Prophylactic post-operative high flow nasal oxygen therapy versus conventional oxygen therapy in obese patients undergoing bariatric surgery: a randomised controlled pilot study

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Obesity, defined as a body mass index (BMI) > 30 kg/m2, has almost doubled since 1980, with more than 671 million people worldwide now classified as obese. Health problems associated with obesity impact on quality of life and impose a significant cost burden to the health service. The total financial cost of obesity is estimated to be $8.3 billion. Obesity is difficult to treat. Diet, exercise and medications being only modestly effective in aiding weight loss. In selected individual’s, bariatric surgery may offer a means of achieving long-term weight loss, improved health and cost reduction. The physiological and pathological changes that arise from obesity predispose to post-operative respiratory complications. Excess pressure exerted by an increased amount of fat tissue on the chest wall and in the abdomen, leads to collapse and closure of small airways within the lungs. This collapse of the small airways is worsened still by general anaesthesia and lying flat both of which are a requirement for surgery. This collapse persists longer into the postoperative period in the obese when compared to the non-obese population. High flow nasal oxygen therapy (HFN02) has been established as treatment for respiratory failure in infants and neonates. Its use has also become more prevalent in the adult population over the last decade, with an expanding list of clinical applications. High flow nasal cannula (HFNC) are designed to deliver a predetermined amount of heated and humidified oxygen to a patient. HFNC deliver oxygen at a much higher flow rate than a conventional face mask or nasal cannula, up to 70L/min vs 6L/min. The use of high flow nasal oxygen (HFN02) has been shown to improve the clearance of mucus from the airways, reduce the amount of energy used to breath, deliver a more accurate and reliable amount of oxygen to the lungs, and provide a degree of positive pressure into the lungs. The provision of positive pressure has been shown to increase the lung volume, this suggests that small airways that were previously closed are splinted open by the pressure provided. All of the positive effects of HFN02 outlined above are of potential benefit to obese patients in the postoperative period. Of particular interest is the provision of positive pressure which helps open up collapsed small airways. By carrying out our proposed study we hope to determine the impact that HFN02 has on postoperative lung volumes when compared to standard oxygen therapy. Lung volumes, specifically end expiratory lung volume (EELV) can be measured using electrical impedance tomography (EIT). EIT is a radiation free functional imaging modality invented over 30 years ago. It is non-invasive and can be used in real time at the patient’s bedside to assess lung volume changes. EIT has been successful validated against a number of other imagining and measurement modalities. It consists of 16 paired electrodes attached to a belt that is placed around the patient’s chest usually between the 4th/5th or 5th/6th rib. It then feeds back information about the patient’s volumes lungs as they breath to a computer for analysis. Both numerical measurements and images are produced. Hypotheses In obese, adult patients, undergoing laparoscopic surgery for weight reduction, prophylactic post-operative HFN02 therapy will increase EELV, improve respiratory function and reduce respiratory morbidity in the post-operative period