnosis of gestational diabetes. All patients underwent uneventful spinal anesthesia. Patient characteristics and quantitative ultrasound results relative to the antrum grade are presented in Tables 1 and 2, respectively.
All subjects complied with current fasting guidelines for solid food. One subject developed an intraoperative episode of vomiting (estimated vomiting volume <100 mL); she had fasted 6.5 hours for solids and 3.5 hours for clear fluids and had a diagnosis of gestational diabetes.
None of the continuous variables followed a normal distribution (Shapiro-Wilk test, all P < 0.05). Subjects fasted for median 3.5 (IQR, 5.5) hours for clear liquids. Although 2 subjects had ingested clear liquids within 2 hours of ultrasonographic examination (a glass of Gatorade 1.5 hours and a glass of water 30 minutes before the examination), both completed at least 2 hours of clear liquids fasting before surgery. Most subjects ingested water as their clear liquid before surgery (72/103, 70%), followed by apple juice (18/103, 17%); a few subjects ingested coffee, tea, Gatorade, or Jello.
No solid gastric contents were observed in any subject. Approximately half the patients had grade 0 antrum (n = 53/103) and half grade 1 antrum (n = 49/103). One subject (1/103 = 0.97%; 95% CI, 0.05%–6.06%) had a grade 2 antrum (distended with clear fluid content in both supine and RLD positions). This subject had not ingested clear liquids for 3 hours before the ultrasonographic assessment. However, she was obese (body mass index = 32.7 kg/m2), very anxious, and had been drinking water “continuously” overnight and in the early morning; the estimated ingested volume was 1.5 L. Because of various reasons, the initiation of the surgery was delayed for 2 hours, but no further ultrasonographic assessments were performed. During her uneventful surgery, there were no vomiting events.
Ninety-five percent of subjects presented with an antral CSA of ≤9.6 cm2 (95% CI, 8.6–10.3). The antral CSA was larger in the RLD position than in the supine position (median [IQR]: 4.5 [3.2] cm2 vs 3.3 cm2 [1.8] cm2; P < 0.0001). In addition, Bland-Altman analysis demonstrated that the antral CSA was similar in both examination positions (RLD and supine) in subjects with a grade 0 antrum (median difference [IQR], 0.3 [1.1] cm2; P = 0.06), but it was usually larger in the RLD position in subjects with a grade 1 antrum (median difference [IQR], 2.8 [2.5] cm2; P < 0.0001). Furthermore, the mean CSA difference (95% CI) between both positions was 0.2 (0.1–0.3) cm2 for grade 0, 1.4 (1.1–1.7) cm2 for grade 1, and 5.9 cm2 for grade 2.
Univariate analysis of associations revealed positive correlations between antral CSA (supine and RLD) and subject characteristics such as age, weight, and body mass index, but not gestational age. However, there were no significant correlations between antral CSA and fasting hours (Table 3). None of the multivariable models of subject characteristics (independent variables) was strongly predictive of CSA (independent variable) (R2 < 0.3 for various models).
The identification of the gastric antrum and CSA measurements in both examination positions was possible in 98% of the scanned patients. The intraobserver variability of the CSA measurements in the RLD was 15% ± 11%, and the measurements were reproducible for the same subject within 0.7 ± 0.5 cm2.
The median estimated gastric volume in our cohort was 48.1 mL (IQR, 45 mL), based on a mathematical model previously validated in nonpregnant adults.19 The 95th percentile of estimated gastric volume was 117 mL (95% CI, 108–127) or 1.5 mL/kg (95% CI, 1.3–1.7). As expected, the estimated gastric volume was larger in subjects with a grade 1 than in those with a grade 0 antrum (P < 0.0001; Table 2).
Compared with the historical control cohort of nonpregnant adults,18 using the Kolmogorov-Smirnov test, our present cohort of fasted pregnant patients presented a similar distribution of antral CSA (median [percentile 25–75], 4.5 cm2 [3.8–5.7] vs 4.5 cm2 [3.2–6.4], respectively; P = 0.09). The estimated gastric volume in the cohort of nonpregnant adults was statistically smaller than our cohort of pregnant patients (median [percentile 25–75], 35 cm2 [20–57] vs 48 cm2 [30–75]; P = 0.01; Figs. 2 and 3).
Our results validate the use of ultrasonography as a bedside tool to determine the gastric content in term pregnant women.12,13 The majority of pregnant women at term had an empty stomach (grade 0 or 1) after following conventional fasting guidelines. Only 1 woman in the cohort presented a gastric volume in excess of what is considered “normal baseline” gastric secretions (grade 2 antrum, estimated gastric volume, 241 mL), possibly due to multiple factors (ingestion of high volumes of fluid overnight, obesity, and anxiety). It could be argued that gastric ultrasound helped screen a subject at risk who had otherwise followed adequate fasting guidelines, although this was not the purpose of the study.
The proportions of grade 0 (51%) and grade 1 (48%) antrum were comparable to those described in the nonpregnant adult population before elective surgery following similar fasting guidelines (43% grade 0, 54% grade 1)18 as well as those described recently in a cohort of fasted severely obese individuals (40% grade 0, 55% grade 1).24 Likewise, the quantitative assessment of the gastric antrum revealed CSA measurements obtained in RLD position that were similar to those obtained in nonpregnant18 and obese adults24 using the same examination technique.
Our results are in keeping with those of 2 studies by Wong et al.,8,9 assessing gastric emptying in fasted pregnant women, which showed a slightly larger antral CSA (mean, 5.18; SD, 2.13 cm2) in obese subjects than nonobese subjects (mean, 4.07; SD, 2.13 cm2). However, these results differ from and do not support the assumptions of a recent study by Bataille et al.13 and by Bouvet et al.21,25 in nonpregnant surgical patients. Both groups of investigators proposed that an antral CSA threshold of 3.2 cm2 in the supine position could discriminate between an “empty stomach” (fasting state) and a “full stomach” associated with a significant aspiration risk. Bataille et al.13 based the cutoff value on a sample of 6 volunteer pregnant women in late pregnancy; no gestational age was reported. Over half of the subjects in our cohort of 103 fasted patients had a supine antral CSA >3.2 cm2, suggesting that the threshold proposed by Bouvet et al. and Bataille et al. cannot be used to categorize patients to “empty” and “full stomach,” and likely grossly overestimates the group of patients at an increased risk of pulmonary aspiration.
Furthermore, our results do not support the cutoff value of 1.51 cm2 proposed by Bataille et al.13 as a maximum value of supine antral CSA observed after overnight fasting. Despite using the same definition and measurement method for antral CSA, only 1 of 103 patients in our fasted cohort fell below this threshold. Use of this threshold would misclassify most “fasted” patients as “nonfasted.” Rather, our results suggest that a value of antral CSA in the RLD of 10.3 cm2 (upper limit of the 95% CI for the 95th percentile) more accurately describes the upper limit of normal findings in the pregnant patient at term.
Multiple clinical studies have demonstrated that residual gastric volume in fasted low-risk patients can be as large as 1.5 mL/kg.26–31 Our results are consistent with those of fasted surgical patients. Ninety-five percent of subjects in our fasted pregnant cohort presented an estimated gastric volume of <1.5 mL/kg, and the actual values of antral CSA measured in the RLD were very similar to those previously reported for nonpregnant adults.18 Moreover, the absolute gastric volumes and volume per kilogram mirror each other. Because these observations are from different studies, any comparisons should be regarded as hypothesis-generating observations, rather than firm conclusions. Future research is warranted to compare both populations in a formal study.
Our study presents some limitations. First, many participants presented with longer fasting intervals than the minimum required by current guidelines, and the findings may not apply to patients who adhere more closely to minimum guidelines before surgery. Although this may limit the generalizability of our results, it strengthens our suggested CSA cutoff values for fasted term pregnant women. Second, the ultrasonographic examinations were performed by 2 anesthesiologists and no formal interrater reliability was attempted. However, both quantitative and qualitative gastric ultrasound have been previously reported to be highly reproducible with low intraobserver and interobserver variability.12,20 Third, currently, there is no mathematical model of estimated gastric volume that has been specifically constructed or validated in pregnant women using a strong gold standard method (such as suctioning during gastroscopic examination). Both mathematical models in current use have been constructed in adult, nonpregnant subjects.19,21 The development of such model specifically for pregnant women may remain a difficult task. The existing models will need to be fitted (or validated) for the pregnant population in future research.
In conclusion, this study provides a sonographic description of the gastric antrum in fasted term pregnant women scheduled for cesarean delivery under neuraxial anesthesia. It confirms the validity of a previously reported semiquantitative classification into a 3-point grading system of the antrum, which resembles the system validated in nonpregnant adults. It also establishes CSA threshold values for fasted term pregnant women. Further research is needed to investigate the feasibility of a predictive model to estimate gastric volumes based on the antrum CSA and patient variables in pregnant women.
The authors thank Kristi Downey, MSc, Perinatal Research Coordinator, and Archana Malavade, MD, Research Assistant, Department of Anesthesia and Pain Management, Mount Sinai Hospital, Toronto, ON, Canada, for their invaluable contribution to all stages of this project.
Name: Cristian Arzola, MD, MSC.
Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.
Attestation: Cristian Arzola has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.
Conflicts of Interest: The author declares no conflicts of interest.
Name: Anahi Perlas, MD, FRCPC.
Contribution: This author helped design the study, conduct the study, and write the manuscript.
Attestation: Anahi Perlas has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.
Conflicts of Interest: Anahi Perlas is an associate editor of the journal Regional Anesthesia and Pain Medicine.
Name: Naveed T. Siddiqui, MD, MSc.
Contribution: This author helped design the study and write the manuscript.
Attestation: Naveed T. Siddiqui has seen the original study data and approved the final manuscript.
Conflicts of Interest: The author declares no conflicts of interest.
Name: Jose C. A. Carvalho, MD, PhD, FANZCA, FRCPC.
Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.
Attestation: Jose C. A. Carvalho has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.
Conflicts of Interest: The author declares no conflicts of interest.
This manuscript was handled by: Cynthia A. Wong, MD.
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