Resumen de Estudio y desarrollo de nuevas tecnicas de medida no invasivas en extremidades inferiores para supervisar parametros fisiologicos en entornos domesticos
Heart and respiratory rates are considered two of the five parameters that can be used for assessing the health condition of any subject. However, most of the systems used to supervise these parameters are intended for clinical use. In this work we proposed noninvasive techniques to measure the heart and respiratory rates in static conditions. In order to make these techniques reliable for home practice, we simplify the measurement procedure by using only the low extremities of the subject.
To detect the heart rate, an active and passive technique has been proposed. The active one detects the electrical impedance variations related to the volume changes in some larger vessels in low extremities. The measurement is based on the impedance plethysmography method, but instead of using the typical band-type electrodes, we use dry platform-type electrodes, similar to those used by some commercial weighing scales that estimate the body fat/composition of a subject.
The passive technique detects the force ballistocardiogram (BCG) of a subject that stands over a common electronic weighing scale. For this, only the load cell used for estimate the subject's weight is needed. We sense the vertical force yielded by the blood flow through the larger vessels of the cardiovascular system in each heart beat. The technique works in subjects with footwear, provided that he/she could be stand still and straight over the scale.
With both methods, the signal-to-noise ratio (SNR) of the cardiac signals detected is high enough to estimate the heart rate by using an adaptive threshold algorithm. However, the SNR depends on movement artefacts, so short-term measurements are recommended.
From the cardiorespiratory interaction present in the force BCG, the respiration of a subject that stands over a common electronic weighing scale has been extracted. We sampled the amplitude modulated BCG synchronously with the heart rate in order to obtain a respiratory signal at real time. The technique works even in subject with footwear and is not necessary to attach any electrode/sensor to the body, which simplifies the measurement procedure.
Coherent demodulation has been used in some of the techniques proposed and we demonstrated that this method reduce the contribution of external interferences, but not those that comes from the subject (movement artefacts).
The electronic noise can be important because of the high gain used in the implemented circuits. When a single-ended ac-coupling network is connected to an amplifier's inputs, at low frequencies, the most noise comes from the noise current of the amplifier. Nevertheless, we have demonstrated that this contribution becomes lower when we use balanced fully- differential ac-coupling networks.
An electronic based on off-the-shelf integrated circuits and the use of the already incorporated sensors on a electronic weighing scale, makes the techniques proposed an alternative to periodically supervise the heart and respiratory rates of a subject in home.
