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Aspiration of gastric content continues to be an anesthetic complication with significant morbidity and mortality, as highlighted by NAP4 of the Royal College of Anaesthetists . Moreover, in a more recent closed claims analysis of perioperative pulmonary aspiration from the United States, mortality reached 57 % and permanent severe injury 14 %, with almost half of the cases occurring in non-emergency surgical settings despite identifiable risk factors . Point-of-care gastric ultrasound has emerged as a key tool for assessing aspiration risk, particularly in patients with uncertain fasting status or when standard fasting guidelines cannot be followed, such as in urgent or emergent surgical procedures ^,. Nowadays, gastric ultrasound is recognized as a reliable and accurate tool to assess gastric content. More recently, the widespread use of GLP-1 RAs and their associated delayed gastric emptying has heightened interest in the perioperative use of gastric ultrasound.

The evolution of gastric ultrasound: The first description of gastric ultrasound in perioperative care to assess gastric content dates back to 1992, when its use was described in pregnant patients . Perlas et al. later described how the ultrasonographic assessment of the gastric antrum—particularly in the right-lateral decubitus position (RLD)— can provide qualitative information about gastric content and its nature . The same research group characterized the antral content qualitatively (empty, clear fluid or solid) and further validated a mathematical model correlating the cross-sectional area (CSA) of the antrum in the RLD position with gastric volume in the presence of clear fluid. This model allows quantitative assessment of the risk of aspiration in the context of clear fluid when such volume exceeded basal secretions (>1.5 mL/kg) .

Since then, further studies have supported the use of gastric ultrasound as a reliable and accurate modality, with validation and expanded use across special populations such as obese, pregnant, and pediatric patients. The impact of gastric ultrasound on perioperative decision-making and airway management was demonstrated by Alakkad et al., who reported that 71 % of anesthesiologists modified their anesthetic plan– as adjusting the timing of surgery, changing the type of anesthesia, or even canceling the procedure– after gastric ultrasound findings were disclosed in patients with unclear fasting status .

In recent years, the widespread use of glucagon-like peptide-1 receptor agonists (GLP-1 RAs) and its association with delayed gastric emptying and increased risk of aspiration, has led to a growing interest in gastric ultrasound ^,,. Several societies have published guidance and recommendations for the perioperative management of patients on GLP-1 RAs management and acknowledge gastric ultrasound as a clinician-directed adjunct for risk stratification when fasting is uncertain or symptoms suggest delayed emptying ^,,. Additionally, the European Society of Anesthesiology and Intensive Care (ESAIC) focuses on fasting intervals but does not preclude clinician-directed use of gastric ultrasound in the pediatric population when fasting guidelines have not been applied or before emergent or urgent procedures . An example of how relevant gastric ultrasound has become for anesthesiologists in the recent years is its incorporation as one of the main POCUS applications to be evaluated for the American Society of Anesthesiologists (ASA) POCUS Certificate of Completion . Moreover, this year, the American Society of Regional Anesthesia and Pain Medicine (ASRA Pain Medicine) published a comprehensive expert practice recommendations for gastric-point-of-care ultrasound to assess aspiration risk in medically complex patients .

As gastric ultrasound becomes increasingly recognized in guidelines and certification pathways, its routine adoption by anesthesiologists will be key to advancing personalized perioperative care.

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The I-AIM framework standardizes the approach to the different POCUS applications outlining why it should be performed, how to properly obtain images, and integrate the findings into clinical care. This simple, thoughtful, and didactic framework makes both teaching and the clinical application structured and reproducible .

• a.

  Indication: The routine performance of gastric ultrasound on all patients coming for surgery is not recommended, as it is not cost-effective and may lead to delays in care. However, gastric ultrasound can assist in guiding the clinical management in patients who don’t follow the fasting instructions, with an unclear fasting history (emergent/urgent surgeries, cognitive dysfunction, language barriers, children) or with conditions associated with delayed gastric emptying (pregnant patients during labor, known gastroparesis, critical illness, patients on GLP-1 RAs) ^,,,,.

The use of gastric ultrasound is not without limitations and its use in patients with altered gastric anatomy, such as hiatal hernia or prior gastric surgery has not yet been validated and is not currently recommended ^,. A recent study by Wrobel et al. proposed a novel adjusted formula for patients with sleeve gastrectomy and compared it to the more established Perlas model. Although their new model reported a mean error of <1 mL vs 9.54 mL with the Perlas model, the authors conclude that the model described by Perlas might be preferable for clinical use in patients with sleeve gastrectomy given its simplicity for bedside application . Further studies need to be conducted to confirm these findings and provide more robust evidence and recommendations.

• b.

  Acquisition: For gastric ultrasound, patients should be scanned in both the supine and RLD positions, with the RLD being particularly important since gastric contents preferentially shift into the antrum– the most dependent region of the stomach in this position– thereby increasing sensitivity for detecting content, and it is the validated position to quantify gastric volume in the presence of clear fluids ^,, ( Fig. 1 ). A low-frequency, curvilinear transducer (2–5 MHz) is indicated in patients over 40 kg, with an approximate depth of 7 cm; however, this varies according to the patient’s body habitus. A high-frequency linear transducer (12–15 MHz) can be used in children or patients under 40 kg ^,,. Initially, the transducer is positioned in a sagittal plane in the subxiphoid region with the marker pointing towards the head. With gentle sliding movements from right to left, rotation and tilting, the gastric antrum will be identified in close proximity to the left hepatic lobe, anterior to the pancreas and at the level of the aorta ^,,,. An empty antrum typically appears as a “bull’s eye” or collapsed, flat structure with a characteristic five-layered wall, showing an alternating echogenicity pattern (4–6 mm thick). From deep to superficial, these layers include mucosa–air interface (thin, hyperechoic), muscularis mucosa (hypoechoic), submucosa (hyperechoic), muscularis propria (thick, hypoechoic), and serosa (thin, hyperechoic). This distinctive appearance is a key feature that differentiates the antrum from adjacent structures ^,. The gastric wall is better visualized in an empty antrum with a high-frequency linear transducer ^,. ( Fig. 2 ). As the stomach becomes distended with content, its wall thins and the normal layered architecture is less distinct; however, the hypoechoic muscularis propria and the hyperechoic serosa can still usually be identified.

  

  Probe positioning and antral landmarks in supine and right-lateral decubitus positions. Reproduced with permission of Giron Arango Medicine Professional Corporation.

  

  Gastric wall and its layers in ultrasound. From superficial to deep: 1. Serosa (thin, hyperechoic), 2. Muscularis propria (thick, hypoechoic), 3. Submucosa (hyperechoic), 4. Muscularis mucosa (hypoechoic) and 5. Mucosa-air interface (thin, hyperechoic). Reproduced with permission of Giron Arango Medicine Professional Corporation.

Localization of the gastric antrum is possible in more than 90 % and inconclusive or indeterminate exams have been reported in only 2–5 % of cases ^,,. In patients who cannot be moved into RLD position, the semi-sitting position is an alternative, although not equivalent .

• c.

  Interpretation: After identifying all relevant anatomical structures, the type of content can be established qualitatively . When the gastric antrum is empty, its walls collapse onto themselves (bull’s eye or flat shape) ^,,,. In an empty antrum, the multilayered appearance of the wall is clearly identifiable with a characteristic thick, hypoechoic muscularis propria. In the presence of clear fluids or basal secretions, the antral wall looks thinner, and the content is anechoic (black). The presence of bubbles of air can be identified in the presence of clear fluid, appearing as hyperechoic and mobile dots inside a dark, anechoic content, giving a “starry night” appearance ^,,,. Thick liquids or non-clear fluids like dairy products create homogeneous hyperechoic content inside the antrum. The early ingestion of solid food, mixed with swallowed air, creates a “frosted-glass” pattern due to the combination of air and solid along the anterior wall of the antrum, obscuring the posterior wall and deeper structures. Once the air dissipates, in the late stages, the solid content appears as a heterogeneous and particulate content with different echogenicity within a clearly distended antrum with a thin wall ^,,,, ( Fig. 3 ).

  

  Qualitative appearances of the gastric antrum: A. Empty (“bull’s eye”), B. Clear fluid (“starry night”), C. Late-Stage Solids and D. Early-Stage Solids with “Frosted-Glass” Appearance. Labels: A: Antrum Ao: Aorta, L: Liver, P: Pancreas, SMA: Superior mesenteric artery. Reproduced with permission of Giron Arango Medicine Professional Corporation.

Moreover, the antrum can be classified in a qualitative manner as: grade 0 when the antrum is empty in both supine and RLD; grade 1 when clear fluid is observed in RLD but not in the supine position, suggesting a volume compatible with basal gastric secretions (<1.5 mL/kg); and grade 2 when clear fluids are visible in both supine and RLD position, suggesting a volume >1.5 mL/kg. The main advantage of this qualitative classification is that it correlates with previously validated formulas and the estimated gastric volume ^,.

A quantitative assessment is required in the presence of clear fluids to differentiate basal gastric secretions <1.5 mL/kg, and a low risk of aspiration, from a higher volume >1.5 mL/kg that exceeds what is expected in a fasting patient and therefore, should be considered a high risk of aspiration. In the presence of an empty antrum, or in the presence of solids or thick fluids, only qualitative assessment is required to guide clinical management.

The antral CSA has a linear correlation with the gastric volume and should always be measured at the level of the aorta in the RLD position. Measurements at the level of the inferior vena cava (IVC) might underestimate the volume since at this level the structure visualized will likely be the gastroduodenal junction or pylorus, rather than the antrum. The image should be obtained between peristaltic contractions, and the free-tracing tool of the ultrasound machine can be used to measure the CSA including the full thickness of the wall (from serosa to serosa) . This method, as described by Kruisselbrink et al., is a highly reliable method and comparable to the traditional two-diameter method . Three measurements should be taken, and the average allows calculation of the volume according to the most established formula described by Perlas et al.: Volume (mL) = 27.0 + (14.6 × RLD CSA)– (1.28 × Age) ^,,,. To facilitate its clinical applicability, the authors included a table in their original paper correlating age in decades with the antral CSA in RLD position ( Table 1 ). This mathematical model has been reported to be highly accurate and reliable and has been validated in morbidly obese patients , but it is not yet applicable in the pediatric and pregnant population ^,,.

Table 1

Cross-sectional area of the antrum and estimated gastric volume by age.

Age-by-decade reference of right-lateral decubitus antral cross-sectional area and estimated gastric volume for clear fluids (Table adapted with authorization from Perlas et al., 2013; Anesth Analg. 116:357–63).

Recently, a meta-analysis of over 1200 patients suggested that a cutoff value of 10 cm ² of antral CSA in the RLD position should be considered a reasonable upper limit in the presence of clear fluids since this number is the 95th percentile of values in the fasting surgical population .

• d.

  Medical-Decision Making: Based on gastric ultrasound findings, anesthetic plans may be adapted to minimize the risk of aspiration of gastric content. An empty stomach (grade 0) or low gastric volumes <1.5 mL/kg (grade 1), suggest a low risk of aspiration, allowing an anesthetic management similar to that of a fasting patient. On the other hand, the confirmation of a full stomach, such as the presence of solid or thick fluid content or clear fluids with a volume >1.5 mL/kg (grade 2), suggests a high risk of aspiration and measures should be taken to reduce such risk. In this scenario, the anesthesiologist must then consider either postponing or canceling the procedure, or if proceeding, employing strategies, such as a rapid sequence induction for general anesthesia ^,,,.

Gastric ultrasound is not intended to replace fasting guidelines, but rather to serve as an adjunct tool to help anesthesiologists make informed decisions and adjust strategy based on patient risk. It should be remembered that the clinical context is fundamental (urgency, history, other risk factors for aspiration) ^, and that the essence of the established mathematical model is to provide a practical application at the bedside to make informed clinical decisions and maximize patient safety, rather than creating a mathematically perfect model .

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Table 2 summarizes the most significant information about gastric ultrasound in special populations.

• a.

  Obese: Global prevalence of obesity has risen dramatically over the past four decades. In children, rates increased from less than 1 % in 1975 to 6–8 % in 2016. Among adults, obesity rose from 3 % to 11 % in men and from 6 % to 15 % in women over the same period . This has significant implications in anesthesia, given the various adaptations required in pharmacological and airway management ^,,. In the obese population, the depth needs to be adjusted to locate the gastric antrum successfully in ∼90–95 % ^,. Initially, all existing mathematical models were validated in patients with BMI <40 kg/m ²; Kruisselbrink et al. tested the performance of the Perlas mathematical model in patients with severe obesity, finding that the model remained statistically significant with a Pearson correlation coefficient of 0.86 and concordance of 0.82, with a tendency to overestimate volume by approximately 35 mL . There is contradictory data regarding delayed gastric emptying in these patients; while some suggest prolonged emptying time, others report faster emptying or no change compared with non-obese patients. Proposed explanations suggest that delay would be multifactorial, associated with prevalent comorbidities in this population (gastroesophageal reflux, hiatal hernia, anatomical changes, and diabetes mellitus) rather than obesity per se ^,. Overall, gastric ultrasound in patients with obesity is feasible and accurate; it allows differentiation between ‘low-risk’ and ‘high-risk’ stomachs for perioperative gastric aspiration, as described by Van de Putte et al., who reported the feasibility of gastric ultrasound in patients with BMI between 35.1 and 68.7 kg/m ² in 95 % with a CI of 0.86–0.99 . Furthermore, Baettig et al. study with 244 obese patients, reports that the use of gastric ultrasound changed management in 18–21 % of cases, highlighting its potential impact in this population. The ASRA expert practice recommendations conditionally support the use of gastric ultrasound in obese patients due to the lack of certainty of the real role of obesity as an aspiration risk factor .

• b.

  Pediatric: In the pediatric population, the use of a linear transducer in patients under 40 kg provides images with a higher resolution and better definition of the multilayered wall of the stomach. The same qualitative grade system described in adults has been described in children. In the pediatric population, a volume of >1.25 mL/kg has been described as the cutoff for high risk of aspiration ^,.

Spencer et al. first proposed a mathematical model correlating antral CSA in RLD position with gastric volume in children (from 1 month to 17 years), limited by an underrepresentation of patients younger than 48 months and a narrow range of gastric volumes. The formula described by the authors incorporates age and antral CSA in RLD (R2 = 0.6): Volume (mL/kg ⁻¹) = [−7.8 + 0.035 × antral area (mm ²) + 0.127 × age (months)]/body weight (kg) . To date, this is the model most commonly used and described in literature despite its limitations. Bouvet et al. used this model in a large cohort of elective children and found that only 1 % had a gastric volume >1.25 mL/kg, confirming that “at-risk” stomachs are uncommon in well-fasted patients. Later, a model for infants (<12 months) was described, incorporating antral CSA in supine and RLD positions. Their formula achieved a narrow limit of agreement (−0.58–0.62 mL/kg) and a mean bias of 0.01 mL/kg, but its clinical applicability requires further studies to validate their findings . In emergency settings, Evain et al. demonstrated that gastric ultrasound can reliably detect high-risk contents when fasting status is uncertain like children with a diagnosis of appendicitis. Most recently, Cercueil et al. assessed the diagnostic accuracy of qualitative gastric ultrasound in children to detect high gastric fluid volumes (>1.25 mL/kg) in the supine 45° semi-recumbent position. The authors report a limited sensitivity (75 %) but high specificity (85 %), being unable to recommend it for clinical practice. Interestingly, when combining it with quantitative assessment measuring the antral CSA in the RLD position using the mathematical model described by Spencer et al. to calculate gastric fluid volume, the sensitivity improved to 86 %. This interesting study challenges the current European guidelines that favor qualitative assessment alone, suggesting the incorporating volume calculations in the pediatric population may enhance diagnostic accuracy.

• c.

  Pregnancy: Airway management in pregnant women, particularly the risk of aspiration remains as one of the most feared complications for anesthesiologists. Gastric emptying in pregnant patients is delayed during active labor. Therefore, knowing the real-time status of the gastric content allows the anesthesiologist to prepare in the scenario of an emergent procedure requiring general anesthesia . Assessment of the gastric antrum in pregnant patients can be challenging and requires certain adjustments due to physiological (tachypnea, hyperdynamic circulation) and anatomical changes (cephalad and rightward displacement of the stomach secondary to the gravid uterus) . A prospective cohort study by Arzola et al. concluded that in the majority of pregnant women who follow standard guidelines gastric ultrasound revealed an empty or minimally filled stomach (51.5 % grade 0, 47.6 % grade 1, and 1 % grade 2). This supports the adequacy of current fasting guidelines in this population and shows that gastric US is feasible and reliable at the bedside. The same author reports a cutoff value of 9.6 cm ² to identify patients at high risk of aspiration, a value similar to what was posteriorly described in the non-obstetric population. Amaral et al. conclude that measuring the cross-sectional area with ultrasound in term pregnant women is feasible and relatively easy, with a positive correlation between the antral CSA and the gastric volume.

The most recent ASRA expert practice recommendations on gastric ultrasound in medically complex patients supports the use of gastric ultrasound as a tool to monitor and adjust management, thereby reducing the risk of aspiration in pregnant women in active labor, urgent cesarean sections or those with comorbidities (preeclampsia, eclampsia) and does not support its use in non-laboring pregnant women or those scheduled to have elective cesarean sections due to evidence suggesting a normal gastric emptying in this population . Three mathematical models have been described to be used in the pregnant population ^,,, nevertheless, the ASRA expert panel recommends the cutoff limit of 10 cm ² for antral CSA for clear fluids in the pregnant population, this recommendation is consistent with the findings of a previous meta-analysis in adults– which included pregnant women– that also suggested this cutoff value ^,.

• d.

  Diabetes: Gastric emptying may be delayed in patients with diabetes due to gastroparesis secondary to autonomic dysfunction . However, the relationship between this phenomenon and the risk of gastric aspiration is not completely clear. Zhou et al. compared type 2 diabetic vs nondiabetic elective surgical patients 2 h after clear fluids or 6 h after a light meal finding that the prevalence of full stomach in diabetic patients was 48.1 % (after clear fluids 44 % had a full stomach and after a light meal 51.9 % had a full stomach) vs 8 % prevalence in nondiabetics. These findings suggest that preoperative gastric ultrasound is useful in diabetic patients-especially those with diabetic complications-to better assess aspiration risk rather than relying purely on fasting history. Velayudhan et al. compared diabetics vs nondiabetic patients after fasting in supine and RLD, finding that diabetic patients had a significantly higher incidence of a “high-risk” antrum (grade 2) compared to nondiabetic patients under similar fasting conditions and suggests that preoperative gastric ultrasound might be useful for adapting anesthetic/surgical planning in diabetic patients.

The ASRA expert practice recommendations on gastric ultrasound in medically complex patients support the use of gastric ultrasound in diabetic patients, especially those with type 2 diabetes .

• e.

  Use of GLP-1 RAs: Initially developed for diabetes, the use of GLP-1 RAs has risen sharply in recent years following their approval as weight-loss medication. One of the most common side effects of this medication is the delayed gastric emptying, by affecting gastrointestinal motility, which could increase the risk of perioperative aspiration ^,. Several case reports were published in the literature reporting aspiration or regurgitation of gastric contents on patients on GLP-1 RAs despite following adequate fasting guidelines ^,,. Prospective cohorts report that 20–56 % of patients on GLP-1 RAs show increased residual gastric content (RGC) despite appropriate fasting (Clear fluid >1.5 mL/kg or solid content) ^,,. One of these studies found that RGC remained higher regardless of whether semaglutide was held for 1–7 days or 8–10 days and use within the prior 10 days was still associated with increased RGC despite adherence to fasting guidelines .

An aspect not yet clarified is whether the effect on RGC is dose-dependent, if it relates to symptoms such as nausea, vomiting, dyspepsia or gastric fullness , and whether it diminishes over time due to tachyphylaxis .

While perioperative management recommendations for patients on GLP-1 RAs have changed significantly over time, and have variations between the different scientific societies, most of them highlight the value of gastric ultrasound as a key tool for risk stratification ^,,,,,,.