Analysis of time series for non-invasive characterization of blood components and circulation patterns

Abstract

Photo-plethysmography (PPG) is frequently used in research on microcirculation of blood. Usually PPG time series are analyzed by conventional linear methods, mainly Fourier analysis. These methods may not be optimal for the investigation of nonlinear effects of the heart circulation system like vasomotion, autoregulation, thermoregulation, breathing, heartbeat and vessels. PPG is a non-invasive procedure and takes minimal time to be carried out. The wavelet analysis of the PPG time series is a specific, sensitive nonlinear method for the in vivo identification of heart circulation patterns and human health status. This nonlinear analysis of PPG signals provides additional information which cannot be detected using conventional approaches. The wavelet analysis has been used to study healthy subjects and to characterize the health status of patients with a functional cutaneous microangiopathy which was associated with diabetic neuropathy. Additionally the fact that the absorption-coefficients μ a and scattering-coefficients μ s for blood differ at different wavelengths has been exploited and is used for calculation of the optical absorbability characteristics of human blood yielding information on blood components like hemoglobin and oxygen saturation. The measured PPG time signals and the ratio between the peak to peak pulse amplitudes are used for a measurement of these parameters. A newly developed PMD device has been introduced. The non-invasive in-vivo multi-spectral method is based on the radiation of monochromatic light, emitted by laser diodes, through an area of skin on the finger. Delays between the proportions of hemoglobin and plasma in the intravasal volume should be detected photo-electrically by signal-analytic evaluation of the signals. The computed nonlinear coefficients are used for the measurement and calculation of the relative hemoglobin concentration change. Results with this photometric method to measure changes in the hemoglobin concentration were demonstrated during measurements with a hemodynamic model and healthy subjects. The PMD is suitable for non-invasive continuous on-line monitoring of one or more biologic constituent values. The objective of this development is to reduce the dependence on measurement techniques which require that a sample of blood be withdrawn from the patient for in-vitro analysis. Any invasive method used on the patient to obtain blood is accompanied by problems of inconvenience, stress, and discomfort. The patient is also exposed to the normal risks of infection associated with such invasive methods.