Cardiorespiratory fitness in patients undergoing elective open surgery for abdominal aortic aneurysm: does it really fail to predict short-term postoperative mortality?

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DM Bailey, GA Rose, RMG Berg et al. Cardiorespiratory fitness fails to predict short-term postoperative mortality in patients undergoing elective open surgery for abdominal aortic aneurysm. Ann R Coll Surg Engl 2020; 102: 536–539.

doi 10.1308/rcsann.2020.0120

Dear Sir,

We read with great interest and anticipation the recent paper by Bailey et al titled ‘Cardiorespiratory fitness fails to predict short-term postoperative mortality in patients undergoing elective open surgery for abdominal aortic aneurysm’.¹ Upon examination of the findings, it is apparent that there were no deaths during the study period. As such, it is impossible to predict an outcome that did not occur! We suggest that this study was not appropriately designed or powered to detect the outcome of interest, and that the conclusion, which is reflected in the title of the paper, cannot be substantiated.

We do acknowledge the difficulties in designing a study to detect hard endpoints such as mortality. This is exacerbated in the setting of abdominal aortic aneurysm (AAA) repair, where outcomes may vary between countries and clinical centres and are influenced by the presentation of patients (eg aneurysm size, symptomatic vs. asymptomatic), and the type of repair procedures used (emergency vs elective, open surgery vs. endovascular repair [EVAR]). Data from the Vascular Services Quality Improvement Programme report that the risk of death after elective open infrarenal AAA repair in the UK is approximately 4%.² The study by Bailey et al included 109 patients, 79 of whom underwent open AAA repair performed by a single surgeon at a single centre.¹ While there is a place for single-centre studies, it is unlikely that the outcomes reported are representative of that across the UK, or globally. Importantly, the low number of patients included in their study would not be sufficient to replicate the findings of previous studies that have included much larger cohorts (n=230-415) and demonstrated that measures of cardiorespiratory fitness are able to identify patients at increased risk of death within 30 days following AAA open repair.^3,4

While the authors suggested that advances in patient treatment and surgery strategies may explain the low death rate and therefore render cardiopulmonary fitness a less sensitive predictor of short-term mortality, we maintain that the study was too small and was underpowered.

Cardiopulmonary exercise testing (CPET) provides the opportunity to assess various markers of cardiorespiratory fitness, including peak oxygen consumption (V̇O₂peak), ventilatory threshold (V̇T), and the ventilatory equivalents for carbon dioxide (V̇E/V̇CO₂) and oxygen consumption (V̇E/V̇O₂). The prognostic sensitivity of cardiorespiratory fitness differs depending on the measurement that is used,⁵ and is specific to the population and the conditions of the CPET protocol^6,7; and in the case of AAA repair it is also specific to the type of repair procedure used (open vs EVAR).⁸ Bailey and colleagues are commended for investigating the influence of clamp position (infrarenal vs. suprarenal) on the sensitivity of cardiorespiratory fitness to detect morbidity outcomes following open AAA repair. However, the authors applied a generic dichotomous threshold (V̇O₂peak <15ml.kg^-1.min^-1) to characterise ‘unfit’ patients, based on previous recommendations that have been established for patients with lung cancer.⁹ The authors did not report V̇T, and notably ventilatory (or anaerobic) threshold measures have previously been associated with 30-day postoperative mortality in patients undergoing open AAA repair.^3,4

Given the emerging and important role of CPET in the risk assessment of patients with abdominal aortic aneurysm, it is essential that clinical decision making is based on robust evidence. While the study by Bailey et al adds some new insight into the impact of cardiorespiratory fitness on morbidity following open AAA repair, the study was not sufficiently designed or powered to investigate the impact on mortality.¹, As such, the conclusion of the study cannot be substantiated, and we suggest that the title and conclusion of the paper should be corrected to more accurately reflect the outcomes of the study.