Association of Obstructive Apnea with Thoracic Fluid Shift and Small Airways Narrowing in Asthma During Sleep

Rationale: Obstructive sleep apnea (OSA) is highly prevalent among patients with asthma, suggesting a pathophysiological link between the two, but a mechanism for this has not been identified. Hypothesis: Among patients with asthma, those with OSA will have greater overnight increases in thoracic fluid volume and small airways narrowing than those without OSA. Methods: We enrolled 19 participants with asthma: 9 with OSA (apnea-hypopnea index (AHI) ≥10) and 10 without OSA (AHI <10). All participants underwent overnight polysomnography. Before and after sleep, thoracic fluid volume was measured by bioelectrical impedance and small airways narrowing was primarily assessed by respiratory system reactance at 5Hz using oscillometry. Results: Patients with asthma and OSA (OSA group) had a greater overnight increase in thoracic fluid volume by 120.5 mL than patients without OSA (non-OSA group) (164.4 ± 44.0 vs 43.9 ± 47.3 mL, p=0.006). Compared to the non-OSA group, the OSA group had greater overnight decrease in reactance at 5Hz (-1.08 ± 0.75 vs 0.21 ± 0.27 cmH₂O/L/s, p=0.02), and overnight increase in reactance area (14.81 ± 11.09 vs -1.20 ± 2.46 cmH₂O/L, p=0.04), frequency dependence of resistance (1.02 ± 0.68 vs 0.05 ± 0.18 cmH₂O/L/s, p=0.04), and resonance frequency (2.80 ± 4.14 vs -1.42 ± 2.13 cmH₂O/L/s, p=0.04). Conclusion: Patients with asthma and co-existing OSA had greater overnight accumulation of fluid in the thorax in association with greater small airways narrowing than those without OSA. This suggests OSA could contribute to worsening of asthma at night by increasing fluid accumulation in the thorax.