Photoplethysmographic heart and respiratory rate monitoring : instrument design and evaluation
Sammanfattning: A new optical heart and respiratory rate monitor utilising the photoplethysmographic (PPG) signal is presented.The PPG method, first developed in the 1930s, is based on the fact that incident light to the skin surface is absorbed, scattered and reflected in tissue and blood. By a photodetector placed adjacent to the light source, a part of the backscattered light is analysed. The intensity variations of the reflected light arise from pulsatile changes in the tissue and blood vessels, synchronous with heart and respiratory rates.The PPG-signal was derived from the forearm skin with an optical sensor. With bandpass filters the heart (AC) and respiratory (RIIV) synchronous signals were extracted. The dependence of the AC and RIIV-signal and their signal-to-noise ratio (SNR) on wavelength, intensity and sample volume was studied. The results suggest that the main contribution to the AC-signals comes from pulsations in the ascending arteriole (560 nm) and from pulsations in the larger vessels deeper in the tissue (940 nm). The RIIV-signal originates mainly from venules and veins at both wavelengths. Light distribution in skin was calculated with a mathematical model. The skin tissue was divided into 9 layers including bloodless tissue layers and vessels with pulsative flow. With this Kubelka-Munk model it is possible to explain the results obtained in in vivo measurements at 560 nm and at 940 nm.Digital Bessel filters were designed to separate the AC and RIIV-signals of the PPG-signal. A technique for modifying the amplitude characteristics of Bessel filters, whilst preserving their phase characteristics and certain important properties of their impulses and step responses has been developed. The newly developed filter, which extracts the RIIV-signal, has a very short and nearly constant group time delay in the passband. High stop-band attenuation and short impulse and step responses with little ringing, characterise the developed filter type. Filters designed by this method facilitate the extraction of the AC and RIIV-signals.Evaluation of the heart and respiratory monitor shows that the heart and respiratory rates can be measured simultaneously by recording from the skin surface with an optical sensor. The accuracy of the new instrument is comparable with existing methods for heart and respiratory rate monitoring. A central apnoeic event (arrest of breathing) can be detected with the new device.
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