ACCURACY OF THERMISTORS AND THERMOCOUPLES AS FLOW-MEASURING DEVICES FOR DETECTING HYPOPNEAS

The aim of this work was to assess the accuracy of thermistors/thermoc ouples as devices for detecting hypopnoeas in sleep studies. Conventio nal thermistor/thermocouples were studied with a respiratory model all owing the simulation of inspiratory (22 degrees C) and expiratory (37 degrees C) flows. The thermistor signal (V'th) was compared with a pne umotachograph (V'): 1) for sinusoidal and square-wave airflows (+/-0.0 5 to +/-0.8 L.s(-1), 10-20 breaths.min(-1) (bpm)); 2) when changing th e distance from the thermistor to the nose (0-20 mm); and 3) when doub ling the section of the nostrils. The thermistor was strongly nonlinea r and flow reductions (hypopnoeas) were underestimated: a 50% reductio n in V' (+/-0.5 L.s(-1), 15 bpm, sinusoidal) resulted in only an 18% r eduction in V'th, V'th depended considerably on the airflow pattern: f or V'=+/-0.5 L.s(-1), V'th increased by 100% from sinusoidal (20 bpm) to square-wave (10 bpm). For V'=+/-0.5 L.s(-1), 15 bpm, sinusoidal flo w. 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 flow-measuring devices when used at the airflow conditions typical of sleep studies. Their use for quantifying hypopnoeas may lead to considerable underde tection of these respiratory events.