Although uncommon in the elective, surgical population, pulmonary aspiration of gastric content is highly prevalent in certain groups of patients (e.g., up to 38% of trauma patients who require surgery) and can lead to significant morbidity and mortality.1 Mortality after aspiration pneumonia can be as high as 5%2 and it accounts for up to 9% of all anesthesia-related deaths.3,4 One important risk factor for aspiration is the presence of residual gastric fluid at the time of anesthetic induction. However, there is no clinical tool to reliably assess the nature and volume of gastric content at the bedside.
We have recently demonstrated that bedside ultrasonography can provide reliable qualitative and quantitative information about the nature (fluid or solid) and volume of gastric content.5 Antral sonography can differentiate an empty stomach from one with fluid or solid content based on qualitative assessment alone. Furthermore, when the stomach contains fluid, a cross-sectional area (CSA) of the antrum measured in the right lateral decubitus position can predict total gastric fluid volume.5
OBJECTIVES
The aim of this prospective study was to provide a qualitative and quantitative description of the sonographic appearance of the gastric antrum in fasted patients undergoing elective surgery. We propose a grading system of the gastric antrum based exclusively on qualitative findings. We evaluated whether this grading system in patients presenting for surgery correlates with predicted gastric volume based on a formula derived in healthy nonsurgical volunteers. Data obtained in this study can serve as baseline information with which to compare future findings and studies of gastric sonography in higher risk populations.
METHODS
After Institutional Research Ethics Board approval and written informed consent, 200 patients were recruited for prospective descriptive study. Inclusion criteria were patients 18 years of age or older, ASA physical status I to III, and scheduled to undergo elective surgery. Exclusion criteria were preexisting abnormal anatomy of the upper gastrointestinal tract (including previous esophageal or gastric surgery, and hiatus hernia), and current or recent pregnancy (within 3 months). Gastroesophageal reflux disease (GERD) by itself was not an exclusion criterion. All patients followed institutional fasting guidelines (solid intake up to 8 hours before surgery, and clear fluids up to 5 hours before surgery). Demographic data and a complete medical history were taken upon hospital admission.
Before anesthetic induction, a focused gastric examination was performed by a certified sonographer following a previously described standardized scanning protocol.5 A curvilinear array, low-frequency (2- to 5-MHz) transducer and a Philips HD11XE system (Philips Healthcare, Andover, MA) were used. Patients were scanned in the supine position followed by the right lateral decubitus position. The transducer was placed in a sagittal plane in the epigastric region. The gastric antrum and body were scanned by tilting the transducer from right to left to obtain an overall qualitative impression of the gastric cavity and its contents. The antrum is usually best visualized in a parasagittal plane just right of the midline, surrounded by the left lobe and caudate lobe of the liver anteriorly and the head or neck of the pancreas posteriorly. The inferior vena cava lies posterior to the pancreas. The antrum has a characteristic multilayered wall (Fig. 1).
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Figure 1: Note 5 distinct gastric wall layers. In an outward direction, a first thin (echogenic) layer is the air/mucosal interface; a second (hypoechoic) layer is the deep mucosa or muscularis mucosae; a third thin (echogenic) layer corresponds to the submucosa; a fourth thick layer (hypoechoic) is the muscularis propria; and a final thin (hyperechoic) layer corresponds to the serosa.
A qualitative assessment of the gastric antrum was performed by the sonographer. The antrum was judged to be empty if it appeared flat, with anterior and posterior walls juxtaposed. The antrum was judged to contain fluid if it appeared to have an endocavitary lumen with hypoechoic content and distended walls.5 The antrum was judged to contain solid if it was distended with an internal “frosted-glass appearance” characteristic of the early phases after a solid meal, or if it contained small specular images of intermediate echogenicity, similar in appearance to the liver parenchyma. Based exclusively on qualitative assessment of the antrum, patients were classified as follows: grade 0—the antrum appeared empty on both supine and right lateral decubitus positions (Fig. 2); grade 1—gastric fluid was visible on the right lateral decubitus position only, suggesting a small fluid volume (Fig. 3); and grade 2—gastric fluid was observed in the antrum in both supine and right lateral decubitus, suggesting a larger fluid volume (Fig. 4). In addition, 3 still images of the antrum were obtained at rest (between peristaltic contractions) in both patient positions. A CSA of the antrum in the right lateral decubitus position was determined based on a formula using 2 perpendicular diameters, the craniocaudal (CC) and anteroposterior (AP) diameters as previously described6:
Figure 2: The empty antrum (grade 0) in the supine (A) and lateral decubitus (B) positions. The empty antrum appears small and “flat” in both patient positions. It is pointed out by 4 x's corresponding to its 2 perpendicular diameters. L = liver; P = pancreas; PV = portal vein; IVC = inferior vena cava; SMV = superior mesenteric vein.
Figure 3: The antrum with minimal, insignificant amount of fluid (grade 1) in the supine (A) and lateral decubitus (B) positions. Note that a small amount of fluid is detectable only in the right lateral decubitus position. L = liver; P = pancreas; IVC = inferior vena cava.
Figure 4: The antrum with significant fluid content (grade 2) in the supine (A) and right lateral (B) positions. Note that fluid is evident in both positions, but more marked in the right lateral decubitus position. These images correspond to the patient who regurgitated on emergence from anesthesia. L = liver; P = pancreas; IVC = inferior vena cava.
A numerical average of the 3 measurements was recorded. Based on antral CSA, total gastric fluid volume was predicted in each patient, using our previously reported mathematical model as follows5:
Anesthesiologists were unaware of the sonographic findings. Anesthetic and surgical management proceeded according to preexisting plans based on standard clinical practice. All data were gathered prospectively and summarized using percentages and ratios for discrete variables, and mean ± SD for continuous variables.
RESULTS
Two hundred patients were included in this study. Demographics are summarized in Table 1. Ninety-two percent were outpatients and 95% received a general anesthetic. Procedures included orthopedics (67%), urology (19%), minor abdominal (12%), varicose vein stripping (1%), and breast surgery (1%).
Table 1: Demographics
No patients were found to have solid gastric content. As expected for an elective fasted surgical population, most patients (193 of 200 or 96.5%) were classified as grade 0 or 1. The predicted volume for these 2 groups was 0 and 16 ± 36 mL, respectively. The remaining 7 patients (3.5%) were classified as grade 2, presenting a clearly distended gastric antrum and fluid content visible in both lateral and supine positions. The predicted gastric fluid volume in this group of patients was significantly larger than in the other 2 groups (180 ± 83 mL). One patient in the study had an episode of regurgitation of gastric content at the time of emergence from anesthesia. This patient was a 35-year-old man with a body mass index of 25.8 and a history of GERD symptoms, but currently asymptomatic while taking pantoprazole. He underwent an ankle arthroscopy under general anesthesia with endotracheal intubation because of his history of GERD. Upon emergence and after tracheal extubation, he regurgitated approximately 100 mL green bilious fluid. He required repositioning and repeated suctioning of the upper airway. He was followed closely in the recovery room and did not develop clinical or radiographic signs of aspiration pneumonia or pneumonitis. He was discharged home on the same day as planned. His gastric scan had revealed a grade 2 antrum, and the predicted gastric fluid volume was 150 mL.
DISCUSSION
Aspiration pneumonia is a potentially preventable anesthetic-related complication that accounts for up to 9% of all anesthetic-related deaths.3 Three major coexisting mechanisms often contribute to pulmonary aspiration of gastric contents. First, there needs to be some residual gastric fluid for aspiration to occur. The critical gastric fluid volume that by itself increases a patient's aspiration risk is unknown. Several well-designed clinical studies have shown that healthy fasted patients frequently have residual gastric volumes larger than previously thought, up to 1.6 mL/kg without a significant increase in aspiration risk.7–11
Sedation and general anesthesia significantly depress or abolish both major physiologic mechanisms that protect against aspiration (the tone of the lower esophageal sphincter and upper airway protective reflexes).12,13 Therefore, under deep sedation or general anesthesia, the presence of gastric content remains an important patient-related risk factor for perioperative aspiration. The adoption of universal “fasting guidelines” before elective surgery attempts to limit gastric content in the immediate perioperative period and has had an important role in mitigating this risk.14 However, fasting guidelines are not applicable in the urgent or emergent surgical patient, and certain physiologic states (e.g., pregnancy) and medical conditions (e.g., diabetes, trauma, renal, or liver dysfunction) may result in delayed gastric emptying and significant residual gastric volume despite recommended fasting times.15 A bedside tool to reliably assess the nature and volume of gastric content would be very helpful to the clinical anesthesiologist. However, existing methods are invasive, and not readily available or applicable in the immediate perioperative period.16
Ultrasonography has several potential advantages relative to other imaging modalities including portability, noninvasiveness, and cost-effectiveness. Previous studies suggest that ultrasonography can differentiate between fluid and solid gastric contents.17 Bolondi et al.6 imaged the gastric antrum in a cross-sectional view and calculated antral CSA. It has been suggested that antral CSA may serve as a surrogate measure of gastric fluid volume.18,19 Sequential measurements of antral CSA have been used to determine gastric emptying time and have been shown to correlate well with scintigraphic evaluation.20
Our recent data assessed by a blinded sonographer on healthy volunteers after ingesting predetermined volumes of water and a standardized solid meal suggest that ultrasonography of the antrum can reliably distinguish an empty stomach from one with fluid or solid contents and that antral CSA measured in the right lateral decubitus position correlates well with total gastric fluid volume, supporting findings of previous investigators.5,21 Furthermore, using logistic regression, we previously built a mathematical model that allows us to predict total gastric fluid volume based on a single measurement of antral CSA.5
In this prospective, descriptive study of 200 adult fasted surgical patients, we propose a simple 3-point grading system (grades 0, 1, and 2) based exclusively on qualitative sonographic assessment of the gastric antrum. We found that 3.5% of our patient population was classified as grade 2 and had a predicted gastric fluid volume larger than what most anesthesia providers would expect for fasted elective surgical patients. The main advantage of this simple grading system is that although entirely based on a qualitative assessment, and therefore relatively simple to perform, it correlates well with predicted gastric volume. Put simply, if a patient's antrum appears empty in both supine and right lateral positions (grade 0), it is likely truly empty. If it appears only mildly distended in the right lateral decubitus position, but not in the supine position (grade 1), the patient likely has a very small volume of gastric fluid within the limits of what is known to be “safe” (16 ± 36 mL; 0.2 ± 0.5 mL/kg). An antrum that is distended in both supine and right lateral decubitus positions (grade 2) denotes a gastric fluid volume beyond the limits of what is considered safe (180 ± 83 mL; 2.8 ± 1.4 mL/kg) (Table 2). Given the low baseline rate of clinically evident aspiration in the fasted elective surgical population (1:5000), it is not possible to comment on the impact of this larger predicted volume on aspiration risk, but it is beyond the limits of what was previously considered “normal” and “safe.”4,7–11 Because of the small number of patients classified as grade 2, it is not possible to make a meaningful statistical comparison of the rates of diabetes, GERD, or other demographic characteristics among groups.
Table 2: Results
This study has several limitations. We are reporting preliminary work on a new proposed diagnostic tool or “biomarker” that is not yet fully developed or validated. The quantitative aspects of our previously built mathematical model need to be validated by an independent method to measure gastric volume, such as gastroscopy and suctioning. Our investigations have focused mainly on patients of normal body habitus. Although the body mass indices of patients in the present study ranged from 17.0 to 42.5 kg/m2, most subjects were in the normal to mildly obese categories (Table 2). Our findings therefore cannot be easily extrapolated to morbidly obese patients, in whom sonographic imaging may conceivably be more challenging. Our results cannot be applied to patients with hiatus hernia, in whom the change in gastric positioning and architecture may alter the relationship between total gastric fluid volume and antral size. The diagnostic accuracy and reproducibility of the present tool (both intraobserver and interobserver variabilities) remain to be determined.22 Furthermore, this biomarker also needs to be characterized by using a Bayesian approach, establishing likelihood ratios, and determining which kind of patient population may benefit most from this tool, based on estimates of pretest probability.23
CONCLUSION
This study provides a prospective description of the sonographic assessment of the gastric antrum in an adult fasted surgical population. We propose a 3-point grading system based exclusively on qualitative sonographic assessment of the gastric antrum that correlates well with predicted gastric volume and could help differentiate patients at higher risk of aspiration. Further research is needed to validate the existing mathematical model, and to establish the diagnostic accuracy, reproducibility, and strength of this new tool as a predictor of aspiration risk.
DISCLOSURES
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, approved the final manuscript, and is the author responsible for archiving the study files.
Name: Liisa Davis, BSc, RDMS.
Contribution: This author helped design the study.
Attestation: Liisa Davis has seen the original study data and approved the final manuscript.
Name: Masood Khan, MD.
Contribution: This author helped conduct the study.
Attestation: Masood Khan has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.
Name: Nicholas Mitsakakis, MSc, PhD.
Contribution: This author helped analyze the data.
Attestation: Nicholas Mitsakakis has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.
Name: Vincent W. S. Chan, MD, FRCPC.
Contribution: This author helped design the study and write the manuscript.
Attestation: Vincent W. S. Chan approved the final manuscript.
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