Table 1:

Detailed summary of polysomnographic physiological features that can be applied to better characterize obstructive sleep apnea subtypes.

Feature		Main PSG sensor	Description	Effect on OSA	References
Traditionally used in clinical practice	AHI	Thermistors, nasal air pressure, and pulse oximeter	Average number of apnea and hypopnea events per hour of TST.	Current index used to classify OSA severity; increased in more severe patients.	(American Academy of Sleep Medicine 1999, Ho et al 2015)
	Arousal index	EEG	Number of identified arousals per hour of sleep, as defined by AASM. Can be a measure of sleep fragmentation.	Increased in more severe OSA.	(American Sleep Disorders Association 1992)
	ODI	Pulse oximeter	Average number of desaturation episodes per hour of total sleep time. Desaturation usually defined as 3 or 4% decreases from baseline.	Alternative metric associated with OSA severity; it is increased in more severe patients.	(Svanborg et al 1990, Dawson et al 2015)
	Percentage of sleep stages	EEG	Proportion of total sleep time spent on sleep stage N1, N2, N3 and REM.	More severe OSA patients spend more time in REM than slow wave sleep and in light sleep than awake.	(Iber et al 2007, Ratnavadivel et al 2009, Bianchi et al 2010)
	Percentage of TST with SpO2 <90%	Pulse oximeter	Measure of time with low levels of peripheral oxygen saturation (<90%). Can be a measure of degree of hypoxia and desaturation.	Direct relationship with the duration and severity of hypoxia in patients with OSA.	(Bostanci et al 2015)
	Sleep efficiency	EEG	Ratio of total sleep time to time in bed. Indicates how much of the recording time spent on bed was scored as sleep.	Usually decreased in more severe OSA, but with controversial results.	(Ng and Guan 2012, Iber et al 2007, Ratnavadivel et al 2009, Bianchi et al 2010)
With potential for clinical use	Arousal intensity	EEG	Intensity of EEG arousal detected and quantified using discrete wavelet transform.	Associated with increased heart rate response to arousal and respiratory control instability in OSA.	(Azarbarzin et al 2014, Amatoury et al 2016)
	Cardiorespiratory coupling features	ECG, respiratory inductance plethysmography	Influence of respiration on heart rate. Include % of synchronized time per unit of sleep time and average duration of synchronization.	Percentage of synchronization decreases and average duration increases in severe OSA.	(Sola-Soler et al 2015)
	CAP phases frequency and duration	EEG	Number/duration of CAP A-phases subtypes (A1, A2, A3) per unit of sleep time. CAP represents a marker of NREM sleep instability.	Associated with inspiratory flow limitation, poor response to CPAP therapy, fatigue and sleepiness.	(Guilleminault et al 2007, Bosi et al 2018, Gupta and Shukla 2018)
	ECG/PPG-derived respiration features	ECG and pulse oximeter	Mean respiratory rate, respiratory frequency and respiratory power spectrum density. Used for apnea detection and ECG-derived AHI calculation.	Mostly used to help automatic classification of respiratory events in patients with OSA.	(Pallás-Areny et al 1989, Penzel et al 2002, de Chazal et al 2009)
	HRV features	ECG	Linear, nonlinear, spectral and wavelet characteristics of ECG. These include mean RR interval, SDNN, RMSSD, SDSD, NN50, pNN50, DFA alpha1, DFA alpha2, entropy, ULF, VLF, LF, HF, Allan factor.	Patients with OSA have changes in specific features of HRV that indicate autonomic dysfunction.	(European Society of Cardiology and the North American Society of Pacing and Electrophysiology 1996, Aeschbacher et al 2016)
	K-complex components and distribution	EEG	Signal characteristics and frequency of K-complexes per unit of sleep time.	Mild airflow limitation, such as in OSA, increases K-complex frequency and amplitude.	(Nguyen et al 2016)
	ORP	EEG	Continuous measure of sleep-wakefulness state based on power spectrum patterns of EEG.	ORP in the immediate 9 seconds following arousals (ORP-9) was associated with sleep instability in OSA.	(Younes et al 2015a, Younes and Hanly 2016)
	Peak and mid-inspiratory flow shape features	Nasal air pressure	Indicate aspects of pharyngeal narrowing and site of collapse. Features include negative effort dependence (NED, percent reduction in inspiratory flow between peak and plateau).	Lower NED indicates tongue-related obstruction and severe indicate epiglottis collapse. Can guide therapy choice.	(Genta et al 2017, Azarbarzin et al 2017a, 2017b)
	PTT	Pulse oximeter	PPG-derived index that reflects peripheral vascular resistance and intrathoracic pressure. Can estimate blood pressure, respiratory effort and arousal index.	Severe OSA patients have more periods of ≥ 10 mmHg increases in baseline systolic blood pressure.	(Gehring et al 2018, Pépin et al 2009, Schwartz 2005)
	Sleep EEG power spectrum density	EEG	Power density and ratios of discretely defined EEG frequencies (e.g. delta, theta, alpha, beta), calculated from short sleep EEG epochs.	Increased delta and theta power in more severe OSA. Increased beta power in NREM and delta power in REM sleep, and correlation with worse driving performance.	(Xiromeritis et al 2011, Vakulin et al 2016)
	Sleep spindles density/frequency	EEG	Number of sleep spindles (high-frequency and short EEG oscillations emerging mostly during NREM sleep) per unit of sleep time	Reduced with more severe OSA and increased after CPAP treatment	(Himanen et al 2003, Carvalho et al 2014, Yetkin and Aydogan 2017)

AASM: American Academy of Sleep Medicine; AHI: apnea-hypopnea index; CAP: cyclic alternating patterns; CPAP: continuous positive airway pressure; ECG: electrocardiogram; EEG: electroencephalogram; HF: high frequency; HRV: heart rate variability; LF: low frequency; NED: negative effort dependence; NN50: Number of pairs of adjacent NN intervals differing by more than 50 ms; NREM: non-rapid eye-movement; ODI: oxygen desaturation index; ORP: odds-ratio product; OSA: obstructive sleep apnea; pNN50: NN50 count divided by the total number of all NN intervals; PPG: photoplethysmography; PSG: polysomnography; PTT: pulse transit time; REM: rapid eye-movement; RMSSD: square root of the mean squared differences of successive NN intervals; SDNN: standard deviation of NN intervals; SDSD: standard deviation of differences between adjacent NN intervals; SpO₂: peripheral oxygen saturation; TST: total sleep time; ULF: ultra low frequency; VLF: very low frequency