Abstract

SIFUENTES, E.; COTA-RUIZ, J.  and  GONZALEZ-LANDAETA, R.. Respiratory Rate Detection by a Time-Based Measurement System. Rev. mex. ing. bioméd [online]. 2016, vol.37, n.2, pp.91-99. ISSN 2395-9126.  https://doi.org/10.17488/rmib.37.2.3.

This paper proposes a system that converts a time-modulated signal from a resistive sensor into a digital signal with the goal to estimate the respiratory rate of a subject. To detect breathing, a known method based on a nasal thermistor, which detects temperature changes near the nostrils, is used. In this work, the thermistor mounted in a Wheatstone bridge, forms a RC circuit which is connected directly to a microcontroller, without using any analog circuit or analog-digital converter. Thus, whenever the subject breathes, it causes a fractional change in resistance x (∆R/R 0) on the thermistor, and this produces a time-modulated signal that is directly digitized with the microcontroller. Measurements were made on 23 volunteers, obtaining changes of x > 0.01. The temperature resolution was 0.2 °C, and the time response was 0.8 s, mainly limited by the thermistor properties; these features were enough to obtain a well-defined waveform of the breathing, from which was easy to estimate the respiratory rate by a compact, low cost and low power consumption system. Unlike interface circuits based on voltage or current amplitude, with this kind of interface, the self-heating of the sensor is avoided since the thermistor does not require any voltage or bias current.

Keywords : time-based measurements; Wheatstone bridge sensors; respiratory rate; temperature measurement; nasal thermistor.

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