Accuracy of thermistors and thermocouples as flow-measuring devices for detecting hypopnoeas

Abstract

The aim of this work was to assess the accuracy of thermistors/thermocouples as devices for detecting hypopnoeas in sleep studies. Conventional thermistor/thermocouples were studied with a respiratory model allowing the simulation of inspiratory (22°C) and expiratory (37°C) flows. The thermistor signal (V'th) was compared with a pneumotachograph (V'): 1) for sinusoidal and square-wave airflows (± 0.05 to ± 0.8 L·s-1, 10-20 breaths·min-1 (bpm)); 2) when changing the distance from the thermistor to the nose (0-20 mm); and 3) when doubling the section of the nostrils. The thermistor was strongly nonlinear and flow reductions (hypopnoeas) were underestimated: a 50% reduction in V' (± 0.5 L·s-1, 15 bpm, sinusoidal) resulted in only an 18% reduction in V'th, V'th depended considerably on the airflow pattern: for V'= ± 0.5 L·s-1, V'th increased by 100% from sinusoidal (20 bpm) to square-wave (10 pneumotachograph bpm). For V'= ± 0.5 L·s-1, 15 bpm, sinusoidal flow, V'th increased by 79% when the distance thermistor-nose varied from 20-0 mm, and V'th decreased by 37% when doubling the nose section. We concluded that thermistor/thermocouples are inaccurate how-measuring devices when used at the airflow conditions typical of sleep studies. Their use for quantifying hypopnoeas may lead to considerable underdetection of these respiratory events.