Thermal Infrared Imaging: A Novel Method to Monitor Airflow During Polysomnography

THE DIAGNOSIS OF SLEEP APNEA TYPICALLY INVOLVES AN OVERNIGHT POLYSOMNOGRAM WITH CONTINUOUS MONITORING OF SEVERAL PHYSIOLOGIC parameters and surrogate measures of airflow (nasal pressure [Pn], oronasal thermistor, expired CO2 waveform [PECO2]) using contact sensors.1–5 A subject can come in contact with at least 20 such sensors during a study. These sensors and the wires can influence not only the usual sleep architecture, but also the posture of the patient during the study night.6–8 Contact sensors like nasal prongs and oronasal thermistors are routinely used as surrogate measures of airflow because they are less obtrusive than the reference-standard airflow sensor, namely, the pneumotachometer.1,9,10 However, the presence of the surrogate airflow sensors and the associated wires in the proximity of the subject's oronasal area can still cause discomfort to the patient during sleep. Contact sensors also have the potential to misalign during the study, requiring repositioning at the bedside by a technician, to obtain good-quality data. We have demonstrated that measurement of breathing rates is feasible at a distance using thermal infrared imaging (TIRI).11 The method is based on second-order statistics of the presence or absence of the hot expiratory plume in the vicinity of the nostrils. We have also studied a more advanced method that is based on wavelet analysis of the thermal imaging signal on the nostrils themselves.12,13 When analyzing the thermal imaging signal, we extracted the full breathing waveform (rate and amplitude). Consistent segmentation of the nostril area and facial tissue-tracking algorithms14 enabled sustained monitoring of breathing, resulting in functionality equivalent to that of a thermistor, delivered in a contact-free manner. In 5-minute recordings of 20 awake healthy individuals, agreement between breathing waveforms from the virtual (imaging) and contact thermistors was found to be greater than 90%. The hardware and software system that was developed to implement the latest method is called automatic thermal monitoring system (ATHEMOS). The centerpiece of the system is a midwave infrared camera (FLIR® SC-6000), supported by a black body for calibration and a pan-tilt mechanism for positioning. The system is controlled and the acquired imaging signals are processed by real-time custom software developed in our labs. ATHEMOS and the imaging method that we reported on in a previous publication12 constitute the TIRI portion of the present study. The objective in this current study was to determine the efficacy of TIRI to detect apnea and hypopnea by using conventional surrogate airflow sensors as comparison standards during polysomnography. Thus, our hypothesis was that there would be a very high degree of chance-corrected agreement (κ ≥ 0.6) between TIRI and each of the conventional flow sensors, such as thermistor, Pn, and PECO2 in the detection of apnea and hypopnea. Some of the data from this manuscript has been presented at scientific meetings and published in the form of abstracts.15,16