Bedside Ultrasound AAA Examination

Author: Ryan Gibbons, MD, Department of Emergency Medicine, Lewis Katz School of Medicine at Temple University
Editor: Matthew Tews, DO, MS, Medical College of Wisconsin

Objectives

  1. Understand the emergent nature of Abdominal Aortic Aneurysms (AAA)
  2. Understand the challenge of diagnosing AAA
  3. Appreciate the anatomy of the abdominal aorta
  4. Learn the bedside ultrasound technique to evaluate the aorta
  5. Understand the challenges and limitations of ultrasound
  6. Appreciate the appropriate ED population at risk for AAA
  7. Introduce the idea of screening for AAA in an ED population

Introduction

Abdominal aortic aneurysms (AAAs) are a common clinical entity with a prevalence of 1.3% among middle aged patients increasing to over 12% among elderly men.1-2  In 2009, there were over 17,000 deaths related to AAAs.3  Mortality rates from ruptured aneurysms remain exceedingly high between 50-95%.4-5

With each passing minute, the mortality increases by 1% necessitating timely diagnosis and treatment.6 Up to 30% of ruptured AAAs are initially misdiagnosed, and physical exam has a very poor sensitivity of less than 65%.7-10  In fact, only 25% of individuals will present with the classic triad of abdominal pain, hypotension, and a pulsatile mass.7-8

Early diagnosis through the use of emergency medicine bedside ultrasound has been shown to have a sensitivity of 94-99%.11-12  It is safe, effective and can be performed in less than 5 minutes.  Most importantly, its use can reduce mortality by 20-60%, in comparison to CT which in one study took an average of 83 minutes with a mortality above 70%.13-14 Furthermore, patients are often too unstable to leave the emergency department to obtain more advanced imaging.  Because of these benefits, the use of bedside ultrasound has become commonplace in evaluating the patient with a suspected AAA.

Anatomy

The abdominal aorta is a retroperitoneal vessel entering the abdomen through the aortic hiatus just below the xiphoid process at the level of T12.  It lies just anterior to the vertebral body and alongside the inferior vena cava (IVC).

Image 1

It extends to the level of L4 where it bifurcates into the common iliac arteries about 1-2cm below the umbilicus.

As it descends through the abdomen, the aorta tapers in size moving more superficial and giving off several branches.  Sequentially, these are the celiac artery, superior mesenteric artery (SMA), renal then gonadal arteries, and finally the inferior mesenteric artery (IMA).

Image 2

IVF=Inferior Vena Cava; AO=Aorta

The celiac artery can be seen branching into the hepatic and splenic arteries creating the classic “seagull sign’’ seen in the transverse view.

Image 3

An aneurysm is defined as a focal dilation >50% of a vessel’s normal diameter.  A diameter > 3cm delineates an abdominal aortic aneurysm.

Image 4

There are two types of AAAs:  fusiform (90%) and saccular.

 

image 5

Image 6

AO=Aorta

Fusiform aneurysms account for the vast majority of AAAs and frequently extend anteriorly and leftward.  Saccular aneurysms are focal outpouchings that are much less common and occasionally associated with a mycotic source.  An aneurysm of the internal iliac artery is typically >1.5 cm.

Nearly 90% of AAAs occur infra-renal.15 The renal arteries are often challenging though to visualize with bedside ultrasound.  By scanning to the level of the aortic bifurcation, the provider ensures complete evaluation of the aorta.

Ultrasound Technique

Imaging of the aorta is done in real time.  The 2-5 mHz curvilinear abdominal probe is utilized given its superior depth penetration.

Image 7

Indicator Circled in Red

Begin by placing the probe just caudal to the xiphoid process in the transverse orientation with the indicator to the patient’s right.

Image 8

Complete a transverse sweep from the level of the celiac artery to the bifurcation of the aorta.

Multiple sequential clips may be needed to visualize the aorta in its entirety.  Then, measure the aorta at its maximum diameter in the transverse plane.  Be sure to measure the diameter from outer wall to outer wall and to include any thrombus visualized.

Image 9

Measurements are most accurate when the probe is directly perpendicular to the vessel.

Next, orientate the probe in the sagittal plane with the indicator towards the head of the patient.

Image 10

Again, begin at the level of the celiac artery just distal to the xiphoid process and obtain a clip(s) in the sagittal plane extending as caudally as possible.

Intraluminal thrombus may be present as well.  Thrombus generally appears as echogenic material within the vessel but can be easily missed.  Typically, it is located along the anterior and lateral walls and can create the appearance of a false lumen which underestimates the true size of the aneurysm.  Be very cautious not to miss these.

Perform a right upper quadrant (RUQ) ultrasound sweep to assess for free fluid when concerned for a ruptured AAA.

Free Fluid Seen Between Liver and Kidney

Most AAA ruptures though are retroperitoneal (70-90%) which ultrasound is unable to evaluate.16 Nonetheless, one study demonstrated a 97% sensitivity for diagnosing ruptured AAA when combining ultrasound and clinical acumen. 17

Finally, the presence of an intimal flap is pathognomonic for an aortic dissection. (Video 6 & Image 11)

Image 11

Aggressive blood pressure control and calling your vascular or thoracic surgeon is warranted immediately.

Technical Considerations

As with any ultrasound study, proper set up is key to obtaining accurate images.  The patient should be placed in the supine position at the level of the scanner’s waist.  Dim the lights as appropriate and apply adequate ultrasound gel.

The 2-5 mHz curvilinear abdominal probe is best utilized.  The 1-5 mHz phased array (cardiac) probe can be used as well.

Image 12

1-5 mHz phased array (cardiac) probe

The initial depth should be maximized in order to visualize the vertebral body and locate the key landmarks discussed above.  Once identified, the depth can be adjusted accordingly to optimize your view.  In particular, as you move caudally, the aorta becomes more superficial and less depth is required.

Firm, constant pressure is best to help displace obstructing bowel, as is appropriate gain adjustments to limit artifact.  Modifying the angle of the probe or moving slightly off midline while angling back may improve visualization.  Establishing a view just caudal to obstructing bowel gas and then sweeping or tilting cephalad may supplement imaging as does the reverse technique.  Finally, placing the patient in the left lateral decubitus position my augment imaging as well.  Nonetheless, up to 5% of patients will not have a visible aorta.  Advanced imaging is then required based on the patient’s hemodynamic stability.

Who to Scan

Remember <25% of patients will present with the classic triad of abdominal pain, hypotension, and a pulsatile mass.

Consider an abdominal aorta ultrasound in the following patients

  • >50 yo with chest, back, flank, abdominal, or groin pain
  • Renal colic
  • Cardiac Arrest
  • Hypotension
  • Syncope
  • Thromboembolic events to the lower extremities
  • Neurologic deficit of the lower extremities

Consider AAA screening in asymptomatic individuals

  • Recommended for men >65 years who ever smoked by AAFP and US Preventative Survey Task Force18
  • Society for Vascular Surgery recommends:19
    • All men age >65
    • Men >55 with family history of AAA
    • Women >65 with family history of AAA or who have smoked

Key Risk Factors20

  • >50 years old
  • Male
  • Hypertension
  • Smoking
  • Family History
  • CAD, DM, Hyperlipidemia, PAD

Pearls & Pitfalls

1. 90% of AAAs are infra-renal.  Scan the entire aorta to its bifurcation to ensure visualization below the renal arteries.

2. Beware the intraluminal thrombus and its false lumen.  Remember to include the thrombus to accurately measure the aortic diameter.

3. Recognize the pathognomonic intimal flap in an aortic dissection.

Image 13

4. The aorta and IVC can be challenging to differentiate.  Typically, the thick-walled aorta is the pulsatile, non-compressible, circular structure to left of the IVC (anatomically and on screen).  The thin-walled IVC is the compressible, oval-shaped structure next to the aorta.  It may appear to pulsate though due to its proximity to the aorta.  If still unsure, use pulsed-wave Doppler to distinguish between the arterial pulsations of the aorta from the venous flow of the IVC, which will only demonstrate mild respiratory variation.

image 15

A “sniff test” may also help.  Have the patient inhale deeply.  The negative intrathoracic pressure created by inhalation will increase venous return to the heart causing the IVC to partially collapse.

Image 14

5. Do not forget to perform a RUQ ultrasound sweep to evaluate for free fluid when concerned for a rupture AAA.

Diagnostic Algorithm

Diagnostic Algorithm

Diagnostic Algorithm for Evaluation of a AAA Using Bedside Ultrasonography

References

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