1. Field Of the Invention
The invention relates to a device for filtering out baseline fluctuations from physiological measurement signals, comprising a sampling stage for the formation of sampled values of the measurement signals, a first low-pass filter connected thereto, a downstream stage for the reduction of the sampling rate by a predetermined factor, a subsequent nonrecursive second low-pass filter, a subsequent interpolation stage for increasing the sampling rate by the predetermined factor and a subtraction stage for the subtraction of the sampled values coming from the interpolation stage from the sampled values of the physiological measurement signal, coming from the sampling stage.
2. Description of the Related Art
Physiological measurement signals which are taken from patients are normally overlaid by disturbance signals such as, for example, the 50 Hz (60 Hz in the United States) alternating voltage induced by the alternating current grid, electrical muscle potentials and artifacts in conjunction with the taking of the physiological measurement signals from the patient. These disturbances are expressed, to the extent that they are of low-frequency type in comparison with the characteristic frequency of the physiological measurement signals, in the form of baseline fluctuations in the recorded measurement signal progression.
"Journal of Clinical Engineering", vol. 7, No. 3, July-September 1982, pages 235-240, discloses a device of the abovementioned type for filtering out baseline fluctuations from an electrocardiogram. In the disclosure, the baseline fluctuations are filtered out from the electrocardiogram signals in a high-pass filter which comprises a first nonrecursive low-pass filter with a downstream stage for the reduction of the sampling rate by the factor 8, a second nonrecursive low-pass filter connected thereto, a subsequent interpolation stage for increasing the sampling rate by the factor 8 and a subtraction stage, in which the baseline fluctuations obtained as a result of the low-pass filtering are subtracted from the electrocardiographic signal affected by the baseline fluctuations. As a result of the use of nonrecursive (FIR) filters having a pulse response with a limited number of pulses (finite impulse response), phase shifts and thus signal distortions in the filtering are avoided. However, the computing effort, i.e. the number of computing operations to be performed in the filter (multiplications) is very great in the case of nonrecursive filters, thus the sapling rate of the electrocardiogram is reduced by the factor 8, before the electrocardiographic signal is fed to the second nonrecursive low-pass filter. In this case, the first low-pass filter serves for the frequency limitation of the electrocardiogram, that is required for the reduction of the sampling rate. The greater the reduction factor, the smaller the computing effort in the case of the second nonrecursive filter, but at the same time the computing effort in the case of the first nonrecursive filter increases. Accordingly, in the case of the known device, there is provided, ahead of the actual baseline filtering, a prefiltering with a reduction of the sampling rate by the factor 2. As a result of this the resolution of the electrocardiographic signal in toto is reduced.