Introduction Currently, there are limited data reporting the pathophysiology of chronic respiratory failure in obese patients, the so called Obesity Hypoventilation Syndrome (OHS). Although a number of hypotheses have been proposed, there are no comprehensive data that have investigated the imbalance between respiratory muscle load, capacity and drive. We aimed to investigate the factors contributing to early chronic respiratory failure in obese patients including body habitus, upper airways obstruction, lower airflow obstruction and lung volume.

Methods A cross sectional study was performed in an obese group of subjects (BMI >30kg/m²) with and without early chronic respiratory failure. Early chronic respiratory failure was arbitrarily defined for this analysis as an arterial base excess > 2 mmol/l as this is a metabolic respiratory biomarker of 24-hour carbon dioxide levels. Arterial blood gas measurements were undertaken in the morning, after an overnight study to determine the 4% oxygen desaturation index and apnoeas hypopnoea index. An overnight auto titrating continuous positive airway pressure study (S9 ResMed, Oxfordshire, UK) was used as another measure of the severity of upper airways loading. Pulmonary function and the forced oscillation technique (MS-IOS, CareFusion, CA, USA) to measure of airway impedance were performed in the upright and supine position.

Results 54 patients, aged 51.9 ± 9.08 years, were recruited with BMI of 46.4 ± 9.53.

Conclusion We are finding considerable heterogeneity in terms of anthropometric and physiological findings within obese subjects with and without early chronic respiratory failure. We have shown that subjects with early chronic respiratory failure, compared to subjects without, have lower lung volumes (upright and supine), a greater airway impedance (seen in FOT at FRC, both upright and supine), and a larger fall in their ERV on lying down. All these differences may be due to the higher BMI, and in particular differences in distribution of fat in subjects with early chronic respiratory failure. However, these obesity differences were not reflected in large differences in the AHI or ODI between the groups.