Name: Exploring Subxiphoid Sagittal Gastric Antrum Views for Quantitative Volume Estimation
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exploring subxiphoid sagittal gastric antrum views for quantitative volume estimation

Gastric Ultrasound Technique for Mitigating Peri-Procedural Aspiration Risk

Pulmonary aspiration of gastric contents can cause pneumonitis, pneumonia, and even death1. Higher volume, the presence of particulate matter, and higher acidity of aspirate have been shown to increase the severity of this scenario. Numerous factors help guide a clinician in assessing the risk of aspiration, including comorbid diseases that may slow gastric emptying times, mechanical gastrointestinal obstruction, and timing of the last oral intake. Historically, the latter relies solely on an assessment of the patient&#39;s history, which can be unreliable and inaccurate. In addition, clinician judgment has been shown to be poor to fair at diagnosing a full stomach2.
In 2011, a special task force appointed by the American Society of Anesthesiology (ASA) first published the guidelines for preoperative fasting, and these were updated in 20173,4. Although the ASA fasting guidelines are helpful, they are population-based and not tailored to specific clinical situations, and they recommend further consideration for patients with altered pathology, such as delayed gastric emptying or bowel obstruction. Furthermore, these guidelines rely on a patient who is an accurate historian and can correctly recall their last oral intake. Finally, the guidelines&#39; recommended fasting intervals may not be sufficient to ensure an empty stomach in urgent or emergency situations.
To address gaps in the published fasting guidelines and identify patients at high risk of aspiration, diagnostic point-of-care ultrasound (POCUS) imaging protocols of the stomach have been developed and validated by two separate author groups: one group consisting of intensive care unit (ICU) physicians, and the other of anesthesiologists. The ICU group5 focused on critically ill patients requiring urgent endotracheal intubation and developed a method to screen these patients for gross gastric distension by evaluating the stomach through the left upper quadrant (LUQ). In the LUQ, the authors used the spleen as a sonographic window to visualize the gastric body in the coronal and transverse planes to screen for qualitative signs of gastric distension. When gross gastric distension was identified, the authors took special airway precautions to minimize the chance of aspiration (e.g., by placing a nasogastric tube for gastric decompression [if not contraindicated] before induction of general anesthesia and endotracheal intubation). Separately, a group of anesthesiologists focused on perioperative patients developed a technique for screening stomach contents that would not be expected in properly fasted patients with normal gastric emptying6. This technique involves placing the ultrasound probe in a sagittal plane in the epigastrium to visualize the gastric antrum. The technique allows for both qualitative detection of high-risk stomach contents and, in cases of clear fluid, quantitative estimation of gastric fluid volume.
By combining these two protocols into a hybrid approach, this manuscript will abide by the I-AIM framework to categorize the key steps in gastric ultrasonography: indications, acquisition, image interpretation, and medical decision-making7. However, since this Special Collection is focused on diagnostic POCUS image interpretation, this manuscript will only briefly cover image interpretation and will largely defer the discussion of medical decision-making, as this falls outside the scope of this Collection.
Indications 
Gastric ultrasound has at least four possible indications. First, gastric ultrasound is indicated to screen for high-risk stomach contents prior to intubation or procedural sedation in situations when the patient&#39;s stomach volume and/or contents are either unknown or the history about gastric volume/contents is unreliable. In this setting, the gastric ultrasound exam is performed to risk-stratify the likelihood of pulmonary aspiration and adjust the patient&#39;s care to minimize this risk. Second, some intensivists have used gastric ultrasound to measure gastric residual volumes (GRV) in patients receiving enteral feeding8. In this case, assessing the gastric antrum can aid in the diagnosis of enteral feed intolerance and subsequently decrease the risk of aspiration pneumonia. Recently, gastric ultrasonography has been evaluated as a tool to measure antral cross-sectional area and has shown a strong correlation with GRV in ICU patients9. Third, gastric POCUS has been used to assess delayed bowel function and post-operative ileus in patients following surgical procedures10. Fourth, in pediatric patients, gastric ultrasound has been utilized to diagnose foreign body ingestion and pathologies such as pyloric stenosis11. For other pediatric applications of gastric ultrasound, readers are referred to other sources. The remainder of this article will focus on gastric ultrasound in adults11.
There are very few contraindications to gastric ultrasound because the exam is a non-invasive test that poses negligible direct harm to patients. The main absolute contraindication is patient refusal. Relative contraindications include any of the following: (1) dressings/wounds in the area of the usual scanning windows; (2) a lack of time to attempt the exam due to rapid deterioration of a patient&#39;s hemodynamic or clinical status; and (3) a very high or very low pre-test probability of a full stomach. Fortunately, the presence of intervening dressings/wounds can sometimes be addressed by choosing an alternate sonographic window. For example, if the anterior abdominal approach is obstructed, one can attempt a left lateral view and vice versa. A shortage of time can also be managed through deliberate practice, as studies have shown that gastric ultrasound can be performed expeditiously in expert hands12. Finally, there are cases where the pre-test probability of a full stomach is either very low (e.g., a healthy patient properly fasted for surgery) or very high (e.g., a patient presenting with a known, fixed intestinal obstruction). In such cases, gastric ultrasound is relatively contraindicated because the test - like all diagnostic tests - is imperfect and has the potential to generate false positive and false negative results that can lead patients in an inappropriate direction.
Acquisition 
For acquisition, it is recommended to begin with the subxiphoid sagittal approach to visualize the gastric antrum. The antrum represents an optimal anatomical location to measure gastric contents due to its consistent and superficial location. It constitutes the most dependent portion of the stomach and can be quickly identified in most patients6. Additionally, the gastric antrum dilates in a linear fashion as its contents increase, making it a suitable target for qualitatively assessing stomach contents and suggesting a risk stratification of gastric aspiration6. If the anterior sagittal window is not accessible (e.g., due to wounds/dressings) or provides indeterminate data, the left lateral window may offer useful qualitative data regarding a patient&#39;s gastric contents.
Regarding image interpretation and medical decision-making, the manuscript reviews a range of potential outcomes and explains how gastric ultrasound can be collaboratively used in distinct patient populations. Finally, the common pitfalls and limitations of these methods will be described.

Author Spotlight: Point-of-Care Ultrasound for Gastric Content Assessment and Risk Stratification in Perioperative Care 

Gastric Point of Care Ultrasound in Adults: Image Acquisition and Interpretation

Current NPO guidelines do not specifically address patients with delayed gastric emptying or bowel obstruction. Utilizing point of care ultrasound to assess the gastric contents allows providers to have more accurate information regarding these patients, and appropriately risk stratify their likelihood of pulmonary aspiration. These protocols will hopefully allow perioperative practitioners to better assess the gastric contents of patients requiring sedation or endotracheal intubation.With more accurate data, the practitioner will be better able to risk stratify and minimize the likelihood of pulmonary aspiration. Portions of this protocol will be utilized to research the gastric emptying patterns of unique patient populations, such as those with diabetes, bowel obstructions, hiatal hernias, or who are taking the novel medication glucagon-like peptide-1 receptor agonists.

Watch this Scientific Journal Video about Exploring Subxiphoid Sagittal Gastric Antrum Views for Quantitative Volume Estimation at JoVE.com

Exploring Subxiphoid Sagittal Gastric Antrum Views for Quantitative Volume Estimation  

Begin by positioning the patient in the supine position. After selecting a low-frequency probe, set the preset to abdominal mode. Place the probe on the patient's subxiphoid region in the sagittal plane, orienting the probe marker in a cephalad direction.Next, fan the probe from the patient's left to right, until the liver appears on the left side of the screen. Then, visualize a pulsatile aorta in the long axis, deep in the image, followed by the gastric antrum just caudal and deep to the superficial liver edge. Ensure that the abdominal aorta is positioned at the deeper edge of the image.If the inferior vena cava is observed instead of the aorta, correct the ultrasound beam angle. If the stomach is visible at the pylorus level rather than the antrum, adjust the probe placement by fanning leftward until the aorta becomes visible. Then, identify the muscularis propria of the stomach.For image acquisition, click Acquire to save a video clip of the current sonographic view. Next, position the patient in the right lateral decubitus. For the quantitative estimation of total gastric volume obtain an image of the gastric antrum at its maximum expansion containing only clear liquids.Freeze the image. Then activate the trace tool and trace out the cross-sectional area of the gastric antrum along the outer hyperechoic layer of the wall, representing the serosa. Click on Save and use the given formula to estimate the gastric fluid volume.In the subxiphoid sagittal view, the gastric antrum lies caudal to the liver's tip and superficial to the aorta. In this study, the appearance of the antrum under different conditions as shown, recently consumed clear liquids create a starry night effect due to refracting light, while solid foods result in a hyperechoic air-fluid boundary called the frosted glass appearance. After one to two hours, the air dissipates from the frosted glass image, revealing a hyperechoic heterogeneous content.

Research

JoVE Journal

Medicine

Over the past two decades, diagnostic point-of-care ultrasound (POCUS) has emerged as a rapid and non-invasive bedside tool for addressing clinical ...