Ultrasound: AAA
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Transcript Ultrasound: AAA
FOCUSED EMERGENCY ULTRASOUND:
EVALUATION OF THE ABDOMINAL AORTA
MARY BETH PHELAN, MD, RDMS
DEPARTMENT OF EMERGENCY MEDICINE
FOREDTERT MEMORIAL HOSPITAL
Lecture Objectives
Describe clinical role of bedside
ultrasound in screening for AAA
Describe the technique of acquiring
sonographic images of the aorta
Describe the sonographic appearance
of the normal aorta
Describe the sonographic appearance
of AAA
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Case History
A 62-year-old man comes to the
emergency department at 11PM
complaining of left flank pain for
approximately 2 hours. He has a
history of hypertension.
His initial vital signs are: HR 98, RR 24,
BP 190/105, Temp 98.0.
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Case History
The emergency medicine resident
equipped with the latest in emergency
medicine ultrasound technology and
training, IMMEDIATELY performs an
abdominal ultrasound on the patient.
This exam reveals the following:
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Case History
The patient is taken to the OR after only
30 minutes in the ED.
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OVERVIEW
Epidemiology
Clinical presentation
Anatomy
US exam
Sonographic anatomy
Scanning techniques
Pitfalls
Epidemiology
AAA present in 2-4% of the population >
50
Incidence increasing
Male > female
10,000 deaths/yr
Rupture has a > 80% mortality rate
Epidemiology: Risk Factors
Cardiovascular disease
Family History increases risk 10-20%
Age > 50
Smoker
Clinical Perspective
Settings in which to perform US in the ED
–
Abdominal/back/flank pain and hypotension
–
Stable elderly patient with abdominal or
back pain
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Clinical Perspective
Rate of expansion variable
– 4-4.9 cm AAA has a 3.3% risk of rupture
– 5cm AAA has a 14% risk of rupture
– > 5cm has a 20-40% risk of rupture
Clinical Perspective
4cm or less: annual US examinations
Between 4-5 cm: US every 6 months
Greater than 5cm: Elective repair
Mortality rate for elective repair is 5%
Clinical Presentation
Highly variable
Classic triad:
– Abdominal/Back pain
– Pulsatile mass
– Hypotension
Less than 1/3 of patients will have the
triad
Clinical Presentation
Diagnosis
– A formidable clinical challenge
– Notorious for masquerading as renal colic
– May be mistaken for:
Diverticullitis
GI bleed
MI
Musculoskeletal back pain
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Clinical Presentation
Stable vital signs
Back or flank pain, left side > right
Testicular or leg pain
Hypertension
Mortality rate same as elective repair
Clinical Presentation
Vast majority are retroperitoneal
10 -30 % intraperitoneal
GI bleeding most often seen in patients with
aortic grafts
Mortality 50%
Does this patient have an abdominal
aortic aneurysm?LEDERLE, JAMA 99
2 groups
Sensitivity of examination for ruptured
AAA
Sensitivity of exam with increasing
size of AAA
CONCLUSION:Cannot be relied on to
exclude AAA
Misdiagnosis of Ruptured Abdominal
Aortic Aneurysms
MARSTON W ET AL J OF VASCULAR SURG 1992
Misdiagnosis= delay >6hr or other diagnosis
Most common physical findings in
misdiagnosed group: ABD PAIN, SHOCK,
BACK PAIN
Pulsatile mass present more often in correctly
diagnosed group
SUSPECTED LEAKING ABDOMINAL AORTIC
ANEURYSM:USE OF SONOGRAPHY IN THE
EMERGENCY ROOM SHUMAN WP, ET AL, RADIOLOGY 88
US IN ED FOR SUSPECTED AAA
1 MIN EXAM
CORRECTLY IDENTIFIED 31/32 AAA
DECISION TO OPERATE BASED ON 3
CRITERIA CORRECT 21/22
DX EXTRALUMINAL BLOOD BY
SONOGRAPHY POOR 4% (1/24)
NO FALSE NEG EXAMS
Diagnosing AAA
Palpation of the abdomen alone
Plain radiographs
Computed tomography
ULTRASOUND
Diagnosis: PE
Absence of mass does not R/O AAA
Obesity
Bleeding into retroperitoneum may create
doughy abdomen.
Hypotension minimizes pulsations
Diagnosis: Plain Radiographs
AAA can be seen in 60-75% of cases
Calcification of aortic wall
Paravertebral mass
Cross table lateral most helpful view
Negative study not helpful
Diagnosis: CT Scan
Near 100% accuracy
Better demonstration of extent of aneurysm
Will detect complications of the aneurysm
– Retroperitoneal blood
– Dissection
Drawbacks
– Contrast
– Patient has to leave the ED
– Delays time to diagnosis
Diagnosis: US
Ultrasound
– Best test for detection of AAA in the ED
– Sensitivity 97% to 100%
– Small percentage can not be imaged due
to bowel gas
6% in one study
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Diagnosis: US
Ultrasound
– In some studies as accurate as CT
– Measurements within 3 mm of surgical
specimens
– Angiography may underestimate AAA
diameter
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Diagnosis: US
Emergency department ultrasound scanning
for abdominal aortic aneurysm: accessible,
accurate and advantageous
Kuhn et al. Ann Emerg Med 2000
“Relative neophytes can perform aortic ultrasound
scans accurately. These scans appear useful as a
screening measure in high-risk emergency patients;
they may also aide in rapidly verifying the diagnosis
in patients who require immediate surgical
intervention”
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Diagnosis: US
ED Ultrasound Improves Time to Diagnosis
and Survival in Ruptured AAA
Plummer D, et al: Abstract at 1998 SAEM, Chicago, IL.
•Average
minutes
•Average
•Average
minutes
•Average
minutes
time to diagnosis by bedside US = 5.4
time to diagnosis by CT = 83 minutes
time to OR for diagnosis by US = 12
time to OR for diagnosis by CT = 90
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US EXAM
Transducer is 2.5-3.0MHz curvilinear
Place the transducer in the subxiphoid area,
using the left lobe of the liver as an acoustic
window
Pressure must be applied to displace bowel
gas
The aorta must be examined in both the
longitudinal and transverse planes
Longitudinal
Orientation
Marker
Transverse
Orientation
Marker
Orientation
is similar to
that of a CT scan
Position
probe is
perpendicular to long axis
of body or to long axis of
object that is being
IVC,Liver
studied
Aorta
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US EXAM
The aorta appears as an anechoic, pulsatile
tubular structure to the left of the spine
After the longitudinal scan, the transducer is
rotated 90 degrees to the aorta to obtain
transverse views.
The key landmark in the transverse view is to
locate the spinal column as a hypoechoic
area at the bottom of the screen.
The aorta is located above and to the left of
the spine
AORTA
Left sided structure
Thick vascular wall
Not compressible
Pulsatile
IVC
Right sided structure
Thin wall
Will collapse
– “Sniff”
– Valsalva
May pulsate from
aortic transmission
US EXAM
Measure from outside wall to outside
wall
An aneurysm is identified as any
measurement of 3 cm or greater
Measure at:
– Epigastric region
– Take off of SMA
– 3-4 cm intervals to bifurcation
Measure any aneurysm
US EXAM
Obesity or excessive bowel gas may obscure
the aorta
A coronal view of the aorta may be a
reasonable alternative
The patient is supine
The transducer is placed in the mid-axillary
line (probe indicator toward the patient’s
head)
The aorta is visualized adjacent to the vena
cava
SONOGRAPHIC APPEARANCE OF
THE NORMAL AORTA: LONGITUDINAL
SONOGRAPHIC APEARANCE OF
THE NORMAL AORTA:
TRANSVERSE
Mid portion
Bifurcation
SONOGRAPHIC APPEARANCE OF THE
NORMAL AORTA
(L LATERAL DECUB/CORONAL)
ABDOMINAL AORTIC ANEURYSM
90% of AAA are infra-renal
70% involve the renal vessels
Thrombus is common, and usually forms on
the antero-lateral walls of the aneurysm
Two forms
– Sacular
– Fusiform – most common
ABDOMINAL AORTIC ANEURYSM
First sign may be loss of normal taper
AP diameter > 3CM
Focal dilitation even if less than 3 cm
Thrombus
Intimal flap
AORTIC ANEURYSM
Large fusiform AAA
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AAA with clot
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Another AAA with clot
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ULTRASOUND EXAM: PITFALLS
Bowel gas can be a major problem
– Apply pressure
– Roll the patient on their left side ( use the liver as
an acoustic window)
Does not detect complications of AAA
– Retroperitoneal rupture
– Dissection
CT/MRI/angiography for stable patients is still
recommended
Pitfalls in Technique
Failure to acquire high resolution images due
to bowel gas
Inaccurate measurements – do not measure
what you cannot see!
Distinguishing the IVC from the aorta
Not identifying extraluminal fluid
Failing to distinguish the normal “tortuous”
aorta from an abdominal aortic aneurysm.