Tidal expiratory flow limitation detected with intra-breath oscillometry in obesity hypoventilation syndrome: effects of body position and positive airway pressure; a cross-sectional study

Background: Altered respiratory mechanics due to increased extrapulmonary mechanical load may be the key to understanding the profound effect of continuous positive airway pressure (CPAP) therapy in obesity hypoventilation syndrome (OHS). Methods: We performed a cross-sectional study to assess intra-breath mechanical changes in an OHS population. 10 Hz intra-breath oscillometry and forced spirometry values were measured in the sitting and supine positions, followed by a supine oscillometry at different CPAP levels (none-5-10-15-20 cmH 2 O). We plotted intra-breath resistance ( Rrs ) and reactance ( Xrs ) values against flow and volume to analyse tidal changes in a more detailed manner. Results: Thirty-five patients completed the study. Intra-breath reactance variables differed by body position (end-expiratory reactance: -1.85 (±1.89) hPa·s·L -1 vs -4.17 (±2.22) hPa·s·L -1 , and by difference between end-expiratory and end-inspiratory reactance: -1.27 (±1.68) hPa·s·L -1 vs -3.56 (±2.02) hPa·s·L -1 , sitting vs supine respectively, p<0.001). Temporal mapping displayed characteristic looping of Xrs against tidal volume and flow, suggesting tEFL. Augmented CPAP levels were associated with improved Xrs values and reduced loop areas, suggesting diminished tEFL (p<0.001). CPAP-induced expiratory glottal interference was observed in 60% of patients parallel with small airway mechanical changes affecting expiratory Rrs values. No correlation was found between forced spirometry values and intra-breath oscillometry variables. We also did not detect correlation between intra-breath oscillometry variables and body mass index (BMI), OHS staging or age. Conclusion: tEFL is frequently present in patients with OHS during quiet breathing, worsening in the supine position. CPAP appears to counteract this effect, suggesting that tEFL is an important factor in the pathomechanism of respiratory impairment and a possible target for optimizing therapy. Trial Registration This study was approved by and registered at the ethics committee of Semmelweis University (SE TUKEB 239/2018)