AAA Flashcards
What is an anuerysm?
An aneurysm is a bulge in a blood vessel caused by a weakness in the blood vessel wall, usually where it branches. As blood passes through the weakened blood vessel, the blood pressure causes a small area to bulge outwards like a balloon.
At least 50% localised and irreversible dilatation of a blood vessel when compared to the normal expected diameter
“Ballooning”
What parts of the arterial wall are affected by aneurysm?
All 3 walls affected
What is normal aortic diameter?
1.4-2.5cm
Important to take into account natural variation of vessel diameter in different patients, largely due to size, age, sex differences.
Cause/ formation of AAA
Results from a combination of factors including:
- degradation of the elastic lamellae
- loss of smooth muscle wall
- enhanced proteolysis (of collagen & elastin)
- leukocytic infiltration & necrotic inflammation
Decreased wall strength/increased wall stress expansion or rupture
Prevalence:
1 in 100 men >65years old UK
5k deaths per year UK
Major risk factors:
Smoking
Family history ~15% incidence (male)
Male gender 6:1
Age
Other risk factors:
Hypertension
Chronic obstructive pulmonary disease
Hyperlipidaemia
Types and shapes of aneurysms
Fusiform- most common (dilation across entire cross section of vessel
Saccular – localised bulging of vessel wall
Bi-lobed
Mycotic aneurysm – an aneurysm caused by an infectious agent, mainly bacteria, which affects the blood vessel walls
Tortuous - due to elongation of aorta
Pseudoaneurysm- A pseudoaneurysm, also known as a false aneurysm, is a pool of blood that forms in a small sac attached to one side of an artery. It occurs when there is a breach to the arterial wall, resulting in an accumulation of blood between the tunica media and tunica adventitia of the artery. Unlike an aneurysm, a pseudoaneurysm only includes one or two layers of your artery wall. Treatment may be necessary to prevent rupture and serious complications
Inflammatory aneurysm – characterised by extensive peri-aneurysmal fibrosis and thickened walls.
Dissected aneurysm – tear occurs within the wall.
where can aneurysms be located compared to the renal arteries?
Suprarenal -above renal arteries
Pararenal -involves the level of the renal arteries
Juxtarenal - starts at the level of the renal arteries
Infrarenal - starts below the level of renal arteries (most common - 90% AAA infra-renal)
Symptoms of AAA
Unruptured AAA - mostly asymptomatic**
- Back or abdomen pain
- pulsatile mass
- limb ischaemia
- Ureterohydronephrosis
Ruptured AAA
- Abdominal, back or loin pain – maybe sudden
- Syncope, shock or collapse
Pain – due to pressure on surrounding structures.
back pain could be due to erosion of vertebral bodies.
Ureterohydronephrosis – dilation of the ureter due to mechanical or inflammatory obstruction - compression of the ureter, by the aneurysm, from the kidney. Mainly occurs on the left side.
AAA risks
Continued growth
Rupture
Limb/ digit ischaemia (from thrombosis)
Growth rate: 0.33cm/ year
Rupture rate:
- 4 – 5.5cm - 1%
6 – 6.9cm - 10%
>6.9cm - 32%
Rupture mortality: 65 – 85%
Rapture rates seem to increase in female patients (4x).
High rupture rates in patients who smoke, with hypertension and those with a family history.
Investigation
NHS AAA screening programme – Ultrasound scan for men at 65 years
If AAA found, patient is referred to vascular surgeon
Why ultrasound?
Cheap
Quick
Non-invasive
Accurate (2mm variation)
No radiation exposure
Other modalities
Computed Tomography (CT)
Used to plan surgical intervention to demonstrate relationship of aneurysm to major branches and structures within the body
Can show rupture
Magnetic Resonance Imaging (MRI)
Safer than conventional angiography, as it does not use nephrotoxic contrast medium
Both modalities allow 360° view which can help to gain a better perspective of shape of aneurysm.
Information required by surgeon
The maximum diameter of aorta (outer-to-outer and inner-to-inner)
Type and shape of aneurysm
Position
Thrombus presence and percentage load
Scan limitations
- If not usual fusiform, mention the type/ shape.
If mobile areas of thrombus are noted mention – could embolise. The appearance of micro-embolic lower limb infarcts in a patient with easily palpable pedal pulses suggests a popliteal or abdominal aneurysm.
Liquefaction within thrombus – may indicate imminent rupture.
Scanning protocol
Probe types – C5-1, 9-2 curved array
Measurements
Diameter in centimetres (cm)
Scanning protocol
Consider manual handling
B-Mode sweep in both Longitudinal (LS) and Transverse (TS) orientation. This allows the sonographer to take into account vessel positon and tortuosity before taking measurements.
Assess full length of abdominal aorta using B-Mode and Colourflow (making sure to optimise image)
Measure the cross-sectional AP and ML diameters in transverse and cross-sectional AP in longitudinal planes
Aorta is aneurysmal if >3cm or if there is a 50% focal increase in diameter. If the aorta is aneurysmal, a scan of the iliac, common femoral and popliteal arteries should be routinely performed
Always measure AP of the aorta even when doing a scan for disease
Challenges/scan limitations
Bowel gas – ask patient to cough, scan from different plan (coronal plane)
Depth
Vessel tortuosity
Poor vessel resolution
Acoustic shadowing from plaque
If using curvilinear C9-2, use a lower frequency probe if available, dynamic control, adjust focus
More accurate in longitudinal plane
Poor posterior wall resolution when tissue btn posterior wall and lumar spine appear to merge.
Patient BMI
Management
Surveillance:
3.0-4.4 cm: ultrasound every 1 year (NICE guidelines recommend following NHS screening pathway)
4.5-5.4 cm: ultrasound every three months
5.5 cm or bigger – consider surgery (Refer people with an AAA that is 5.5 cm or larger to a regional vascular service, to be seen within 2 weeks of diagnosis. - NICE 2020)
Growth action:
>5mm in 6 months – flag up to vascular surgeon
>1cm in 1 year – flag up to vascular surgeon
AAA surgery - open repair
1) general anaesthesia
2) midline incision
3) AAA identified
4) proximal control - clamp aorta (preferably below renal arteries)
5) Distil control - clamp common iliacs
6) aneurysm opened and any thrombus removed
7) a prosthetic graft is anastomosed to either end of affected section
8) clamps removed and blood flow restored
Risks and comps - open repair AAA
infection
blood cot in legs
bleeding
kidney failure
respiratory failure
injury to colon’s blood supply
erectile dysfunction
spinal cord injury
heart attack, stroke or death
Endovascular aneurysm repair (EVAR)
Endovascular aneurysm repair (EVAR) involves introducing a stent-graft system through the femoral arteries, which relines the aneurysm, diverts blood flow through the endograft and allows the aneurysm to thrombose. This requires that the aneurysm should have an adequate (1.2 cm) ‘neck’ below the renal arteries for stent fixation.
- EVAR has been the more preferred surgeries over the past few years (65% patients were considered suitable).
Ideal case for EVAR
Neck length – >10-12mm
Neck diameter – 18-32mm
Aortic angulation – ≤60
Minimal vessel calcification
Iliac diameter:
CIA – 8-22mm
EIA - >7mm
CIA length - >20mm
Anatomical constraints limit the use of EVARs in 30-40% of patients
- short neck
- excessive angulation
- aneurysm involvement with aortic side branches
Non-ruptured AAA management
2020 NICE guidelines: open repair if suitable (EVAR falling out of favour)
Offer EVAR if open repair is unsuitable due to anaesthetic/medical condition
Open repair doesn’t require follow-up, no risk of endoleaks
Cons for open repair:
Extensive dissections
Prolonged visceral ischaemia
Require higher clamp site
More extensive reconstruction
Fenestrated-EVAR (FEVAR)
Holes in main graft allow insertion of stents into side branches
When is FEVAR used?
AAA with inadequate/ short necks
Renal arteries or target arteries have to arise from a normal segment of the aorta to ensure no gap between the fenestration and target vessel.
Fenestrated EVAR – holes in the fabric of the stent-graft, reinforced by nitinol ring to facilitate catheterization, prevent fraying of the fabric and allow attachment of a side alignment stent.
Alighnemt stents are typically inserted into the side branches to prevent vessel occlusion or stenosis from misalighnemnt between the fenestration and origin of th target vessel.
- Fenestration branches are usually connected with balloon-expandable stents.
Branched-EVAR (BEVAR )
Pre-sewn cuffs for placement of stent-grafts into side branches
When is BEVAR used?
AAA with involvement of side branches
Target branches originate from the aneurysm (there is a gap between the main stent-graft and the aortic wall).
Pros and cons of EVAR
Advantages:
Avoids open abdominal surgery
Avoids aortic cross clamping
Reduced mortality in the first four years of follow-up; longer-term results are unknown
Disadvantages:
Follow-up with ultrasound or CT scans is essential to monitor the endograft (to ensure no sac growth or endoleak)
Failure of the endograft can occur - this was a common problem with early grafts but the newer designs are more durable
Randomised controlled trials (RCTs) suggest reduced mortality in the first four years of follow-up. However, rates of death from any cause appear to equalise over the years, due to stent graft-related complications
No long-term survival benefit of EVAR compared to open repair
Significantly higher risks of re-intervention and aneurysm rupture after EVAR
EVAR endoleaks
An endoleak is defined as flow outside the endograft but within the aneurysm sac
Can cause continued sac growth: surveillance is important
5 types of EVAR endoleak:
I) Leak from proximal/distal graft attachment sites
Treated by stent extension/cuff/surgical intervention/conversion to open repair
II) Branch or reconstitution leak, whereby the excluded sac fills and empties via one or multiple aortic side branches. (IIa- single vessel, IIb- two vessels or more). Treated by embolization/open surgery to tie off branch. Might not need treatment if leak is stable
III) Structural failure of the endograft (IIIa- Limb disconnection, IIIb- Graft fabric disruption) Treated by relining/extending with covered stent, or open conversion
IV) Graft porosity. Often occurs immediately after procedure. Not seen as frequently with modern materials. Usually settles by itself once clotting settles – no treatment required
V) Endotension - (continued expansion of the aneurysm sac greater than 5 mm,without radiographic evidence of a leak site). Diagnosed if growth seen but no other cause found. Treated by endograft reinforcement or open conversion
EVAR surveillance
Pre-discharge - Duplex
1 mo - Duplex
3 mo -CT
6mo - Duplex
9mo - Duplex
1 year - CT
6mo/annually - Duplex and x-ray
CT is done if significant problem is detected during Duplex scan eg >0.5cm in sac size.
EVAR scan protocol
Probe types – C5-1MHz curved array, C9-2MHz curved array
Measurements – velocities in cm/s, diameter (anterior-posterior AP, medial-lateral ML) in centimetres
The stent graft/aneurysm is visualised and traced to the proximal margin. The neck of the aneurysm is measured in a longitudinal plane, to ensure oblique angles are not obtained, (AP). With a low colourflow velocity or power Doppler colourflow setting care is taken to ensure no leak is present at the proximal attachment site.
The aortic aneurysm sac diameter is measured in both transverse (AP x ML) and longitudinal (AP) planes.
Measurement of the aortic diameter should be taken by both inner wall to inner wall and outer to outer wall. The diameter is measured at the transition between the bright echogenic echoes of the inner or outer anterior and posterior aortic wall boundaries and the more echolucent lumen or thrombus.
B-Mode images, Colorflow images and Doppler spectral waveforms are obtained of the aneurysm, graft body/neck, bilateral graft limbs and bilateral external iliac arteries. This is to ascertain that there are no impingements, compression, kinking, thrombosis, stenosis, effacement of attachment sites or dislocation of limbs and endoleaks.
Expected waveforms in abdominal aorta
-Has high resistance flow (Aorta perfuses both low resistance abdominal organs and high resistance lower extremities).
Classification of abdominal arterial stenosis
Diameter reduction
0-49% Mild
50-74% Mod
75-99% Severe
Renal arteries
2-4% of hypertensives adults caused by Renal Artery Stenosis (RAS)
US can be used as first line imaging
Many limitations including: Lack of agreement on what constitutes a significant stenosis
