Decision tools in vascular surgery - nhmrc.gov.au

Decision tools in vascular surgery

Rob Fitridge1, Prue Cowled1, Nicholas Dawson2, Maggi Boult1, Mary Barnes3

1. University of Adelaide, Department of Surgery, QEH, Adelaide, SA
2. CSIRO , ICT, The Australian e-Health Research Centre

3. CSIRO, Mathematics Informatics and Statistics , Glen Osmond, SA

ABDOMINAL AORTIC ANEURYSMS 2

• Abdominal aortic aneurysm (AAA)
is a condition where the aorta
dilates from about 1.5-2.5cm to
3.5-10cm or larger

• The risk of rupture increases with
increasing size (30-50% of
aneurysms over 8cm will rupture /
year)

• Rupture typically results in death
• Treatment options include

watchful waiting, open repair and
endovascular repair

In Australia the preferred treatment modality has changed significantly:
• In 2000 there were 2479 procedures of which 35% were EVAR
• In 2010 there were 2847 procedures of which 75% were EVAR

University of Adelaide

Who is affected by abdominal aortic aneurysms?

• Men (5% men over 65)
• Increasing age increases risk
• Smoking
• Family history
• Degenerative not atherosclerotic
• 1000 deaths from rupture per year in

Australia

University of Adelaide 3

OPEN AAA REPAIR

University of Adelaide 4

ENDOVASCULAR ANEURYSM REPAIR (EVAR)

University of Adelaide 5

Consequences of AAA repair

Perioperative mortality:
• After emergency repair for a rupture is 30-60%
• After elective open repair ranges from less than 2% to more than 12% (mean

pooled value 5.5%)*
• After elective EVAR repair ranges from 0.5% to more than 10% (mean pooled

value =3.3%)*

But we have found that much of the variation in outcomes after EVAR can be
attributed to differences in factors that were known before the operation.

Our aim is to provide information about an individual patient’s risk and
make this information available to surgeons and their patients in order to
assist the preoperative decision making.

*Brown LC, Powell JT, Thompson SG, Epstein DM, Sculpher MJ, Greenhalgh RM. The UK Endovascular Aneurysm Repair Trials: 6
randomised trials of EVAR versus standard therapy. Health Technol Assess 2012;16(9)

University of Adelaide

Development of the Australian EVAR Risk Assessment
Model (ERA MODEL)

• Data from an Australian audit of endovascular repair of abdominal aortic
aneurysms

• 961 patient enrolled and progress followed several years
• Statistical analysis of results by CSIRO statistician to determine whether

pre-operative variables were strongly associated with any adverse outcomes
• Showed linkage of adverse outcomes with 8 readily available

preoperative variables
• 17 outcomes were predicted in the ‘ERA’ Model
• Developed a simple decision tool that surgeons and patients can use
• Available as an i-phone application through Apple store

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The infrarenal abdominal aortic aneurysm

Infrarenal neck 8
Aneurysm

Iliac arteries

University of Adelaide

Pre-operative variables used to develop each outcome model

Pre-operative variable Aneurysm diam.
Age
Outcome . ASA

Gender
Creatinine
Aortic Neck Angle
Infrarenal neck diam.
Infrarenal neck length

3 year survival <0.001 <0.001 <0.001 0.002
0.008 <0.001 <0.001 <0.001
5 year survival <0.001
0.001 0.030
Aneurysm related death 0.070

Early death 0.057

Initial re-interventions

Mid-term re-interventions 0.045 0.029 0.014

Initial endoleak Type I 0.007

Mid-term endoleak Type I 0.005 0.130

p-values displayed as easier to understand than reduction in AIC, which was criterion used for inclusion.
Some p-values >0.05, but reduction in AIC was significant and therefore included.

University of Adelaide 9

ERA Model (EVAR) 10
The app for iPad looks like this

• Patient details are entered into
the left hand column

• The predicted outcomes are
displayed in the right hand
column

• By selecting an outcome,
additional information is
provided at the base of the right
hand column

• This information can be enlarged

University of Adelaide

This is the information displayed 11
when you press

University of Adelaide

This is the information displayed 12
when you press

University of Adelaide

And this is the information 13
displayed when you press

University of Adelaide

ERA MODEL VALIDATION

• Internal validation – uses bootstrapping
• External validation – uses data from other sources

Internal validation :

Barnes M, Boult M, Maddern G, Fitridge R. A Model to Predict Outcomes for Endovascular Aneurysm Repair Using Preoperative Variables.
European Journal of Vascular and Endovascular Surgery. 2008;35(5):571-579.

External validation:
St George’s Vascular Research Institute (London, UK)

Barnes M, Boult M, Thompson MM, Holt PJ, Fitridge RA. Personalised predictions of endovascular aneurysm repair success rates:
validating the ERA model with UK vascular institute data. European Journal of Vascular and Endovascular Surgery. 2010; 40(4) 436-441.

Royal Brisbane & Women’s Hospital (QLD, Australia)

Brendan Wisniowski B, Barnes M, Jenkins J, Boyne N, Kruger A, and Walker P. Predictors of outcome after elective endovascular
abdominal aortic aneurysm repair and external validation of a risk prediction model. Journal of Vascular Surgery. 2011;54(3):644-653.

University of Adelaide 14

Comparing predictor variables: Australia vs St George’s

Outcome Australia St George’s p-value
N=961 N=312

Male ratio 86% 90% <0.001
Age 75±6.9 77.4 ±7.8 0.79
ASAII 32% 24% <0.001
ASAIII 59% 48% <0.001
ASAIV 6% 27% <0.001
Aneurysm size 58mm 64mm <0.001
Aneurysms <55mm 44% 19% <0.001
Creatinine µmol/L 115 118 0.48
Infrarenal neck length 25.7mm 23.7mm 0.018
25% 54% <0.001
≤20mm 23.6mm 23.7% 0.70
Infrarenal neck diameter 15.6% 30% <0.001
Aortic neck angle ≥45

Results in percentages unless otherwise indicated | Bolding denotes statistically significant differences 15

University of Adelaide

Comparing outcome rates: Australia vs St George’s

Outcome Australia St George’s p-value

Death 1.8% 4.2% 0.003
Early 2.6% 4.8% 0.03
Aneurysm related
81% 69% <0.001
Survival 68% NA
3-year 0.12
5-year 2.9% 3.2% 0.118
4.0% 3.2%
Endoleak I 0.05
Initial 7% 4.8% 0.055
Mid-term 12% 9.7% <0.001
32% 41% 0.07
Endoleak II 11.6% 11.3%
Initial 16
Mid-term

Initial re-intervention
Mid-term intervention

Results in percentages | Bolding denotes statistically significant differences

University of Adelaide

Validation methods for St George’s data

• Predictions made using St George’s data predictor variables and
Australian ERA model coefficients

• Goodness of fit of models assessed with Frank Harrell’s Design Package
(val.prob function used to compare predicted values with actual observed
outcomes)

• Goodness of fit of St George’s outcomes assessed using area under ROC
curves and R2 statistic

• Predictions were made for actual value of each predictor for each patient
(11% of UK data was higher than the upper limit of the Australian region
of applicability)

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Validation results for St George’s UK vascular unit

Even though St George’s patients were sicker, had larger aneurysms, more difficult anatomy and were
more likely to die the ERA model provided a comparable fit for early death, aneurysm related death, 3-
year survival and mid-term type I endoleaks

Evidence: higher area under ROC curves and /or R2 goodness of fit statistic

1

Area under ROC curve 0.5
Australia
St George Vascular

0 Aneurysm 3-year Initial type- Mid term Close to 1 suggests a good model
related survival 1 endoleaks type-1 Close to 0.5 is a poor model
Early death
death endoleaks

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Current directions – ERA Model

• New Australian data has been collected (operations between 2009-2012)
• CTs obtained for most patients
• Additional data from overseas to aid with validation
• Aim to determine whether original model can be improved on by using data

from more recent procedures
• Adding in new outcomes such as 12 month survival
• Reviewing whether other preoperative variables can be useful indicators for

outcomes

We are also reviewing measurements of iliac tortuosity and
calcification for modelling purposes …

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TORTUOSITY AND CALCIFICATION

• The EVAR graft is inserted via the iliac arteries which vary from person to
person.
– Some are full of twists and turns (tortuous)
– Some have a lot of hardening (calcification)
– Some are narrower than others

• We propose that treatment outcomes may be linked to the amount of
calcification and tortuosity found in a patient’s iliac arteries

• Currently surgeons make subjective assessments of tortuosity and calcification

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TORTUOSITY & CALCIFICATION PROJECT

Objective:
• Develop algorithms to measure severity of calcification and tortuosity from

contrast-enhanced CT images that are repeatable and accurate
• Apply measures to EVAR trial data and examine for statistical associations

with adverse outcomes following surgery
• If measures are predictors of outcomes, determine how to make tools

widely available

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MATERIALS AND METHODS

• Initially… 22

– Classic tortuosity measures using
curvature

• Max curvature
• Sum of curvature
• Straight line ratio: 1/kL

– Classic calcification measures

• Looking for correlations with:

– Early problems
– Late problems
– Problems 1/6/12/24/36 months

Early problems: failed deployment, graft complications (i.e. kink / occlusion), endoleak, unplanned procedures, conversion to open

University of Adelaide

Initial Results. Correlation between:
Max curvature and Early problems

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PROGRESS TO DATE

• Seeded vessel segmentation, with medial line and surfaces, working
reliably on high contrast CT images

• Three “classical” tortuosity measures implemented

• Working on further speed-ups for data processing and on
improvements to handle CT data without visible contrast agent

• Assessing different methods of measuring calcification

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NEXT STEPS

• Statistically review measures with known outcomes for
patients (underway)

• Compare measures with surgeon subjective measurements

Known outcomes include:
• Operative problems such as failed deployment, misplaced

deployment, endoleaks, graft migration
• Post operative problems such as mortality, endoleaks, kinking

& migration of graft, stenosis, surgical procedures related to
graft problems etc

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Contacts 26

Professor Rob Fitridge [email protected]
OR

Tania de Loryn (project officer)
Ph: 08 8133 4015
F: 08 8222 7872

Email: [email protected]

Basil Hetzel Institute
28 Woodville Rd

Woodville South, SA 5011

Information about the audit can be obtained from website:
http://health.adelaide.edu.au/surgery/evar/

University of Adelaide

Acknowledgements

NHMRC for funding this research
AIHW National Death Index

Professor Phil Walker & Gillian Jagger (QLD)
Professor John Fletcher & Kerry Hitos (NSW)

Professor Michael Grigg (VIC)
&

All contributing surgeons, both past and present

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Questions?

Thank you for your time