probe selection and machine presets for point-of-care ultrasound of the abdominal aorta

Imaging the Abdominal Aorta with Point-of-Care Ultrasound

Point-of-care ultrasound (POCUS) has increased in use over the last several years and is being increasingly incorporated into various residency training programs1,2. POCUS has great utility in critical care areas such as the emergency department and the intensive care unit, specifically to aid in the rapid diagnosis of life-threatening intraabdominal emergencies such as acute aortic dissection, as well as abdominal aortic aneurysms, especially those at risk for rupture and those that have ruptured into the peritoneum.
AAA rupture and acute aortic dissection are associated with high mortality. The mortality of ruptured aortic aneurysms ranges from 67% to 94%3,4. The mortality associated with type A aortic dissection increases at a rate of 1% per hour after acute dissection and the mortality of type B aortic dissection ranges from 10% to 25% at 30 days5. Abdominal aortic dissection in isolation is rare and accounts for only 0.2% to 4% of all aortic dissections6,7,8,9,10. Since most abdominal aortic dissections occur as an extension of thoracic aortic dissections, evaluation of the abdominal aorta for evidence of dissection may aid in the diagnosis of thoracic aortic dissection11.
Computed tomography with angiography (CTA) is the gold standard for imaging pathology associated with the abdominal aorta; however, it has several drawbacks. It may be time-consuming, especially in an unstable patient, and requires a technician to perform and a radiologist or vascular surgeon to interpret the images. CTA uses ionizing radiation and requires the use of intravenous contrast dye for optimal detection of pathology. Furthermore, the performance of CTA requires potentially unstable patients to leave the critical care area. In contrast, POCUS is non-invasive, cost-effective, and lacks the ionizing radiation and contrast dye that CT requires. It can also be performed and interpreted by the same individual in real time and does not require the patient to leave the monitored area.
A systematic review of emergency department POCUS for diagnosing AAA by Rubano et al. revealed a sensitivity of 99% and specificity of 98%, with a positive likelihood ratio of 99 and a negative likelihood ratio of 0.0112. This pooled analysis evaluated the test characteristics over a varied group of operators, including resident and attending physicians with a wide range of training in POCUS.
The test characteristics for the POCUS evaluation of abdominal aortic dissection are different from those of AAA and may vary depending on the origin of the dissection. Sonographic findings of an intimal flap separating the true and false lumens have a sensitivity of 67%-79% and a specificity of 99%-100% for aortic dissection13,14. As most aortic dissections found in the abdomen are an extension of a thoracic aortic dissection, additional POCUS applications of the heart and lungs to evaluate for pericardial effusion, aortic root dilatation, and left pleural effusion may be performed, but will not be the focus of this paper13.
Finally, it is important to note that the United States Preventative Services task force provides a Grade B recommendation for a one-time ultrasound screening for AAA in men aged 65-75 who have ever smoked. This is particularly relevant to the primary care setting.
This review will describe a step-by-step protocol for the performance of POCUS in the bedside evaluation of the abdominal aorta, specifically to evaluate for AAA and abdominal aortic dissection. This protocol assumes a basic knowledge of diagnostic ultrasound, including physics, instrumentation, as well as medical knowledge of anatomy and pathologic states of the abdominal aorta and major branching arteries. Readers are advised to refer to other sources for prerequisite knowledge.

Author Spotlight: Using Point-of-Care Ultrasound for Comprehensive Evaluation of the Abdominal Aorta

An Approach to Point-Of-Care Ultrasound Evaluation of the Abdominal Aorta

We seek to demonstrate the use of point-of-care ultrasound or POCUS to evaluate the anatomy and pathology of the abdominal aorta. POCUS has excellent sensitivity and specificity for the evaluation of abdominal aortic aneurysm, and excellent specificity for the evaluation of abdominal aortic dissection. Some of the challenges include ensuring POCUS users have adequate training and ensuring they can identify pathology and artifacts with accuracy.This protocol is comprehensive and can be performed and interpreted by the same individual in less than five minutes. This is faster than computed tomography angiography, especially when the patient is critically ill and not stable enough to leave the critical care area for an imaging test.

Probe Selection and Machine Presets for Point-of-Care Ultrasound of the Abdominal Aorta

To perform a point of care ultrasound evaluation of the abdominal aorta, select a 2.5 to 3.5 megahertz curvilinear probe, which provides the best visualization for most adults. Alternatively, use the phased array probe of 1 to 5 megahertz, commonly called cardiac probe, which is typically used for echocardiography. Use the abdominal preset on the machine regardless of the probe used.Set the mode to B mode or two-dimensional gray scale and the depth to 20 centimeters. Adjust the depth once the aorta is visualized to keep the aorta in the midfield of the screen. Choose a lower frequency range for patients with a high body mass index to improve image acquisition.

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Research

JoVE Journal

Medicine

Disorders of the abdominal aorta, including aneurysms and dissection, have potentially high rates of morbidity and mortality. While computed tomography (CT) is the current gold standard to image the abdominal aorta, the process of obtaining a CT may be time-consuming, requires the use of intravenous contrast dye, and involves exposure to ionizing radiation. Point-of-care Ultrasound (POCUS) can be performed at the bedside and has excellent sensitivity and specificity for the diagnosis of abdominal aortic aneurysm and excellent specificity for the diagnosis of abdominal aortic dissection. Additionally, POCUS is non-invasive, cost-effective, lacks ionizing radiation, requires no intravenous contrast dye, and can be performed without taking the patient from a critical care area. Screening for abdominal aortic aneurysm (AAA) can be done in primary care settings as well. 
This article will review the approach to POCUS of the abdominal aorta to evaluate such critical pathology. In this paper, we will review the sonographic anatomy of the abdominal aorta as well as the choice of the ultrasound probe, description of POCUS image acquisition, and some pearls and pitfalls of using POCUS to aid in the diagnosis of potentially life-threatening abdominal aortic pathology.

This protocol reviews the steps to image the abdominal aorta with point-of-care ultrasound. We discuss image acquisition, troubleshooting imaging pitfalls and artifacts, and the recognition of life-threatening abdominal aortic pathology.

Disorders of the abdominal aorta, including aneurysms and dissection, have potentially high rates of morbidity and mortality. While computed tomography ...

Imaging the Abdominal Aorta in the Transverse Plane Using Point-of-Care Ultrasound

To begin scanning, position the patient's supine with the abdomen exposed. Hip flexion, if tolerated by the patient, relaxes the abdominal muscles and may improve image acquisition. Apply ultrasound gel to the transducer.Orient the transducer and the transverse plane with the indicator toward the patient's right. Ensure that the indicator position matches the one displayed on the screen. Place the transducer just distal to the patient's xiphoid process and apply light pressure to visualize the anterior aspect of the vertebra with its hyperechoic shadow casting arch.Slide the transducer coddly until the celiac trunk is visualized. The celiac trunk is short and quickly bifurcates into the hepatic artery and splenic artery, which together form the seagull sign. Capture these images for later review by clicking the button on the system that records clips.Slide the transducer coddly to view the superior mesenteric artery or SMA. Coming off the anterior aorta, SMA follows a path parallel to the aorta to course inferiorly. The splenic vein courses anterior to the SMA and the left renal vein courses between the SMA and the aorta.Measure the AP diameter of the suprarenal aorta by optimizing a live image of the aorta in this location, and then pressing the system's freeze button. Press caliper or measure and move the track ball or touch pad of the system to the outer edge of the anterior wall, the adventitia and click select. Next, move the track ball or touch pad again to the outer edge of the posterior wall and click select.Wait for the system to generate a measurement. Click the button on the system that saves still images to save this image as a still image containing the measurement. Continue scanning in a transverse plane to visualize the entirety of the aorta through the bifurcation and capture the images.When a live image of the distal aorta is optimized, measure the AP diameter of the infrarenal aorta. Press caliper or measure and move the track ball or touch pad of the system to the outer edge of the anterior wall, the adventitia and click select. Move the track ball or touch pad again to the outer edge of the posterior wall and click select.Wait for the system to generate a measurement. Save the still image as demonstrated previously. Adjust the depth as the abdominal aorta courses coddly through the abdomen becoming more superficial and tapering slightly.Continue scanning coddly through the aortic bifurcation into the left and right iliac arteries. Adjusting the depth to maintain the aorta in vertebral body in the middle of the screen. Capture images while scanning through the bifurcation.When imaging the abdominal aorta in the transverse plane, an aorta measuring greater than 3 centimeters was considered aneurysmal. Such aneurysms also showed the presence of a hematoma filling most of the lumen. The presence of an intimal flap within the lumen of the aorta indicated aortic dissection.Depending on the chronicity of the dissection, the flap was either thin and moved with the pulsations of the aorta or thickened and had an adjacent hematoma. An acute thoracic aortic dissection was found to extend through the abdominal aorta and into the right iliac artery.

Imaging the Abdominal Aorta in the Longitudinal and Coronal Plane Using Point-of-Care Ultrasound

To begin scanning, position the patient's supine with the abdomen exposed. Hip flexion, if tolerated by the patient, relaxes the abdominal muscles and may improve image acquisition. Apply ultrasound gel to the transducer.Place the transducer transversely in the proximal abdomen, and the transverse plane, with the indicator toward the patient's right. Ensure that the indicator position matches the one displayed on the screen. Starting in the subxiphoid area, scan to obtain a transverse view of the aorta.Once the transverse view of the aorta is optimized, rotate the probe clockwise with the indicator pointing towards the patient's head. Following the aorta as the image becomes longitudinal on the screen. Acquire images while scanning caudally, examining for aneurysmal dilations, and save these images for later review by clicking the button on the system that records clip.To obtain a longitudinal view of the aorta in the coronal plane, if possible, position the patient in the left lateral decubitus position for better image acquisition. Place the transducer in the coronal plane at the mid axillary line on the patient's right with the indicator pointing cranially. Scan the aorta in the coronal plane.The aorta will be visualized deep into the IVC if both vessels are imaged. The abdominal aortic pseudo thrombus was a reverberation artifact resulting from the reflection of the ultrasound beam between the anterior and posterior walls of the SMA.