Point-of-care Ultrasound to Screen for Gastrointestinal Dysfunction: Image Acquisition and Interpretation

Summary

Screening for gastrointestinal (GI) dysfunction using point-of-care ultrasound (POCUS) has been reported but remains underutilized. This manuscript reviews the relevant literature and describes a protocol that permits POCUS screening for dilation/dysfunction of the small bowel and/or stomach, which can arise as the final common pathway of various GI pathologies.

Abstract

A major cause of morbidity and healthcare resource utilization is gastrointestinal (GI) dysfunction. Although GI symptoms are frequently evaluated with computed tomography (CT), this modality subjects patients to ionizing radiation, high costs, and high resource utilization (including sometimes inter-facility transfers from non-hospital settings). Thus, alternative methods of screening for GI dysfunction that do not subject patients to radiation and transport risk are highly desirable. One such emerging option that may help with bedside decision-making is GI point-of-care ultrasound (POCUS). However, utilization of GI POCUS is limited by a lack of training of providers and standardization in image acquisition and interpretation. To address this unmet need, we propose an evidence-based image acquisition protocol using point-of-care ultrasound to screen for two important types of GI dysfunction: ileus or obstruction. This protocol includes guidance on probe selection, patient positioning, and image acquisition sequence. Further, we review image interpretation and the limitations of this protocol.

Introduction

Gastrointestinal dysfunction is a common cause of significant morbidity and mortality^1,2. Abdominal pain alone contributes to close to 18 million emergency department visits annually, and total healthcare expenditures related to GI pathology are conservatively estimated at $119.6 billion annually³. Computer tomography, which is often the imaging modality of choice for gastrointestinal concerns, is associated with ionizing radiation, increasing the lifetime risk of malignancy⁴. Moreover, many causes of GI dysfunction can be addressed conservatively if surgical or life-threatening conditions are appropriately ruled out. For all these reasons, rapid bedside identification of GI dysfunction has the potential to improve care and promote goal-congruent decision-making.

Over the last decade, point-of-care ultrasound (POCUS) has become an increasingly popular tool for rapid, bedside evaluation of multiple organs, including the heart, lungs, and peritoneal cavity, to name just a few⁵. However, while POCUS of many organs has been codified into guidelines, curricula, and certificate pathways⁶, POCUS of the intestines has yet to reach such milestones. Nevertheless, a growing body of literature suggests that intestinal POCUS may help to narrow the differential diagnosis of GI dysfunction. In the emergency department, POCUS has been shown to aid in the diagnosis of bowel obstruction with excellent accuracy and specificity, greater than can be achieved with abdominal plain film. Ultrasound findings of decreased peristalsis or dilated bowel showed 94% sensitivity and 81% specificity, while abdominal plain film showed 46% sensitivity and 67% specificity for small bowel obstruction, respectively^7,8,9,10. Moreover, literature has shown that trainees with minimal hands-on training can reach a surprising level of accuracy in diagnosing bowel obstruction¹¹.

While most of the literature on POCUS-assisted diagnosis of bowel obstruction originates from the specialty of emergency medicine (EM), current EM guidelines do not require trainees to achieve proficiency in intestinal ultrasound as part of standard training¹². Further, patients with bowel dysfunction are cared for in multiple settings other than the emergency department by non-EM providers. Such providers acquire POCUS training through residency-embedded curricula or workshops and certificates sponsored by their institution or national organizations such as ACP, ASA, SHM, and CHEST¹³. Most of these trainings do not include dedicated modules on GI dysfunction. Moreover, there is no set of standardized acquisition techniques for clinicians to follow when attempting to image GI dysfunction.

Given the significant morbidity and mortality of ileus and mechanical obstruction and the proven benefits of POCUS as a screening modality, increased use of GI POCUS by providers, in or outside the emergency department, has the potential to improve care. Therefore, in hopes of bridging this gap, this article primarily aims to outline an image acquisition protocol to screen for GI dysfunction, with a focus on ileus and SBO.

A secondary aim of this paper is to propose the integration of gastric and bowel bedside ultrasound. This aim is unique in that gastric POCUS is typically limited to use in preoperative settings, in which anesthesiologists perform gastric evaluations to assess for stomach contents, which, if present, increase the aspiration risk^14,15. Though some authors have explored whether gastric POCUS can be used to predict the risk of GI dysfunction postoperatively¹⁶, much of the gastric ultrasound literature is focused solely on periprocedural applications. Thus, small bowel POCUS and gastric POCUS remain two siloed entities. We propose a protocol linking these two gastrointestinal ultrasound applications for identifying and characterizing GI dysfunction.

Protocol

All procedures performed in studies 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. Illustrative images were obtained from a de-identified educational database devoid of protected health information. Inclusion criteria: any patient with suspected GI dysfunction or healthy volunteers with presumed normal bowel function. Exclusion criteria: patient refusal.

1. Transducer selection

1. Select a low-frequency (1-5 MHz), large footprint (i.e., curvilinear) transducer to visualize organs deep in the body. If the curvilinear probe is not available, select any low-frequency transducer (e.g., a sector array transducer)^{17,18,19,20,21,22,23}.
   NOTE: If the curvilinear probe's spatial resolution is not adequate to permit measurement of bowel wall thickness, switch to a linear high-frequency probe for that portion of this protocol (Step 8.2.1).

2. Machine settings and machine placement

1. Mode
   Select abdominal mode (indicator on screen left, high spatial resolution, low temporal resolution).
2. Machine placement
   1. Place the ultrasound machine on either side of the patient and position the machine close enough to the patient to minimize how far the sonographer has to turn their head to look back and forth between the ultrasound machine screen and the patient.
3. Image acquisition preset
   1. Set the ultrasound machine's image acquisition technique to archive clips after the Acquire button is pressed (aka prospective collection). If, on the other hand, the operator prefers retrospective collection, reverse the order of any paired steps involving fanning the transducer and clicking Acquire (e.g., steps 5.1, 5.2, 5.3, etc.).
   2. Set the recording loop length to at least 2 s.

3. Patient positioning

1. Position the patient supine with the abdomen exposed. If the patient cannot tolerate lying supine, elevate the head of the bed to 30°.

4. Scanning technique

1. Because parallel sections of the abdomen will be scanned systematically (see Step 5), apply gel directly on the patient in parallel sections in the cranial-caudal orientation as opposed to repeated application of gel to the ultrasound probe (Figure 1).
2. Point the probe's indicator toward the patient's head for coronal or sagittal views and toward the patient's right side for transverse views.

5. Systematic abdominal scan ("Lawnmower technique")

1. Scan initiation
   1. Place the probe on the anterolateral abdominal wall just cranial to the right iliac crest, with the probe positioned between the mid- and anterior axillary lines.
   2. Orient the ultrasound beam aligned with the body's transverse plane with the indicator to the patient's left side (Figure 2).
   3. Adjust the screen depth to be able to visualize loops of bowel, including their anterior and posterior walls and the space just deep to their posterior walls.
      NOTE: In most adults, this will result in a screen depth between 12 cm and 18 cm. However, in patients at extremes of body habitus, the optimal depth may be shallower or deeper than this.
   4. Slide the probe cranially towards the left upper quadrant of the abdomen until a loop of bowel is visible.
      NOTE: If tolerated by the patient, slight compression (also known as "graded compression") of the abdomen with the ultrasound probe may help displace bowel gas.
   5. Differentiate the small bowel from other segments of the GI tract using as explained in Table 1.
2. Image archiving
   1. Short-axis view of the small bowel
      1. Once a loop of small bowel is visualized, attempt to adjust the probe position (sliding, rocking, rotating, or fanning) until the bowel is visualized in short-axis cross section.
         NOTE: The probe adjustments may result in the probe being in other planes of the body (e.g., sagittal, coronal, or oblique).
      2. When a circular cross-section of small bowel is visualized (Figure 3A and Figure 4A), click acquire or equivalent button that permits video archiving.
         NOTE: Video clips will be reviewed after the exam is completed to screen for peristalsis, measure bowel lumen diameter, and evaluate for other relevant findings. Some ultrasound devices do not allow retrospective measurements once videos or still images have been saved. For these situations, we recommend performing Section 6 of the protocol immediately after image acquisition.
   2. Long-axis view of the small bowel
      1. While maintaining the probe in the same location on the body, rotate the probe 90° clockwise or counterclockwise from the short-axis view to obtain a long-axis view of the small bowel.
      2. When a long-axis cross-section of small bowel is visualized (Figure 3B and Figure 4B), click acquire or equivalent button that permits video archiving.
         ​NOTE: Short and long axis cross-sections of the same segment of bowel may have different diameters; this is expected as the lumen diameter is dynamic due to phases of peristalsis.
3. Scan continuation ^24,25,26
   1. Continue scanning parallel sections of the abdomen by sliding the probe in the cranial direction until just upper abdominal organs or ribs come into view, then stop and slide the probe to the patient's right, then continue sliding caudally.
   2. Continue until the entire abdomen has been scanned as defined by the following boundaries: mid-axillary lines bilaterally, ribs/xiphoid cranially, and bladder caudally.
   3. When a loop of bowel is visualized that shows different characteristics than the previous bowel seen, repeat steps 5.2 (e.g., if initially decompressed bowel loops were visualized in one portion of abdomen and now a different section of abdomen shows dilated loops of bowel or vice versa).
   4. Optional gastric antral ultrasound: If the small bowel was not visualized in the previous steps, attempt gastric ultrasound to screen for gross gastric distension.
      1. Place the curvilinear probe in the subxiphoid location, indicator pointing cranially, and probe aligned with the body's sagittal plane.
      2. Obtain a view that contains the liver cranially and the aorta in the deep portion of the image (Figure 5).
      3. Within this view, find a circular hollow viscus that is surrounded by a hypo-echoic (dark) outer rim. This is the stomach.
      4. Click acquire or equivalent button that permits video archiving.
   5. Click End Exam or equivalent step that transmits ultrasound exam to a permanent digital archive where the video clips can be reviewed later.
      NOTE: The remaining steps are performed after image acquisition is completed.

6. Bowel diameter measurement

1. Review acquired ultrasound clips to identify two clips where small bowel loops were clearly visible, including both their superficial and deep boundaries (see Figure 3 and Figure 4).
2. For the first clip, do the following:
   1. Play the clip until the bowel diameter appears the largest.
   2. Select Freeze to pause the clip.
   3. Use the caliper function to measure the superficial-to-deep diameter of the bowel from the outer wall to the outer wall⁹ (see Figure 3 and Figure 4).
   4. Select Save or equivalent function to archive this measurement.
3. For the second clip, repeat step 6.2 (including sub-steps).
4. Calculate the average of these two measurements and use this value as the small bowel diameter measurement for this exam.

7. Peristalsis evaluation

1. Referring to the clips obtained in steps 5.1-5.3, evaluate the loops of bowel for the presence of peristalsis (Video 1).
2. Evaluate peristalsis.
   1. Categorize peristalsis as hyperkinetic, normal, or decreased/absent using the following criteria: (1) hyperkinetic is >10 contractions/min; (2) normal is 3-10 contractions/min; (3) decreased/absent is <3 contractions/min^27,28,29.
   2. If decreased/absent, evaluate for "to-and-fro" motion of bowel contents, which is abnormal, indicating bidirectional movement which is abnormal (Video 2).

8. Additional findings associated with gastrointestinal dysfunction

1. Extraluminal free fluid
   1. Examine video clips obtained in steps 5.1-5.3 for extraluminal fluid. When present, this appears as anechoic material between loops of bowel, often in a triangular shape. This is sometimes referred to colloquially as the "Tanga Sign" (Figure 6 and Video 3)^25,30.
2. Bowel wall edema
   1. Measuring bowel wall thickness: use the caliper function to measure the distance between the parietal outer layer to the boundary between the endothelium and the bowel lumen (Figure 7 and Video 4).
   2. Click Acquire (or equivalent button) to save the measurement.
3. Prominent circular folds
   1. Evaluate the acquired video clips for bowel wall edema. When present, this appears as prominent circular folds, also known as plicae circulares or valvulae conniventes, projecting into the intestinal lumen, known colloquially as the "keyboard sign" (Figure 8)²⁶.
4. Gross gastric distension
   1. Screen for gross gastric distension in two locations on the abdominal wall: (1) the subxiphoid region with the probe in the mid-sagittal plane looking for gastric distension of gastric antrum (Figure 5, Figure 9, and Video 5) and (2) the left mid-axillary line, approximately 4-7^th intercostal spaces looking for gross distension of the gastric body/fundus (Figure 10 and Video 6)^14,31.

Results

Differentiating normal versus abnormal gastrointestinal function is best performed using a multi-parameter model. The model that has been the most robustly tested includes the following criteria, sometimes referred to as "diagnostic criteria": 1) bowel diameter ≥ 2.5 cm and abnormal peristalsis. Additional criteria, sometimes referred to as "staging criteria" vary in the literature, but those commonly used and easily identifiable on bedside ultrasound include interloop free fluid and bowel wall edema⁸

Discussion

The proposed GI POCUS image acquisition protocol has several key steps. First, given the irregular orientation and dynamic location of the bowels in the body, it is important to have a systematic image acquisition approach. Specifically, the "lawnmower" technique allows for systematic visualization of the abdominal cavity and identification of optimally localized loops of bowel.

Second, although the optimal criteria for identifying intestinal dysfunction with ultrasound remain under ac...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
CX50	Philips	n/a	Used to obtain a subset of the Figures & Videos
Edge 1	Sonosite	n/a	Used to obtain a subset of the Figures & Videos
Epic 7C	Philips	n/a	Used to obtain a subset of the Figures & Videos
HS60	Samsung	n/a	Used to obtain a subset of the Figures & Videos
Logiq E10	GE	n/a	Used to obtain a subset of the Figures & Videos
LX	Sonosite	n/a	Used to obtain a subset of the Figures & Videos