Gastric ultrasonography (GUS) is as a noninvasive tool to examine stomach contents at the bedside as a determinant of pulmonary aspiration risk.1–8 It provides qualitative information about stomach contents (empty, clear fluid, thick fluid/solid).1,2,4,5 In addition, the volume of gastric fluid can be estimated based on a measurement of the cross-sectional area (CSA) of the antrum,3,6 and a recently proposed 3-point grading system is an easy screening tool to discriminate between low and high gastric volumes.4,6 However, previous research has focused on individuals with normal or close-to-normal body habitus, and the feasibility of GUS in patients with severe obesity (body mass index [BMI] ≥35 kg/m2) has yet to be established.
The main goal of this prospective cohort study was to establish the feasibility of GUS assessment in severely obese patients (BMI ≥35 kg/m2) who have fasted overnight before elective surgery.9 We defined feasibility (our primary outcome) as the ability to identify a full cross section of the gastric antrum in at least 80% of subjects in the right lateral decubitus (RLD) position. Secondary outcome measures included the proportion of subjects in whom it was possible to use a 3-point grading system (antral grades 0–2) and the image-capturing time. In addition, we compared antral CSA and estimated gastric volume with historical data from a previously published study in nonobese subjects.4
After approval by the Institutional Ethics Board and obtaining written informed patient consent, 60 patients were invited to participate. Inclusion criteria were 18 years of age and older, ASA physical status I–III, BMI ≥35 kg/m2, and elective surgery. Exclusion criteria were pregnancy and preexisting abnormal anatomy of the upper intestinal tract (previous lower esophageal or gastric surgery, hiatal hernia, gastric cancer). The presence of gastroesophageal reflux was not an exclusion criterion. As per standard institutional practice, all patients fasted for both fluids and solid food 8 hours before surgery. A complete medical history and demographic data were obtained.
A previously described standardized scanning protocol was followed.1 A curved low-frequency (2–5 MHz) probe and a Philips HD11XE ultrasound system (Philips Healthcare, Andover, MA) were used. All patients were first scanned in the supine followed by the RLD position. A sagittal scanning plane in the epigastrium was used to locate the antrum using the left lobe of the liver anteriorly and the pancreas posteriorly as anatomical landmarks. The aorta or inferior vena cava served as additional reference points.1,2,4–6,10,11 All scans were performed before induction of general anesthesia by a single clinical anesthesiologist with 8 years’ experience using sonography for other clinical applications and 2 years’ experience in GUS (>200 previous gastric scans).
The visibility of the antrum was evaluated in a binary manner (visible or not) in both supine and RLD positions. If the antrum was visible, it was judged to be empty if it appeared flat with juxtaposed anterior and posterior walls. If the antrum was distended, with thin walls and hypoechoic content, it was judged to contain fluid. It was judged to contain solid food or thick fluid if it appeared distended with a content of mixed echogenicity.1 The antrum was classified as grade 0 when empty in both supine and RLD positions, suggesting an empty stomach (Fig. 1, A and B). A grade 1 antrum was defined as the presence of fluid only apparent in the RLD, suggesting a low fluid volume (Fig. 2, A and B). In a grade 2 antrum, fluid is apparent in both supine and the RLD positions, suggesting a higher fluid volume (Fig. 3, A and B).4,6 In addition, 3 still images of the antrum were obtained in each patient position with the antrum at rest between peristaltic contractions. The image acquisition time was documented. The antral CSA was measured for every subject with a visible antrum in the RLD using the free tracing tool of the ultrasound equipment and including the whole thickness of the antral wall. Total gastric volume was estimated only for subjects with a BMI <40 kg/m2 using a previously reported mathematical model (Volume [mL] = 27.0 + 14.6 × Right-lat CSA − 1.28 × age).6 This model has been validated only for subjects with a BMI <40 kg/m2 (R2 = 0.731). The thickness of the antral wall and the depth of the anterior wall of the antrum were also measured. Antral CSA and baseline gastric volume were compared with data from an earlier study by one of the authors in nonobese surgical subjects.4 From this original study (N = 200), only patients with a BMI <35 were included as a comparative control cohort (N = 179).
To demonstrate a minimum feasibility of 80%, assuming a conventional true feasibility of 92% in the general population (based on our clinical experience), we used the z test for binominal proportions to estimate a sample size of N = 55 (nominal power = 0.8, α = 0.05). Sixty subjects were recruited to account for possible patient exclusions or missing data. The assumption of normal distribution was checked with the Shapiro-Wilk test. Categorical data (such as gender, grade classification, history of gastroesophageal reflux, diabetes, and obstructive sleep apnea [OSA]) are expressed as incidence or ratios and analyzed with the Fisher exact test. Continuous variables (such as age, CSA, and volume) are expressed as mean ± standard deviation (SD). Means were compared using 1-way analysis of variance test. Weight and BMI were compared using the Kruskal-Wallis nonparametric test. Differences were considered significant if P < 0.05.
The distribution of the data on antral size and gastric volume for the purpose of comparison with the historical cohort was visually inspected with Q-Q plots and tested with the Kolmogorov-Smirnov test. Statistical analysis was performed using SAS 9.1 for Windows (SAS Institute Inc., Cary, NC).
Sixty patients with BMI from 35.1 to 68.7 kg/m2 were enrolled in this study. Demographic data are summarized in Table 1. Preoperative comorbidities included diabetes (30%), gastroesophageal reflux (21.7%), and OSA (11.7%). The surgical procedures were orthopedics (35%); bariatric surgery (31.7%); other abdominal (8.3%), gynecologic (8.3%), and urologic (5%) interventions; endoscopic procedures (6.7%); and other surgery (5%). Following the World Health Organization International Classification, 32 patients (53.3%) were class II (BMI >35–39.9 kg/m2), 22 patients (36.7%) were class III (BMI >40–49.9 kg/m2), and 6 patients (10%) were super-obese (>50 kg/m2) (Table 2).a
GUS was feasible in 95% of subjects in the RLD (95% CI, 0.86–0.99) and in 90% of subjects in the supine position (Table 2). The antrum could not be imaged in either position in 1 subject (1.7%). The antrum was graded in 53 patients (88.3%) (95% CI, 0.77–0.95). Twenty-one subjects (39.6%) presented a grade 0 antrum, 29 subjects (54.7%) presented a grade 1 antrum, and 3 patients (5.7%) presented a grade 2 antrum (Table 3). No patients had thick fluid or solid gastric contents. The thickness of the gastric wall was 4.8 ± 1.3 mm (N = 59). The depth of the antrum was 7.1 ± 1.4 cm in the supine (N = 55) and 7.2 ± 1.6 cm in the RLD position (N = 57). The median image acquisition time was 3.5 minutes. Image acquisition took <5 minutes in 76% (N = 45) of patients (95% CI, 0.63–0.86).
As expected, increasing antral grade was associated with larger CSA and higher predicted gastric volume (24 ± 16 mL, 69 ± 19 mL, and 165 ± 35 mL for grades 0, 1, and 2, respectively). There was no correlation between antral grade and age, gender, weight, BMI, or incidence of diabetes, gastroesophageal reflux, and OSA (Table 1).
Compared with the historical control cohort of nonobese subjects,4 this current cohort of severely obese individuals presented larger antral CSA and larger baseline gastric volumes (P < 0.001). However, gastric volume per unit of weight was similar in both cohorts (P = 0.141), and all values were within previously reported ranges in the general population (Table 4). The data did not follow a normal distribution as visualized by the Q-Q plot (Fig. 4) and were analyzed with the Kolmogorov-Smirnov test.
This prospective study suggests that GUS for the perioperative evaluation of gastric contents is feasible in the severely obese patient. Previous studies of bedside GUS in both the anesthesia and intensive care literature have focused on subjects with a normal to mildly obese body habitus (BMI 17–42.5 kg/m2).1–6,11–15 The feasibility of GUS has not been systematically documented in the severely obese. Only 1 previous small study on 10 obese pregnant patients (prepregnancy BMI of 42 ± 9 kg/m2) reported successful scanning in that sample.15 In addition, 2 mathematical models have been described to calculate total gastric volume based on antral CSA in subjects with BMI <31 kg/m2 and BMI <40 kg/m2, respectively.3,6 This study was performed in Belgium where the prevalence of severe obesity is lower than in North America. To complete the study within a reasonable timeframe, we decided to choose a BMI ≥35 kg/m2 as a cutoff for the inclusion criteria, which also coincides with the lower limit of class II obesity following the World Health Organization classification.
The incidence of OSA in our study population (11.7%) is less than quoted in studies of severely obese males (up to 55%).16 However, patients scheduled for bariatric surgery are not routinely tested for OSA at our institution. For the purpose of this study, OSA was noted if there was a previously known diagnosis. The true incidence of OSA may therefore be higher than reported.
Following a standardized scanning protocol,1 the gastric antrum was successfully imaged in 95% of subjects in the RLD and 90% of subjects in the supine position. These findings are comparable with those from previous studies in nonobese patients. Bouvet et al.2,3 imaged the antrum in 98% of subjects in the semisitting position whereas Perlas et al.4 reported a 100% success rate in the RLD position. This latter position is particularly useful because a larger proportion of gastric contents move preferentially toward the more dependent areas of the stomach, i.e., the antrum in the RLD.1 In our sample of severely obese individuals, antral scanning was suboptimal in 7 patients (11.7%). Of these, the antrum was not visible in the RLD in 2 patients after locating it without difficulty in the supine position. In 1 of these 2 cases, the surgeon reported a thick fat omentum and a lean small stomach intraoperatively. In our patient sample, the antrum was relatively deep (anterior wall at 7.2 ± 1.6 cm from the skin), which may have made imaging more challenging. We did not find any solid or thick fluid content in our patient sample. This is expected in a fasted population (mean fasting time 12 ± 3 hours). Several reports suggest that one can determine if the gastric content is nil, clear fluid, or thick fluid/solid based on qualitative characteristics of the substance in the gastric lumen.1,5 However, the minimum volume that gastric sonography would detect with a high degree of sensitivity is still unknown, and it is possible that a very small amount of fluid or solid may be undetectable. It has been previously shown that scanning in either the right lateral or the semisitting position helps increase the sensitivity of this test to detect small volumes compared with the supine position since a larger proportion of gastric content moves preferentially to the more dependent antrum.1,17,18
A 3-point grading system (grades 0-1-2) based on qualitative antral sonography has been shown to correlate with gastric fluid volume in nonobese individuals.3,6 A grade 0 antrum corresponds to an empty stomach. It has been previously shown that a grade 1 antrum corresponds with a gastric volume of <100 mL 75% of the time and is commonly seen in fasted subjects.3,6 Conversely, a grade 2 antrum corresponded with volumes >100 mL 75% of the time in nonobese subjects and is uncommon in fasted subjects.3,6 The distribution of antral grade in this cohort of fasted severely obese individuals (39.6% grade 0, 54.7% grade 1, and 5.7% grade 2) was comparable with that of a previous cohort of nonobese individuals (43% grade 0, 53.5% grade 1, and 3.5% grade 2).4 Our current cohort presented significantly larger antrums at baseline with significantly larger baseline gastric volumes (Table 4). However, the volume per unit of weight was no different between the 2 cohorts and the volume levels were within previously reported ranges in the general population.19–23
In a previous study on pregnant patients, Wong et al.24,25 reported a slightly larger fasting CSA in obese (5.2 ± 2.1 cm2) versus nonobese patients (4 ± 2.5 cm2). The mean antral CSAs per grade in our current study were also modestly higher than the values reported in a previous study of nonobese patients (4.8 ± 1.3 cm2 vs 3.6 ± 1 cm2 for grade 0, 7.0 ± 1.4 cm2 vs 5.6 ± 1.4 cm2 for grade 1, and 12.1 ± 2.5 cm2 vs 11.6 ± 3.2 cm2 for grade 2).4 However, these modest differences are unlikely to be clinically significant and may be explained by other factors, including minor technical differences among studies and investigators. The thickness of the gastric wall in our patient cohort was 4.8 ± 1.3 mm, which is consistent with that of nonobese subjects (3–5 mm).26–28 Similarly, a previous endoscopic ultrasound study reported no correlation between gastric wall thickness and BMI.29
As expected, increasing antral grade was associated with increasing antral CSA and gastric volume (P < 0.0001) (Table 3). The reported volumes were estimated on the subset of patients with a BMI <40 kg/m2 using a validated mathematical model.6 These observations are in line with previous studies in nonobese subjects.19–23 Although the minimum threshold of gastric volume that increases aspiration risk is controversial, it has been well established that healthy fasted individuals with low aspiration risk commonly present gastric volumes of up to about 1.5 mL/kg (or 100 mL in the average-weight adult). Using aspiration of gastric fluid through a nasogastric tube immediately after induction, 5 studies reported upper limits of residual gastric fluid between 75 and 130 mL (total N = 802).19–23 In a separate study, the upper limit of normal baseline gastric volume in fasted patients was 103 mL by nasogastric aspiration and 163 mL by a marker dilution method (N = 252).30 Another study using magnetic resonance imaging to study gastric emptying in 20 volunteers reported an upper gastric volume of 95 mL at baseline.31
Limitations of this study include a single operator. A recent cohort study describing learning curves in patients with BMI of 25 ± 3 kg/m2 concluded that a mean of 33 examinations is required to achieve a 95% success rate with qualitative assessment of gastric content.32 Even though the operator in our study had substantial experience with GUS, it is still unknown how many examinations are required to obtain similar levels of competence for gastric volume calculation or for qualitative assessment in severely obese patients. In addition, this is a descriptive observational study of a single-patient cohort, and the results have to be evaluated in the context in which the study was made.
Further research is warranted to fully determine the best way to apply GUS to severely obese subjects. In particular, a mathematical model that allows gastric volume assessment in patients with a BMI ≥40 kg/m2 has yet to be described. The application of 3-dimensional ultrasound could be another promising method to measure gastric volume accurately, but data remain preliminary.33
This prospective cohort study of 60 patients suggests that gastric ultrasonography assessment is feasible in fasted severely obese patients (95% CI, 0.86–0.99). Our data also suggest that absolute antral size and baseline gastric volume are larger in severely obese individuals.
Name: Peter Van de Putte, MD.
Contribution: This author helped design and conduct the study, collect the data, and prepare the manuscript. He has approved the final manuscript.
Attestation: Peter Van de Putte attests to the integrity of the original data and the analysis reported in the manuscript. He is the archival author.
Conflicts of Interest: The author has no conflicts of interest to declare.
Name: Anahi Perlas, MD, FRCPC.
Contribution: This author helped design the study, analyze the data, and prepare the manuscript. She has approved the final manuscript.
Attestation: Anahi Perlas attests to the integrity of the original data and the analysis reported in the manuscript.
Conflicts of Interest: Anahi Perlas received support for academic time from a University of Toronto, Department of Anesthesia Research Merit Award 2011–2013.
This manuscript was handled by: Maxime Cannesson, MD, PhD.
The authors thank C. Bertrand for her much appreciated help with the statistical analysis.
a Available at: http://apps.who.int/bmi/index.jsp?introPage=intro_3.html. Accessed October 31, 2013.
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