ABDOMINAL AORTIC ANEURYSM

AIMS

• Understand the indications for AAA PoCUS

• Identify the normal abdominal aortic and vascular anatomy

• Be able to image the abdominal aorta in short and long axes from the diaphragm to the bifurcation

• Be able to take accurate measurements of the abdominal aorta in 2 planes at the proximal, mid and distal aorta

• Be able to recognise aneurysmal change in the aorta

• Be able to recognise limitations of bedside aortic ultrasound and potential pitfalls

EPIDEMIOLOGY

• The prevalence of AAA ( >/= 3cm in size) is probably around 5-10% in men aged 65-79 yrs and about 1-2% in women (Jones et. al. 2016).

• The mortality rate from a ruptured AAA is around 90%, with less than 50% of those with an acute rupture making it to hospital.

• In New Zealand mortality for those receiving emergent AAA repair (many do not receive operative care) is around 35% compared to an elective repair mortality of 7% (NZMJ) 2012).

  • Mortality increases with age

• Maori experience AAA events at a younger age, with an event rate 1.5 times NZ Europeans, and a mortality around twice that of NZ Europeans (NZMJ 2012)

RISK FACTORS for AAA

• Older age (Younger in NZ Maori men and women)

• Male

• Cardiovascular disease

  • Risk increases with burden of cardiovascular disease

  • Risk of AAA at a younger age

• Smoker past or present

• Hypertension

• Family history

CLINICAL PRESENTATIONS OF AAA

Ruptured AAA can present in a myriad of ways. Less than 50% of patients present with the classic triad of hypotension, pulsatile abdominal mass, and back/abdominal pain.

Emergency doctors should have a low index of suspicion for AAA especially in the middle/older age groups.

Complications of AAA include:

• Rupture usually with pain

  • retro-peritoneal - may be relatively more stable, may extend

  • intra-abdominal with shock

• Dissection

  • pain

  • organ ischemia

  • limb ischemia

• Thrombo-embolic event

  • limb ischemia

  • distal embolic events

• Aorto-enetric fistula (rare)

SIGNS AND SYMPTOMS

• Collapse or syncope

• Hypotension and shock

• Back and or abdominal pain

• Pulsatile abdominal mass - sensitivity only 68% (Fink et. al. 2000)

• Flank pain - renal colic mimic

• Limb ischaemia, pulse deficits, and embolic events

• Non specific back pain (usually non mechanical in nature)

• Sciatica mimic

• Massive GIB (aorto-enetric fistula)

AORTIC and iliac ANEURYSM

Abdominal:

• A normal abdominal aorta is 2cm or less

• Conventionally an aneurysm is diagnosed when the abdominal Aortic diameter exceeds 3cm or more.

• There are two main types of aneurysm fusiform (the most common) and saccular.

  • The majority occur in the mid-distal aorta

  • Surgically defined in relation to renal arteries as infra-renal (90%), juxta-renal, or supra-renal

  • When involving the distal aorta they may extend into the iliac arteries (10-20%)

Iliacs:

• Isolated iliac aneurysms account for only 2% of abdominal aneurysms

  • 70% of these are the common iliac artery (CIA)

  • 30% Bilateral

• CIA > 2.5cm is considered high risk

  • Male > 1.7cm is abnormal

  • Female > 1.5cm is abnormal

Risk of rupture:

• Risk of rupture for a AAA is dependent on the size and also the rate of growth

  • small aneurysms can rupture !

• < 4cm has a 5yr rupture risk of around 2% (AHA)

• > 5cm the 5yr rupture risk is 25% (AHA) - risk increasing with size

  
  

INDICATIONS FOR AAA POCUS

Bedside ultrasound for AAA is indicated whenever there is clinical suspicion for AAA (see above).

Usually incorporated with bedside imaging for free fluid in the abdomen in the unwell patient.

• Risk factors, signs or symptoms for AAA

• Unstable patient not suitable for transfer to CT

• Setting of undifferentiaed shock

It is an effective, highly accurate, and sensitive tool when carried out in the correct manner:

• Early detection of AAA in the ED can significantly reduce mortality (Costantino et. al. 2005)

  • CT is gold standard but is often not safe or feasible in the unstable or undifferentiated shocked patient

• PoCUS for AAA has a sensitivity of 99% and specificity of 98% (Rubano et. al. 2013)

LIMITATIONS

• Patient and other factors may limit imaging completeness or interpretation

• User dependent

• Limited imaging of any potential retro-peritoneal rupture

• Cannot exclude secondary or other complications such as dissection, organ infarction etc.

Any new abnormal bedside abdominal aortic ultrasound finding should be discussed with a senior ED doctor, and consideration for a CT made.

Even with an apparently normal bedside aorta scan, if there is significant clinical suspicion for aortic pathology a CT should be carried out.

anatomy of the aorta

The Aorta passes through diaphragm at T12 the level of the xiphoid process. It runs to the left of the midline anterior to the vertebral bodies before bifurcating approximately at L4, the level of the umbilicus.

• Proximal aorta: Diaphragm-celiac-SMA

• Mid aorta: SMA to renal arteries

• Distal aorta: Renal arteries to bifurcation
  

MAJOR branches FOR POCUS

Celiac Trunk

• Just below diaphragm

• Common hepatic and spleenic arteries arise off the trunk - Seagull sign

 Superior Mesenteric Artery (SMA)

• 1cm below celiac trunk

• Runs anterior and parallel to aorta in caudal direction

• Good point for a “suprarenal measurement”

 Renal Arteries

• Distal to SMA

• Usually not seen in sagittal views

• <3cm from diaphragm

• 90% of aneurysms originate distal to this point
  
  

ABDOMINAL AORTIC POCUS

PREPARATION

Machine:

• Curvilinear probe with lots of gel

• Abdominal preset

• Initial depth setting around 15cm

Patient:

• supine

• if feasible knees up can relax abdominal muscles

INITIAL ORIENTATION TO THE ABDOMINAL AORTA

Start at the epigastrium in the transverse plane (short axis of the aorta) with the probe marker to the patients right side. Use the liver as a sonic window to help attain image.

• Identify the vertebral body

• Identify the aorta:

  • Superficial to the vertebral body slightly to the patients left

  • To the right of the midline on the screen (check probe marker orientation)

  • Circular in the short axis

  • Pulsatile

  • Thick walled

• Identify the IVC

  • To the left of the midline on the screen - the patients right side

  • Less circular

  • Usually displays collapsability with respiration

Optimise the image:

• Depth - reduce depth so that the vertebral body is just at the bottom of the screen

• Gain - adjust gain so that the aortic lumen is relatively anechoic (black)

• Focus - ensure focus is at the depth of the aorta

Long axis localisation of the aorta:

• If difficulties localising or differentiating the aorta in the short axis the probe can be rotated into the long plane at the epigastrium (probe marker to the patients head)

• Tilt the probe keeping it at the midline sweeping from the patients right to left

• IVC seen to the patients right side passing in close association with the liver receiving the hepatic vein, before passing through the diaphragm into the base of the heart.

• Aorta seen slightly to the patients left side identified by the branches SMA and celiac

  • Sitting anterior to the vertebral bodies

  • Seen by rocking the probe cephalad, the most proximal extent of the the abdominal aorta will dive deep to the liver passing behind the diaphragm

Continue imaging of the aorta (see protocol) from this point in an ordered sequence imaging the aorta in short axis and then long axis. Taking measurements in both planes at the proximal, mid, and distal aorta down to the bifurcation. Ideally the right/left CIA should also be imaged and measured.

ABDOMINAL AORTA PROTOCOL

SHORT AXIS

Start in the short axis (transverse plane) and identify the aorta:

• An initial short axis scan from the xiphoid level distally down to the aortic bifurcation can be done to get a general impression

• A video clip may be taken of this

Measurements of the aorta in short axis should be made in the transverse (Tx) and anterior-posterior (AP) dimensions at the proximal, mid and distal aorta.

• Must be outer wall to outer wall

• Images should show the maximal dimension of the aorta at that level i.e. measure the largest dimension seen

• Save images at each identified important anatomical level with measurements

Proximal Aorta

Locate the aorta at the epigastrium, as described above, in the transverse probe orientation:

• Idenitfy the ‘SEAGULL SIGN’ made by the celiac trunk branches the hepatic and spleenic arteries

  • The celiac trunk may be hard to see - tilt the probe cephalad to help

• Slide down distally to identify the SMA a small circle sitting superficial to the aorta

  • Just distal to the SMA origin, the left renal vein may be seen passing across the midline between the SMA and aorta to enter the IVC

  • The spleenic vein passes across the midline superficial to the SMA to the venous confluence which forms the portal venous system.

• Measure the short axis proximal aorta at the largest dimension between the celiac and SMA (AP and Tx measurements)

Mid Aorta

This is the SMA down to the level of the renal arteries. The renal arteries can be hard to identify so measurements are taken just below the SMA origin

• Renal arteries often < 1cm from SMA origin

• 90% of AAA are infra-renal (below renal arteries)

• Involvement of the renal vessels has surgical consequences but is not an important part of your PoCUS exam

• Measure the mid aorta short axis just below the SMA origin, if identified measure dimensions at the level of the renal arteries

Distal Aorta

This is below the renal arteries to the aortic bifurcation:

• Scan down until the bifurcation is noted

• Measure the largest dimensions noted proximal to the bifurcation

LONG AXIS

Start again in the epigastrium turn the probe 90 degrees clockwise from the transverse orientation to the long axis (probe marker towards the head).

Attain an images from the most proximal aorta (near diaphragm) down to the bifurcation:

• Often can be attained in 2 images

  • Elongate the aorta by rotating the probe slightly to attain the the best/most complete long axis view (opening up the long axis view)

  • Ectatic aortas can be hard to fully visualise across the long axis length so manipulation of the probe may be required to attain dimensions at each level.

• Identify celiac and SMA

• Ensure visualisation down to bifurcation, normal tapering (smaller dimension) of the aorta should occur

Measure AP dimension at proximal, mid and distal aorta

• Measure outer wall to outer wall

• Measure perpendicular to the walls

• Ensure in the midline of the long axis of the aorta

  • A measure off the midline will underestimate the AP dimension

  • This is the cylinder tangent effect (below)

COMMON ILIACS

If possible the common iliacs should be included in your beside aorta ultrasound

• Up to 20% aortic aneurysms extend into the iliacs (unilateral or bilateral)

• Iliac aneurysms account for 2% of primary aneurysms of aorta of which 70% involve the CIAs

  • > 1.5cm female abnormal

  • > 1.7cm male abnormal

• Ideally measure in short axis (AP + Tx dimension) and long axis (AP dimension)

Short axis view:

• Starting at the distal aorta scanning distally until the aortic bifurcation into the right and left CIAs.

  • Measure the AP and Tx dimension

Long axis view:

This can be attained from rotating the probe perpendicular from the short axis view of the each CIA, or by using a coronal view of the aortic bifurcation.

From the short axis probe position for each CIA

• R CIA - rotate the probe clockwise to the long axis, probe marker cephalad to patients left

• L CIA - rotate the probe clockwise to the long axis, probe marker cephalad to patients right

  • Care is required not to confuse the common iliac veins (CIV) with the CIA which can be seen running parallel

  • Colour Doppler can be used to determine directionality of flow to distinguish between the two (Probe marker must be toward patient head)

Coronal view of the CIA

• Identify the level of aortic bifurcation (usually the umbilicus) from normal midline short axis view

• Place the probe in a coronal position at the identified level (probe marker to the head)

TROUBLE SHOOTING ABDOMINAL AORTIC VIEWS

Aortic views can be hindered by a number of issues. The most important of these being bowel gas and significant abdominal obesity. A number of techniques, and alternative sonographic windows can be utilised to help attain views.

Bowel gas

Often encountered in the upper abdomen causing dirty shadowing

Techniques to overcome bowel gas:

• Utilise the liver as a sonic window to help visualise the proximal/mid aorta

  • A deep breath hold can help bring the liver down to increase your window

• Graded constant downward pressure at a static probe position (check patient comfort) applied for a few minutes can help disperse the bowel gas

• Left para-midline short/long axis views

  • Probe placed to left of the midline and angled towards the aorta

• Start imaging distal and return to the proximal aorta, bowel gas can often disperse over time

• Position patient in left lateral decubitus and scan in usual planes

Coronal plane ALTERNATE WINDOWS to the abdominal aorta

Due to bowel gas or obesity sometime alternate windows may be required to visualise the aorta. The most common being right or left coronal views, using the liver or spleen as acoustic windows.

• Limited to single axis measurement

• Often only visualise proximal portion of the abdominal aorta

OBESITY

Sometimes significant abdominal obesity can limit visualisation, a number of methods can help to identify the aorta.

• Place patient in left lateral decubitus moving abdominal mass away from the midline

• Utilise lower frequency probe setting (turn to penetration mode) will help improve depth of penetration

  • Will lead to deterioration of image resolution

  • Machine has limited maximum depth of penetration

• Use colour power Doppler to help identify aorta

  • Still provides information even if probe near 90 degrees incidence to the aorta

  • Will not show any directionality of flow

• Use coronal windows as above

Often imaging of the aorta will be incomplete- if there is clinical concern about potential aortic pathology a CT scan should be discussed and considered with senior ED doctor advice.

PITFALLS AND MEASURING AAA

Measuring AN AORTIC ANEURYSM

• Outer wall to outer wall

• Include any aortic thombus in your measurements

• Ensure maximal dimensions are measured

  • Ensure on plane measurements i.e. At the midline of the short and long axes of the aorta

PITFALLS TO AVOID

• Excluding an aneurysm on the basis of an incomplete ultrasound

  • If your ultrasound views are incomplete you cannot conclude a normal exam

• No AAA does not exclude other aortic pathology - in particular dissection

• An aorta > 3cm is abnormal

  • small aneurysm can rupture, interpret in clinical context

• Not identifying the aorta correctly

  • Ensure accurate orientation and visualisation of both the IVC and aorta at the start of your scan

• Measuring in a single plane

  • May miss eccentric, ectatic, or saccular aneurysms

  • May be difficult to visualise aortic anatomy in a single plane

• Measuring only the true lumen of an aneurysm

  • Outer wall to outer wall including thrombus - best to over estimate