Degree Name

Doctor of Philosophy


School of Medical, Indigenous and Health Sciences


Chapter 1 provides an overview of the themes of preoperative assessment and risk prediction in patients with severe obesity presenting for non-bariatric surgery.

Chapter 2 describes the natural history of patient weights while on waiting lists for surgery. Patients can spend 3 months or longer on surgical waiting lists between the date of booking for surgery and performance of surgery. Whether or not they tend to gain or lose weight during this time is not known. In this chapter, the course of weight change over a surgical waiting period for all patients who had Category C (more than 3 months) or D (patient not yet ready) surgery in ISLHD in 2016 was explored. Booking dates, surgery dates, booking category, name of operation, and weight recorded at both booking of surgery and performance of surgery were extracted from the electronic medical record system for all patients. Weight changes of more than one kilogram were defined as significant weight loss or gain.

The study sample comprised 1 622 operations in patients booked for a variety of surgical specialties. Twenty seven percent of patients had a missing booking weight so could not be further analysed. The overall mean weight change for the cohort was +0.8 kg (standard deviation -3.7 - +5.3kg). Analysis showed that 22% of patients lost weight while waiting for surgery, 44% stayed the same, and 35% increased weight. The influence of initial booking weight, duration of waiting period, age, and type of surgery on weight change was minimal.

Chapter 3 describes the systematic literature review that was performed to investigate methods of preoperative assessment specific to patients with obesity. Due to the paucity of literature on patients undergoing non-bariatric surgery, bariatric surgery papers were included in the review. Twenty one papers reporting on a total of 5 090 patients were included, with two thirds reporting on bariatric surgery patients. The only preoperative assessment method with any evidence of benefit for either preoperative diagnosis or postoperative risk complication was polysomnography, which arose from only five papers. This systematic literature review was unable to make any strong recommendations as to best assessment practice in patients with obesity presenting for non-bariatric surgery. There was therefore an opportunity to explore the value of other approaches such as functional capacity. This thesis investigated the six minute walk test as well as disability as methods of preoperative assessment.

Chapter 4 describes the analysis of the clinical and functional characteristics of the main study sample of patients with severe obesity (BMI ≥35 kg/m2) presenting for routine non-bariatric surgery. These are presented in terms of their preoperative comorbidities, biomarkers, functional capacity, and disability scores. A cohort of such patients (n=293), with a mean body mass index of 42 kg/m2, were investigated using their medical records, a six minute walk test, N-terminal pro B-type natriuretic peptide measurement, and the World Health Organization Disability Assessment Schedule 2.0.

Cardiorespiratory disease and diabetes were common, with blood tests revealing a high probability of additional unexpected and undiagnosed renal and cardiac disease in a significant proportion of patients. One third of patients had NT pro-BNP values that identify early left ventricular dysfunction in the community. Only 10% of patients walked a distance within 10% of that predicted, and 22% did not complete the test. One third of patients (34%) had a clinically significant level of disability, with those unable to walk for 6 minutes describing higher levels of disability. Functional capacity as measured by the six minute walk test was significantly lower than would be expected from age, sex, and weight alone, and was related to age and degree of disability, but notably not to weight or body mass index.

Chapter 5 presents the primary aim of the main study: can the six minute walk test (6MWT) predict functional outcomes of postoperative disability and quality of recovery in a cohort of patients with severe obesity presenting for non-bariatric surgery?

Overall, the patients recovered well after surgery, with few medical complications. The proportion of patients free from clinically significant disability increased from 66% prior to surgery to 90% at 6 months after surgery. The distance walked in six minutes was weakly predictive of poor recovery at one month and significantly increased disability at six months. Weight and BMI were not. The area under the receiver-operating-characteristic curve was 0.65 (95% confidence interval [CI] 0.51-0.78) for poor recovery and 0.64 (95% CI 0.51-0.77) for increased disability. A preoperative 6MWT distance of 308m was the best cut-off value for predicting increased postoperative disability (sensitivity 0.68, specificity 0.63). The level of functional capacity was once again more important than the degree of obesity in predicting postoperative outcomes.

Chapter 6 presents an in depth analysis of the patient physiology underpinning the results of the main study. The literature contains few detailed reports of the physiology underlying the 6MWT, especially in individuals with obesity. This study explored some of the physiological principles relevant to the test, and related them to this group of patients with severe obesity booked for surgery. One notable finding was that reduced respiratory function was more common in patients unable to sustain a walking speed above 0.9m/s.

Chapter 7 presents an investigation into the longer term implications of one area of the main study results: kidney disease. The incidence of undiagnosed kidney disease preoperatively was much higher than expected, so an analysis of the postoperative course of kidney function was undertaken to investigate this further.

The primary aim of this sub-study was to determine the frequency of postoperative acute kidney injury (AKI), chronic kidney disease (CKD), and kidney failure (KF) in this cohort of study patients. The secondary aim was to investigate the potential association between postoperative kidney impairment and preoperative variables of disability, functional capacity, and comorbidities.

Baseline impaired kidney function was present in just over half of the cohort. One hundred and thirty two patients (48%) had biochemistry within 7 days postoperatively, with 7 (5.3%) of these meeting the criteria for AKI. One hundred and ninety eight (72%) had blood tests at 3 months or more postoperatively, of whom 40 patients (20.2%) moved down a GFR category to a worse CKD stage. One quarter of patients with normal baseline kidney function developed chronically worsened kidney function at three months.

No significant association between preoperative comorbidities, functional capacity or disability level and development of either an AKI or CKD was found.

Chapter 8 summarises the findings from the previous chapters in relation to the overall aims and hypotheses of the thesis. This chapter includes a discussion of the strengths and limitations of the research and provides recommendations for clinical practice and future research work in this field.

Patients do not appear to be at increased postoperative risk simply because of obesity itself. Assessment of functional capacity can be done simply and may contribute more to the accuracy of risk predication than anthropomorphic measures alone. An individual with severe obesity and multiple comorbidities, who additionally displays low fitness and high disability, is likely to be at a higher risk of poor postoperative outcomes.

FoR codes (2008)

1103 CLINICAL SCIENCES, 110301 Anaesthesiology



Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.