Ultrasound Assessment of Gastric Content in the Obese Patient

Pulmonary aspiration of gastric contents is one of the most feared anesthesia-related complications, especially as it was reported to be one of the major causes of mortality related to general anesthesia.1 Apart from very typical clinical circumstances (i.e., digestive tract obstruction), the recognition of patients at risk of regurgitation and aspiration may be difficult. For example, the risk of pulmonary aspiration in the traumatized patient may remain unclear several hours after the initial accident. Even as time since the last meal increases, multiple factors still confound the status of a patient's stomach contents. One must also consider the volume and caloric value of the last ingested meal, administered medications, and the level of pain. In such clinical situations, noninvasive preoperative assessment of the gastric contents and volume may help the anesthesiologist to define the appropriate anesthetic technique and mode of anesthesia induction to minimize the risk of perioperative pulmonary aspiration of gastric contents. In this issue of Anesthesia & Analgesia, Van de Putte and Perlas2 assessed the feasibility of an ultrasound examination of the antrum for the qualitative assessment of gastric contents in severely (body mass index >35 kg/m2) obese patients. The use of ultrasound is flourishing in anesthesia, most notably for regional access and the guidance of central or peripheral venous and arterial access. This trend has increased the availability of small, portable, high-resolution ultrasound devices in the operating theaters. Direct imaging of the stomach contents by positioning the ultrasound probe under the left costal margin and scanning up to the xiphisternum may be difficult because of excessive gastrointestinal air, an empty stomach, or a gastric fundus located behind the costal margin.3 Fortunately, visualization of the gastric antrum is easier as it has a relatively constant anatomic location. Bolondi et al.4,5 successfully imaged the gastric antrum in a cross-sectional view using the left lobe of the liver and the aorta as internal landmarks and first reported that the decrease in antral cross-sectional area after ingestion of a test meal demonstrated a gastric emptying time that highly correlated with scintigraphic evaluation. These data suggested that a single standardized measurement of the antral cross-sectional area was able to discriminate between a fasting and nonfasting stomach.6 This hypothesis was confirmed in a subsequent clinical study, reporting that the measurement of antral area allowed for the diagnosis of a "risk stomach," defined as the presence of solid particles or a volume of gastric contents >0.8 mL·kg−1. This approach achieved a sensitivity of 91% and a negative predictive value of 94% using a cutoff value of 340 mm2.7 Complementary to this semiquantitative method, Cubillos et al.8 described the sonographic appearance of the gastric antrum in the fasted state and following ingestion of clear fluid, milk, and solid meal. They then built a score based on the qualitative sonographic assessment of the antrum in the supine and right lateral decubitus position.9 The main clinical benefit of this scoring system lies in its simplicity, especially for the diagnosis of significant liquid contents (grade 2, defined by distended antrum with fluid visible in both supine and right lateral decubitus). Such a diagnosis alerts the anesthesiologist to the potentially increased perioperative risk of regurgitation and pulmonary aspiration. However, the use of this ultrasonographic tool in clinical practice requires, among other things, that it can be easily applied for most patients, and especially for those patients for whom the risk of aspiration is increased. Obesity has been reported to be a predisposing factor for pulmonary aspiration of gastric contents during general anesthesia.10,11 Unfortunately, it currently affects a large and increasing part of the world population. In 2011 to 2012, the prevalence of obesity was 16.9% among 2- to 19-year olds and 34.9% among adults in the United States.12 Internationally, the worldwide prevalence of obesity has nearly doubled between 1980 and 2008 (World Health Organization, Global Health Observatory, accessed May 31, 2014). One could suppose that obesity may impair ultrasound imaging of the antrum, because of both the reduced echogenicity of the abdomen and the change in the internal landmarks for the location of the antrum as a result of excess adipose tissue. Therefore, it is relevant to focus on the feasibility of the quantitative ultrasound assessment of gastric contents in obese patients. Until now, the feasibility of the ultrasonographic measurement of the antral area has been assessed in nonobese patients only, with a complete view of the antrum provided for >98% of the patients.7 Van de Putte and Perlas2 report that the antrum can be visualized in >90% of obese patients (right lateral decubitus or supine) while also measuring the mean depth of the antrum and the mean thickness of the gastric wall. Thus, they demonstrate that the ultrasonographic preoperative assessment of gastric contents is feasible in severely obese patients (body mass index range 35.1–68.7 kg/m2) despite the predictable difficulties, such as poor transmission of the signal because of significant adipose tissue. Positioning for the ultrasonographic examination is important. Understandably, when the supine patient turns to the right lateral position, gastric contents tend to fall into the antrum, making it more easily visible when performing ultrasound imaging of the antral section. Previous studies also reported that the relationship between the antral area and gastric liquid content was more accurate when the patient was positioned in the right lateral decubitus rather than in the supine position. Such positioning and repeated measurements allowed for the construction of a linear model describing the relationship between liquid gastric volume, antral area, and patient age.13,14 However, it was not obvious that the right lateral decubitus position would allow for easier viewing of the antral region in obese patients. This is especially true when considering that abdominal anatomical landmarks are more difficult to detect in such patients and that such superficial landmarks move significantly when the patient is turned laterally. The confirmation of feasibility for the grading score described by Perlas et al.9 in obese patients is therefore one of the other major findings from the study by Van de Putte and Perlas.2 Nevertheless, in current clinical practice, positioning the patient in right lateral decubitus may be difficult or even impossible. For example, the traumatized patient may be unable to rotate, and the status of the stomach often remains unclear. Further studies should therefore focus on improving the analysis of the antral area measured in supine position for the preoperative assessment of gastric contents, through the use of either a semiquantitative method (critical threshold for antral area) or, if that approach is not possible, a qualitative method. The similar distribution of antral grades in obese patients to those previously reported in nonobese patients and the absence of thick fluid or solid gastric contents is not surprising, as it is well known that obesity in itself does not delay gastric emptying. However, 21.7% of the included patients suffered from gastroesophageal reflux, without any correlation with antral grade. In fact, many comorbidities (such as gastroesophageal reflux or difficult airway anatomy) associated with obesity are also risk factors for pulmonary aspiration of gastric contents. The combination of such comorbidities, in addition to other anesthetic factors (i.e., insufflation of air into the stomach during facemask ventilation),15 with significant gastric contents may lead to an even more pronounced risk of gastric regurgitation. As such, it should be stressed that the volume of gastric contents is only one of the physiologic factors involved in the occurrence of pulmonary aspiration during general anesthesia. Ultrasonographic preoperative assessment of gastric contents may be a useful, although not sufficient, part of the aspiration risk assessment for each patient. To conclude, this work by Van de Putte and Perlas2 contributes to the validation of the preoperative ultrasonographic assessment of gastric-content status in clinical practice. Furthermore, it opens up opportunities for additional clinical research to improve the understanding to ultimately reduce the incidence of pulmonary aspiration of gastric contents. DISCLOSURES Name: Lionel Bouvet, MD, PhD. Contribution: This author helped write the manuscript. Attestation: Lionel Bouvet approved the final manuscript. Name: Dominique Chassard, MD, PhD. Contribution: This author helped write the manuscript. Attestation: Dominique Chassard approved the final manuscript. This manuscript was handled by: Maxime Cannesson, MD, PhD. ACKNOWLEDGEMENTS The authors thank Brenton Alexander, MD, Department of Anesthesiology and Perioperative Care, UC Irvine, for assistance in the preparation of the manuscript.