Comparison of Apnea Detection Using Oronasal Thermal Airflow Sensor, Nasal Pressure Transducer, Respiratory Inductance Plethysmography and Tracheal Sound Sensor

Abstract

Study objectives: Evaluation of apnea detection using a tracheal sound (TS) sensor during sleep in patients with obstructive sleep apnea.

Methods: Polysomnographic recordings of 32 patients (25 male, mean age 66.7 ± 15.3 years, and mean body mass index 30.1 ± 4.5 kg/m²) were analyzed to compare the detection of apneas by four different methods of airflow signals: oronasal thermal airflow sensor (thermistor), nasal pressure transducer (NP), respiratory inductance plethysmography (RIPsum) and TS. The four used signals were scored randomly and independently from each other according to American Academy of Sleep Medicine rules. Results of apnea detection using NP, RIPsum and TS signals were compared to those obtained by thermistor as a reference signal.

Results: The number of apneas detected by the thermistor was 4,167. The number of apneas detected using the NP was 5,416 (+29.97%), using the RIPsum was 2,959 (-29.71%) and using the TS was 5,019 (+20.45%). The kappa statistics (95% confidence interval) were 0.72 (0.71 to 0.74) for TS, 0.69 (0.67 to 0.70) for NP, and 0.57 (0.55 to 0.59) for RIPsum. The sensitivity/specificity (%) with respect to the thermistor were 99.23/69.27, 64.07/93.06 and 96.06/76.07 for the NP, RIPsum and TS respectively.

Conclusions: With the sensor placed properly on the suprasternal notch, tracheal sounds could help detecting apneas that are underscored by the RIPsum and identify apneas that may be overscored by the NP sensor due to mouth breathing. In the absence of thermistor, TS sensors can be used for apnea detection.

Clinical trial registration: Registry: German Clinical Trials Register (DRKS), Title: Using the tracheal sound probe of the polygraph CID102 to detect and differentiate obstructive, central, and mixed sleep apneas in patients with sleep disordered breathing, Identifier: DRKS00012795, URL: https://www.drks.de/drks_web/navigate.do?navigationId=trial.HTML&TRIAL_ID=DRKS00012795.

Keywords: flow measurement; home sleep apnea test; obstructive sleep apnea; polysomnography; sleep-disordered breathing; tracheal sound.

Figure 1. Presentation of a tracheal sound transducer.

(A) Diagram of the PneaVoX sensor that uses both an acoustic sensor and a pressure sensor. The sensors are inserted in a protective plastic housing to ensure an airtight acoustic chamber between the skin and the transducer. (B) Placement of tracheal sound sensor right above the sternal notch using a double-faced tape. If necessary, an adhesive bandage could be used over the sensor to hold it in place.

Figure 2. Example of apneas detected separately on four different signals.

Example of apneas—central (A), mixed (B) and obstructive (C)—detected separately on four different signals (flow sound intensity, nasal pressure, thermistor and RIPsum). The American Academy of Sleep Medicine definition of apnea in terms of signal amplitude decrease and duration was applied to all four signals. The esophageal pressure signal confirms the characterization of the illustrated apneas. Note how obstructive apneas could be mistaken for hypopneas when using the RIPsum signal. The duration of the detected apneas could also vary from one signal to the other. RIP = respiratory inductance plethysmography.

Figure 3. Results of the Bland-Altman analysis for apnea detection outcomes considering the thermistor method as the reference.

The mean difference value of the number of detected apneas between the thermistor and the tracheal sounds was smaller than between the thermistor and the nasal pressure and between the thermistor and the RIPsum. NP = nasal pressure, RIP = respiratory inductance plethysmography, SD = standard deviation, Therm = thermistor, TS = tracheal sound.

Figure 4. Examples of apnea detection errors by different signals.

(A) Obstructive apnea missed by the RIPsum signal. Sometimes during obstructive apnea, the RIPsum signal is not reduced more than 90% as it should during apneas because the thoracic and abdominal signals are not necessary in paradoxical movements. Thus, the thorax and abdominal belt signals do not exactly sum up to zero. (B) Central apnea missed by the thermistor signal. Based on the thermistor signal, there is flow and an apnea should not be scored. However, when examining the esophageal pressure, there was no respiratory effort which is interpreted as a presence of central apnea. Note that the event is clearly identified by the nasal pressure, tracheal sound and RIPsum signals. This discrepancy could be due to high sensitivity of the thermal flow sensor. (C) Oral breathing mistaken for apnea by the nasal pressure signal. In the absence of the thermistor and based solely on the nasal pressure, one could score an apnea given that the nasal flow amplitude is reduced more that 90%. Note that respiratory cycles persist on the thermistor, tracheal sound and RIPsum signals. NP = nasal pressure, RIP = respiratory inductance plethysmography.