## 1 Introduction  
Obstructive sleep apnea-hypopnea syndrome (OSAHS) is a major public health concern characterized by recurrence of airflow reduction (hypopnea) or cessation (apnea) due to upper airway collapse during sleep, resulting in oxygen desaturation and fragmented sleep [1; 2; 3]. In clinical practice, the severity of OSAHS is categorized as mild, moderate, or severe, with apnea-hypopnea index (AHI) thresholds of 5, 15, or 30, respectively, following gold standard polysomnography (PSG) diagnosis [4]. The prevalence of OSAHS for the general population ranges from 9% to 38% [5]. Global estimates suggest that 936 million people worldwide are affected by mild to severe OSAHS, and 425 million people aged between 30 and 69 years worldwide have moderate to severe OSAHS [6]. It is associated with systemic hypertension and chronic inflammation and leads to impaired quality of life [7; 8; 9]. Studies have reported that OSAHS severity increases patient morbidity and mortality from cardiovascular diseases, end-stage renal disease (ESRD), and hypertension [10; 11; 12]. Due to the fact that OSAHS causes excessive daytime sleepiness and decreased alertness, it increases the risk of automobile accidents [13]. To treat moderate to severe OSAHS as early as possible, portable monitoring (PM) using level 3 or 4 PSG with fewer sensing channels is recommended to screen for OSAHS and shorten waiting lists for the gold standard PSG [14]. Airflow is an essential indicator used for scoring apnea and hypopnea [14; 15; 16]. It is measured using a thermistor/thermocouple (Th) or nasal pressure (Np) [17]. Digital biomarkers provide a novel approach for risk management. Digital biomarkers in health and disease management are collected using digital health technologies to explain, influence, or predict health-related outcomes [18]. Due to the fact that the dynamic response of a Th is slow and the relationship between electrical signal and airflow is nonlinear, a single channel airflow biomarker is used to detect apnea [15; 17]. By contrast, Np exhibits an excellent dynamic response and is suitable for accurately quantifying the magnitude of flow, and thereby detecting hypopnea. Unattended PM with Np in conjunction with a Th is more sensitive than a Th or Np alone in detecting sleep-disordered breathing [19; 20]. Moreover, studies have found that PM with Np alone significantly improves apnea and hypopnea detection and classification over PM with a Th [21; 22]. However, one study concluded that if only an Np sensor was used to detect apnea and hypopnea, apnea index and AHI were overestimated. Hence, an algorithm with a Th is potential to produce results that conform precisely to guidelines [19]. Positive airway pressure (PAP) is recommended for the treatment of moderate to severe OSAHS [23]. To determine optimal pressure, PAP titration is manually performed with standard overnight PSG. According to the clinical signs of apnea or hypopnea, PAP may be increased to eliminate these obstructive respiratory events and to identify optimal pressure for an AHI of <5. However, when the titration reference is based on the airflow measured by the device or by the pressure difference between the mask and the outlet of the machine, as measured by a pressure transducer, the optimal titration point is overestimated [24]. In this case, the use of a Th may affect titration reference choice, because its electrical characteristics depend on temperature change caused by breathing, and a small Th may be placed under the mask with less leakage when PAP is being conducted. Due to the use of manual scoring, apnea and hypopnea duration time within a subject can vary markedly by from 10 to 120 s [15]. Accordingly, time domain features for the detection of apnea and hypopnea are now fully accepted by physicians. The time domain algorithm with amplitude changes of peaks (ACP) of Np has been reported in [25; 26].