Night to night variability in obstructive sleep apnea severity, the physiological endotypes and the frequency of flow limitation

Study objectives: There is substantial night-to-night variability (NtNV) in obstructive sleep apnea (OSA) severity in some individuals, however predictors for this remain incompletely understood. This study aims to quantify the degree of NtNV in the apnea-hypopnea index (AHI), hypoxic burden, airflow limitation, and OSA endotypes; to determine if a relationship exists between the degree of NtNV in AHI and in endotype expression; to assess whether the degree of flow limited breathing is predictive of the degree of NtNV of the AHI.

Methods: 71 patients with OSA underwent two polysomnograms (PSGs). OSA endotypes, hypoxic burden and flow limitation frequency were extracted from PSG data. Intra-individual agreement was assessed and associations with the NtNV of the AHI were calculated. Patients were grouped into High Variability vs Low Variability based on the degree of difference in AHI between each night.

Results: Despite wide limits of agreement, at the group level most PSG and endotype variables were not statistically different between first and second night. Flow limitation frequency was 7.7% (2.1 to 13, P<0.01) higher on the second night compared to baseline. There were weak linear associations between NtNV of endotypes and NtNV of the AHI. In sub-group analysis, there was greater difference between nights for Vactive (5%eupnea, P=0.01), Vpassive (3.1%eupnea, P=0.03), Vcomp (3.2%eupnea, P=0.01) and arousal threshold (4.1%eupnea, P=0.04) in the High Variability compared to the Low Variability group.

Conclusions: There is high NtNV in AHI, OSA endotypes and flow limitation in some individuals, however no strong linear relationship exists between these changes.