Anaerobic Threshold and Risk of Peri-operative Adverse Events in AAA Surgery

1. Does Anaerobic Threshold predict risk of peri-operative adverse events following Abdominal Aortic Aneurysm surgery? Dr Sian Davies SpR Anaesthetics James Cook University Hospital, Middlesbrough

2. Anaerobic Threshold Represents the oxygen consumption at which anaerobic metabolism begins to supplement aerobic pathways to generate energy.

3. Background • Cardio-Pulmonary exercise testing (CPET) used to define anaerobic threshold (AT) levels to risk stratify patients • Older (1999) – AT > 11 Low risk AT < 11 High risk • Carlisle, Swart (2007) –mid-term survival correlated most closely with Ve/VCO2 and AT to a lesser degree (open AAA repair).

4. Aim To investigate if AT values derived from our patient population undergoing AAA surgery (open or EVAR) define risk of adverse outcome.

5. Methods • Patients who had undergone pre-op CPET and subsequent AAA repair were identified • Surgical intervention, post-op morbidity + mortality, and length of stay (LOS) data were collected • AT values established for all patients by a single blinded observer (V slope method) • Statistical analysis – simple descriptive statistics and ROC analysis

6. CPET testing

7. Adverse event • Cardiac –acute coronary syndrome, arrhythmia, LV dysfunction • Respiratory – failure, infection • Metabolic / Renal –need for dialysis or CVVH • Surgical complications NOT included in analysis

8. Results • 115 patients – 62 open repair 53 EVAR • 30 day mortality: 2.6% (3/115) • Mean AT = 10.3mlsO2/kg/min (sd 3.3)

9. Open AAA repair 62 patients no morbiditywith morbidity 30 32 30 day mortality 3 patients 30 patients29 patients Mean AT (SD) 11.7 (3.2)9.4 (3.5) Median LOS (range) 11.0 (7 – 31)13.5 (8 – 39)

10. EVAR 53 patients No morbidityWith morbidity 42 patients11 patients Mean AT (SD) 11.2 (3.3)10.5 (1.8) Median LOS (range) 4.0 (3 – 10)11.0 (5 – 21)

11. ROC analysis for open AAA • AT cut off at 11.1mls/O2/kg/min • Sensitivity 71% (low AT & morbidity) • Specificity 62% (high AT &no morbidity)

12. Open AAA AT ≥ 11.1 AT < 11.1 Number patients 24* 26* Incidence morbidity 7/24 = 29.1% 17/26 = 65.4% LOS (median) 10 days 13 days * = AT not achieved, unreadable or data missing for some patients, therefore not included in analysis.

13. EVAR AT ≥ 11.1 AT < 11.1 Number patients 20* 26* Incidence morbidity 4/20 = 20% 6/26 = 23% LOS (median) 4 days 5 days * = AT not achieved, unreadable or data missing for some patients, therefore not included in analysis.

15. Discussion • Adverse outcome after both types of aneurysm repair was associated with lower mean AT and increased LOS

16. Discussion – open AAA • Cut off for stratification between low and high risk is AT of 11.1mlsO2/kg/min in our patient population • Consistent with previous work • Reinforces AT values currently used to assess risk utilising CPET for open AAA patients

17. Discussion - EVAR • Incidence of post-operative morbidity was low after EVAR • Patients with low AT seemed to do well • Further work based on larger patient numbers is needed to define the risk stratification of EVAR patients.

18. References • Older P, Hall A, Hader R. Cardiopulmonary exercise testing as a screening test for perioperative management of major surgery in the elderly. Chest 1999. 116: 355 – 363 • Carlisle J, Swart M. Mid-term survival after abdominal aortic aneurysm surgery predicted by cardiopulmonary exercise testing. British Journal of Surgery 2007. 94/8: 966 - 999

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