Background
Obesity, defined as a body mass index (BMI) > 30 kg/m
2, has almost doubled since 1980, with more than 671 million people worldwide now classified as obese [
1,
2]. Health problems associated with obesity impact on quality of life and impose a significant cost burden to health services. Being obese is associated with a higher frequency of cardiovascular disease [
3], of metabolic diseases such as type II diabetes mellitus [
4], of respiratory morbidity secondary to obstructive sleep apnoea (OSA) and obesity hypoventilation syndrome (OHS) [
5]. In Australia, health expenditures of those with a BMI between 30 and 35 kg/m
2 are 19% higher than those of a normal-weight individual. This increases to 51% in those with a BMI > 35 kg/m
2 [
6]. Overall, the total financial cost of obesity is estimated to be 8.3 billion AUD [
7].
Obesity is difficult to treat. Diet, exercise and medications are only modestly effective in aiding weight loss [
8]. In selected individuals, bariatric surgery may offer a means of achieving long-term weight loss, improved health outcomes and a reduction in healthcare spending [
9‐
11]. However, bariatric procedures are complicated by the additional risks associated with anaesthesia and surgery in the obese patient. Obesity and its associated co-morbidities, have been shown to increase the rate of post-operative myocardial infarction, peripheral nerve injury, wound and urinary tract infection and the requirement for re-intubation [
12].
The evidence that obesity increase the incidence of PPCs is mixed. A retrospective observational study by Baltieri et al. described a 37% prevalence of atelectasis in obese patients following bariatric surgery [
13]. Several studies have identified obesity as an independent risk factor for post-operative respiratory complications [
14‐
17], while others have failed to find an association [
18‐
20]. However, respiratory complications are not infrequent amongst the general surgical population and have been shown to increase hospital length of stay and mortality [
21].
Physiological and pathological changes that occur because of obesity adversely affect both lung mechanics and gas exchange. First, there is a reduction in total respiratory system compliance and an increase in airway resistance [
22,
23]. Increased mass loading on the chest wall, cephalad displacement of the diaphragm and an increase in pulmonary blood flow contribute to this [
24,
25]. Taken together, these changes lead to higher work of breathing [
22] and a tendency toward shallow, rapid tidal volumes (Vt) [
26]. Second, breathing at a lower Vt leads to a reduction in end expiratory lung volume (EELV) [
27], increasing the potential for atelectasis and ventilation/perfusion mismatching [
28,
29]. The physiological alterations outlined above are further exacerbated when the patient is in the supine position and by general anaesthesia (GA), with functional residual capacity (FRC) falling by up to 50% on induction [
30]. This reduction persists longer into the post-operative period in obese compared to non-obese individuals [
16].
High flow nasal oxygen therapy
High flow nasal oxygen (HFNO
2) therapy was first shown to be an effective treatment for acute respiratory failure in the paediatric and neonatal populations [
31,
32]. Recently, it has gained popularity as a therapy in adult patients, with an expanding list of clinical applications [
33].
High flow nasal cannulae (HFNC) are designed to deliver an air/oxygen blend at a predetermined fraction of inspired oxygen (FiO
2). The heating and humidification of inspired gases allows for higher flow rates to be tolerated when compared to conventional oxygen delivery devices. Flow rates of up to 70 L/min can be achieved. The process of heating and humidification also reduces mucosal drying, improves muco-ciliary clearance and reduces energy expenditure and work of breathing [
34]. Higher flow rates reduce room air entrainment during inspiration and flush expired air from the upper airway during expiration, leading to the delivery of higher and more consistent FiO
2 [
34]. There is additional evidence to suggest that the higher flow rates achievable with HFNC generate a degree of positive airway pressure, which increases end expiratory lung volume (EELV) and alveolar recruitment [
35,
36].
Electrical impedance tomography
Electrical impedance tomography (EIT) is a non-invasive, radiation-free, functional imaging modality [
37]. It uses changes in bio-impedance across lung tissue during the respiratory cycle to provide information on ventilation distribution, lung volumes, and regional lung mechanics [
38]. EIT has been successfully validated against several other imaging and measurement modalities and strong linear correlation between the change in end expiratory lung impedance (EELI) and the EELV has been described [
39‐
42].
Rationale for the study
A rise in the incidence of obesity has resulted in a rapid escalation in the number of bariatric procedures being carried out worldwide. Obese patients undergoing surgery of any kind are at a higher risk of developing post-operative atelectasis secondary to the physiological and pathological changes outlined above. Study data from our group suggests that the use of HFNO
2 when compared to low-flow oxygen devices significantly increases EELI (which in turn is associated with an increase in EELV). This relationship appears to be enhanced at higher BMIs. In the study of Corley et al., a mean increase in EELI of 1517 ± 46.6 units was associated with a reduction in respiratory rate, an increase in P/F ratio, and a reduction in a standardised dyspnoea score [
35].
We propose to carry out a randomised, controlled pilot study to evaluate the effects of post-operative HFNO2 therapy on EELV in the obese population undergoing laparoscopic weight reduction surgery. Data gathered will be used to explore the mechanism of action of HFNO2 therapy in this setting and to inform the design of a larger trial with a patient-centred primary outcome.
Discussion
HFNO2 has become more accessible in recent years and is now commonplace in the ICU, emergency departments and operating theatres. HFNO2 therapy is minimally invasive, safe and has few contraindications. We aim to test the hypotheses that respiratory support, in the form of HFNO2, leads to an increase in EELV, conferring a reduction in the incidence and duration of post-operative atelectasis.
HFNO
2 provides a degree of positive airway pressure, which may aid alveolar recruitment and prevent de-recruitment [
35,
36]. Corley et al. showed a significant increase in EELV in patients with respiratory compromise, treated with HFNO
2, following cardiac surgery. This increase in EELV was greater in those with a higher BMI, suggesting a benefit in this cohort.
Trial recruitment began in April 2017 at St Andrews War Memorial Hospital, Brisbane and is anticipated to be completed by the end of April 2018. We aim to publish the results as a single manuscript. It is anticipated that the results of this study will be used to inform the operational design and feasibility of a larger randomised controlled trial.
Trial status
This trial is ongoing and actively recruiting.
Date recruitment started, 3 April 2017.
Date of anticipated completion, 31 June 2018.
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