Metrics of sleep apnea severity: beyond the apnea-hypopnea index

Abstract

Obstructive sleep apnea (OSA) is thought to affect almost 1 billion people worldwide. OSA has well established cardiovascular and neurocognitive sequelae, although the optimal metric to assess its severity and/or potential response to therapy remains unclear. The apnea-hypopnea index (AHI) is well established; thus, we review its history and predictive value in various different clinical contexts. Although the AHI is often criticized for its limitations, it remains the best studied metric of OSA severity, albeit imperfect. We further review the potential value of alternative metrics including hypoxic burden, arousal intensity, odds ratio product, and cardiopulmonary coupling. We conclude with possible future directions to capture clinically meaningful OSA endophenotypes including the use of genetics, blood biomarkers, machine/deep learning and wearable technologies. Further research in OSA should be directed towards providing diagnostic and prognostic information to make the OSA diagnosis more accessible and to improving prognostic information regarding OSA consequences, in order to guide patient care and to help in the design of future clinical trials.

Keywords: apnea; cardiovascular; hypopnea; hypoxia; lung; sleep.

Figure 1.

Timeline of hypopnea definitions.

Figure 2.

Examples of different patterns of obstructive respiratory events for people with AHI in the range 20–30 events/hour: (a) Short events (AHI = 21.3 events/hour, Average hypopnea duration 20 seconds, maximum duration 55 seconds), (b) Long events (AHI = 25.6 events/hour: Average hypopnea duration 31 seconds, maximum duration 92 seconds), (c) REM predominant (AHI= 21.5 events/hour: REM AHI = 67.5 events/hour, NREM AHI = 5.5 events/hour). The images show cut-down PSG montage signals (a, b) = EEG, sub-mental EMG and ECG signals at 30 seconds per page and pulse oximeter (SaO₂), nasal pressure transducer (prongs), oronasal thermal flow sensor (AIRFLOW), respiratory effort thoracic (THOR), respiratory effort abdominal (ABDO), sound intensity and body position at 5 minutes per page or hypnogram signals (c) = study time (Time), Epoch number (Epoch), Sleep stage summary, body position, pulse oximetry (SaO₂), scored Central (Cn. A), Obstructive (Ob. A), Mixed (Mx. A), hypopnea (Hyp), unsure (Uns), and respiratory event-related arousals (RERA). Abbreviations: AHI: apnea-hypopnea index, REM: rapid eye movement, NREM: non-rapid eye movement, PSG: polysomnography.

Figure 3.

Kaplan-Meier survival curves across categories of apnea-hypopnea index (AHI-4%). [Reproduced from Punjabi, et al., PLoS Med 2009;6(8): e1000132 under Creative Commons Attribution License.]. The association of OSA with mortality was significant after adjustment for age, sex and race for AHI-4% 15–29.9 (HR 1.20, 95% CI 1.00–1.44) and AHI-4% ≥30 (HR 1.38, 95% CI 1.08–1.75).