Patients who underwent PSG examination at the Sleep Center of the Third Affiliated Hospital of Anhui Medical University (Hefei First People's Hospital) from March 2019 to October 2020 were screened for study inclusion. Inclusion criteria were: (1) age >18 years old; (2) complete autonomous behavior and cognitive ability; (3) arterial blood gas analysis and PSG monitoring performed during hospitalization; and (4) ability to answer the questionnaire completely and accurately. Exclusion criteria were: (1) associated diseases such as chronic obstructive pulmonary disease, acute attacks of asthma, interstitial pulmonary disease, laryngospasm, vocal cord paralysis, tracheal foreign bodies, anemia, and electrolyte disorders that may affect blood gas analysis; (2) other underlying diseases that may affect HCO3− concentration, such as liver, kidney, lung, and heart dysfunction; (3) use of a ventilator in the past three months; (4) other common sleep respiratory disorders besides OSA; (5) pregnancy, lactation, puerperium, and mental disorders（including a history of depression or anxiety）; (6) use of sedatives or antipsychotic treatment in the past three months; and (7) patients with an abnormal EEG. This study was approved by the ethics committee of the third affiliated hospital of Anhui Medical University, and patients gave their informed consent to all the monitoring and questionnaire contents.
PSG recording and analysis
All subjects were monitored employing an Alice6 PSG instrument (Philips Viagra, USA) and analyzed manually by sleep technicians to confirm or exclude OSA. The apnea hypopnea index (AHI), minimum blood oxygen saturation (LSaO2) and mean blood oxygen saturation (MSaO2) were recorded. Respiratory events were determined. OSA was diagnosed according to the Chinese Guidelines for Primary Diagnosis and Treatment of Adult Obstructive Sleep Apnea (2018 Edition). These include (1) clinical symptoms of any one or more of the following: daytime sleepiness, non-recovery of energy after waking, fatigue, or insomnia; waking up because of breathlessness, poor breathing, or suffocation at night; habitual snoring and breathing interruption; and hypertension, coronary heart disease, stroke, heart failure, atrial fibrillation, type 2 diabetes, mood disorders, or cognitive impairment; (2) PSG monitoring: AHI ≥5 times/h, mainly blocking events; (3) none of the above symptoms and PSG monitoring: AHI≥ 15 times/h, mainly blocking events. Adult OSA can be diagnosed if criteria (1) and (2) are met or only criterion (3) is met. Grade of disease: AHI: 5–15 times/h is mild, > 15–30 times/h is moderate, and >30 times/h is severe. The percentage of total sleep time when blood oxygen is less than 90% (CT90%), duration of apnea hypopnea in total sleep time (AHT%), average duration of apnea hypopnea (MAD), and duration of apnea hypopnea per hour (HAD) were calculated based on the reported data. PSG is the gold standard for the diagnosis of OSA.
Blood gas analysis
Two milliliters of radial artery blood were collected with a special syringe for blood gas analysis (American Westmed). Samples were obtained from study subjects following PSG monitoring and when they were awake and seated in a quiet room. The sample was mixed and sealed, and blood gas analysis was performed within 30 min to detect HCO3−. The process of specimen extraction strictly followed the requirements of aseptic operation.
The ESS asks individuals to grade their sleepiness during several routine activities including sitting and reading, watching TV, sitting in a public place, a long ride (more than 1 h), talking with people, resting after dinner (not drinking), driving, and reposing at rest in the afternoon. Each condition is divided into four grades: never (0), rarely (1), sometimes (2) and often (3). The subjects will score according to their own conditions, and the researchers will calculate the total score. An ESS score ≥9 was associated with high risk of OSA, and an ESS score <9 was associated with a low risk of OSA.
The questionnaire comprised eight characteristics: S, snoring; T, tiredness; O, observed apnea; P, high blood pressure; B, body mass index >35 kg/m2; A, older than 50 years; N, neck circumference >40 cm; and G, male gender. Patients answered the first four questions (STOP questions), and the staff in the sleep room measured the height, weight, blood pressure, and neck circumference of the patients. Then, the respondents answered the last four questions (BANG questions). The answer was yes (1 points) or no (0 points), and the total score was calculated. A score ≥3 was defined as a high risk of OSA, while a score < 3 was defined as a low risk of OSA.
This consisted of snoring frequency, snoring loudness, daytime sleepiness, apnea, wakefulness fatigue, history of hypertension, and body mass index. Some of the questions had to be answered by the patient's family members or co-residents to ensure the accuracy of the answers. Depending on whether the final results were positive or negative, OSA was classified as high-risk or low-risk.
Clinical data of the patients were retrospectively analyzed. All patients received PSG monitoring, blood gas analysis, and completed the ESS, SBQ, and Berlin questionnaires. According to AHI and symptoms, the patients were divided into the non-OSA group or the OSA group, and the latter group was further divided into mild (5 times/h≤AHI≤15 times/h), moderate (15 times/h<AHI≤30 times/h), and severe (AHI > 30 times/h). Patients were divided into two groups according to the SBQ: the high-risk OSA group (STOP-Bang questionnaire score ≥3 points) and low-risk OSA group (STOP-Bang questionnaire score < 3 points). Patients were divided into two groups according to ESS: ESS ≥9 was classified as high-risk for OSA, and ESS <9 was classified as low-risk for OSA. According to the Berlin questionnaire, the patients were divided into two groups: positive for the high-risk OSA group and negative for the low-risk OSA group. A correlation analysis between HCO3− concentration and AHI, LSAO2, MSAO2, CT90%, AHT%, MAD, and HAD was carried out to judge the correlation degree, and the diagnostic value of HCO3− concentration on OSA was analyzed and evaluated. The sensitivity and specificity of the three kinds of questionnaires for screening OSA were compared. Then, the diagnostic value of the STOP-Bang questionnaire score alone and SBQ combined with the HCO3− concentration for OSA was further compared.
SPSS19.0 statistical software was used for data analysis. Patient information was analyzed by descriptive statistics. The normal distribution data is represented by ±s, and the non-normal distribution data is represented by the median (M) and the interquartile range (P75–P25). Correlation analysis was conducted between the HCO3− concentration and AHI, CT90%, AHT%, MAD, and HAD, and the correlation coefficients were calculated. Then, calculation was done for the degree of sensitivity, specific diagnostic test evaluation of the four tables, and sensitivity to the specific degrees of comparison between different methods using the matching c2 test (with P<0.05 for the difference being statistically significant). The optimal truncation value of the HCO3− concentration for the diagnosis of OSA was analyzed by Youden index. A ROC curve was used to compare the scores of SBQ alone and the diagnostic value of SBQ combined with HCO3− concentration for OSA.