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

Summary

This protocol introduces two methods for image acquisition in gastric ultrasonography. Additionally, tips are provided for interpreting this information to assist in medical decision-making.

Abstract

Over the past two decades, diagnostic point-of-care ultrasound (POCUS) has emerged as a rapid and non-invasive bedside tool for addressing clinical inquiries related to gastric content. One emerging concern pertains to patients about to undergo sedation and/or endotracheal intubation: the elevated risk of aspiration from the patient's stomach contents. Aspiration of gastric contents into the lungs poses a serious and potentially life-threatening complication. This occurs more frequently when the stomach is considered "full" and can be affected by the techniques employed for airway management, making it potentially preventable. To mitigate the risk of peri-procedural aspiration, two distinct medical specialties (anesthesiology and critical care medicine) have independently developed techniques to utilize ultrasonography for identifying patients requiring "full stomach" precautions. Due to these separate specialties, the work of each group remains relatively unfamiliar outside its respective field. This article presents descriptions of both techniques for gastric ultrasound. Furthermore, it explains how these approaches can complement each other when one of them falls short. Regarding image acquisition, the article covers the following topics: indications and contraindications, selection of the appropriate probe, patient positioning, and troubleshooting. The article also delves into image interpretation, complete with example images. Additionally, it demonstrates how one of the two techniques can be employed to estimate gastric fluid volume. Lastly, the article briefly discusses medical decision-making based on the findings of this examination.

Introduction

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'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 2017^3,4. 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' 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'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'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'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'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.

Protocol

All procedures performed in this study involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards. The exams can be performed using either a sector array (sometimes colloquially referred to as a "phased array") or a low-frequency curvilinear transducer based on the sonographers' preference. For the figures and scans, a curvilinear probe was used for the anterior sagittal views, while a sector array probe was used for the gastric body scans. The commercial details of the probes are provided in the Table of Materials.

1. Subxiphoid sagittal gastric antrum views (aka "Subxiphoid Views")

1. Supine view
   1. Patient positioning: place the patient in the supine position. If the patient is unable to tolerate being fully supine, up to 30 degrees head up is an acceptable alternative.
   2. Probe selection: select any low-frequency probe (e.g., curvilinear or sector array) (see Table of Materials).
   3. Mode selection: set the preset to abdominal mode.
   4. Probe placement
      1. Place the probe on the patient's sub-xiphoid region in the sagittal plane (Figure 1).
      2. Orient the probe marker in a cephalad direction.
   5. Image optimization
      1. Fan the probe from the patient's left to right until one can visualize the following structures on the screen: (1) liver on the left side of the screen; (2) pulsatile aorta in the long-axis deep in the image; and (2) gastric antrum just caudal and deep to the superficial liver edge (Figure 2).
      2. Ensure the abdominal aorta is positioned at the deeper edge of the image. If the inferior vena cava (IVC) is seen instead, correct the ultrasound beam angle if angled too far towards the patient's right. In this case, visualize the stomach at the pylorus level rather than the antrum. If that happens, adjust by fanning leftward until the aorta becomes visible. This is the appropriate position to analyze the gastric antrum (Figure 3).
         NOTE: If only the stomach is visible at the IVC level and not the aorta, the qualitative sonographic data is still valuable, but remember that this view has lower sensitivity for detecting a "high-risk" stomach. In such cases, a volume assessment will probably underestimate gastric volume (Figure 3).
      3. Identify the muscularis propria (hypoechoic, thickest gastric wall layer) to ensure focusing on the stomach. Large, dilated bowel can be misidentified as the gastric antrum (Figure 4).
         NOTE: The pancreas may be visible posterior to the gastric antrum (Figure 2).
   6. Image acquisition: click on Acquire to save a video clip of this sonographic view.
2. Right lateral decubitus view
   1. Patient positioning: position the patient in the right lateral decubitus (Figure 5).
   2. Probe selection: follow step 1.1.2.
   3. Mode selection: follow step 1.1.3.
   4. Probe placement: follow step 1.1.4.
   5. Image optimization: follow step 1.1.5.
   6. Image acquisition: follow step 1.1.6.
   7. Quantitative estimation of gastric volume
      1. If the gastric antrum appears to contain only clear liquids, quantify the total gastric volume by obtaining an image of the gastric antrum as described above, with the patient in the RLD position (refer to step 1.2.5).
      2. When a view of the gastric antrum is obtained during its maximal expansion, freeze the image.
      3. 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 (Figure 6).
         NOTE: An antral area greater than 10 cm²is unlikely consistent with normal baseline volume and suggests a "full stomach".
      4. Click on Save.
      5. Estimate the gastric fluid volume using the following formula¹³: stomach volume (mL) = 27 + 14.6 ACSA (in RLD) cm² − 1.28 x age (years).
         ​NOTE: This formula has been validated for clear liquids in the antrum. This formula was also validated in a separate study of morbidly obese individuals¹⁴.

2. Left upper quadrant (LUQ) gastric body views

1. If subxiphoid views are inadequate or inaccessible, proceed to evaluate left upper quadrant views (step 2.2 and step 2.3).
2. LUQ coronal gastric body view
   1. Patient positioning: follow step 1.1.1.
   2. Probe selection: follow step 1.1.2.
   3. Mode selection: follow step 1.1.3.
   4. Probe positioning
      1. Place the probe on the left mid-axillary line in the longitudinal plane with the orientation marker pointed toward the patient's head (Figure 7).
         NOTE: With the probe marker oriented in this direction, the thoracic cavity will appear on the left side of the ultrasound scanner and the right side will be the abdominal cavity.
   5. Image optimization
      1. Identify the spleen and left hemidiaphragm. The spleen is identified by its rounded capsular shape. The left hemidiaphragm appears as a hyperechoic line, superior to the spleen, and moves with the normal respiratory cycle.
      2. Once the spleen is identified, fan posteriorly to identify the kidney, which will appear as the oblong structure inferior and posterior to the spleen.
         NOTE: The spleen offers less of an acoustic window than the liver does on the right abdomen
      3. Once these landmarks are identified, angle the transducer in an anterior direction to obtain a view of the gastric body (Figure 7).
         NOTE: The anatomic structures will likely shift as the diaphragm moves. If possible, consider asking the patient to briefly hold his/her breath while the scans are being performed.
   6. Acquire the images following step 1.1.6.
3. LUQ anterior transverse gastric body view
   1. Patient positioning: follow step 1.1.1.
   2. Probe selection: follow step 1.1.2.
   3. Mode selection: follow step 1.1.3.
   4. Probe positioning
      1. Place the probe on the anterior surface of the patient's abdomen at approximately the mid-clavicular line, just caudal to the ribs (Figure 8). In order to fully assess the gastric body, it might be necessary to rotate the probe clockwise to an oblique view.
   5. Image optimization: fan the probe cranial-to-caudal until the gastric body is seen deep into the spleen.
   6. Acquire images folllowing step 1.1.6.

Results

To ensure accurate visualization of the gastric antrum, it is necessary to probe deep enough to identify the aorta. Detecting the IVC instead places the scan at the gastric pylorus level, causing findings to underestimate genuine gastric content. While diagnostic utility remains for the gastric pylorus level, it iss less straightforward to interpret compared to the gastric antral view (Figure 3). Hence, a crucial task is to scan the great vessels and distinguish between the gastric antrum an...

Discussion

As stated previously, the primary purpose of gastric ultrasound is to evaluate gastric contents and assess risk before airway management or procedural sedation. The provided protocol outlines two main methods to capture images of the gastric antrum and body, aiding in this risk assessment. Additionally, reviewing the representative results section assists gastric sonographers in developing image interpretation skills. While a comprehensive discussion of medical decision-making is beyond this protocol's scope, this manusc...

Materials

Name	Company	Catalog Number	Comments
High Frequency Ultrasound Probe (HFL38xp)	SonoSite (FujiFilm)	P16038
Low Frequency Ultrasound Probe (C35xp)	SonoSite (FujiFilm)	P19617
SonoSite X-porte Ultrasound	SonoSite (FujiFilm)	P19220
Ultrasound Gel	AquaSonic	PLI 01-08