Using intra-breath oscillometry in obesity hypoventilation syndrome to detect tidal expiratory flow limitation: a potential marker to optimize CPAP therapy

Background Continuous positive airway pressure (CPAP) therapy has profound effects in obesity hypoventilation syndrome (OHS). Current therapy initiation focuses on upper airway patency rather than the assessment of altered respiratory mechanics due to increased extrapulmonary mechanical load. Methods We aimed to examine the viability of intra-breath oscillometry in optimizing CPAP therapy for OHS. We performed intra-breath oscillometry at 10 Hz in the sitting and supine positions, followed by measurements at increasing CPAP levels (none-5-10-15-20 cmH 2 O) in awake OHS patients. We plotted intra-breath resistance and reactance ( Xrs ) values against flow ( V’ ) and volume ( V ) to identify tidal expiratory flow limitation (tEFL). Results Thirty-five patients (65.7% male) completed the study. We found a characteristic looping of the Xrs vs V’ plot in all patients in the supine position revealing tEFL: Xrs fell with decreasing flow at end-expiration. Intra-breath variables representing expiratory decrease of Xrs became more negative in the supine position [ end-expiratory Xrs (mean ± SD): -1.9 ± 1.8 cmH 2 O·s·L − 1 sitting vs . -4.2 ± 2.2 cmH 2 O·s·L − 1 supine; difference between end-expiratory and end-inspiratory Xrs : -1.3 ± 1.7 cmH 2 O·s·L − 1 sitting vs. -3.6 ± 2.0 cmH 2 O·s·L − 1 supine, p < 0.001]. Increasing CPAP altered expiratory Xrs values and loop areas, suggesting diminished tEFL ( p < 0.001). ‘Optimal CPAP’ value (able to cease tEFL) was 14.8 ± 4.1 cmH 2 O in our cohort, close to the long-term support average of 13.01(± 2.97) cmH 2 O but not correlated. We found no correlation between forced spirometry values, patient characteristics, apnea-hypopnea index and intra-breath oscillometry variables. Conclusions tEFL, worsened by the supine position, can be diminished by stepwise CPAP application in most patients. Intra-breath oscillometry is a viable method to detect tEFL during CPAP initiation in OHS patients and tEFL is a possible target for optimizing therapy in OHS patients.