Parameters of Respiration
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Breathing or external respiration is based on the expansion and contraction of the lungs. This mechanical activity can be picked up at appropriate locations on the surface of the skin by a laser Doppler vibrometer. To extract the breathing signal from the raw vibrometer signal, a low-pass filter is required since the bandwidth of the breathing signal is limited to an upper frequency which is below the range of the heartbeat activity. Because of its low distortion, a liner-phase FIR filter is used with an order high enough to suppress the influence of the heartbeat. There are several possibilities to calculate the breathing frequency on the basis of the low-pass filtered vibrometer signal in the frequency and time domain, respectively. The filtered signal is cut into data blocks of fixed length within which the estimation of the breathing frequency is executed. Customarily this frequency with the unit Hz which is identical with the respiration frequency measured in the unit breaths per minute or bpm. Thanks to the overlapping of the processed data blocks every second a new value of the respiration rate is calculated. To achieve a high resolution and a parameter independent implementation, the averaged zero-crossing method and the FFT method with zero-padding is preferred. But also the estimation based on the autocorrelation function is investigated because of its noise resistance based on the inherent averaging. Besides the respiration signal, the ventilation, i.e., the inhalation and exhalation is of interest which can be derived from the route signal of the measurement point. The route signal is the integral of the vibrometer signal which is a speed signal. The breathing can be measured on the thorax, i.e., closest to the lungs, but also at the neck or at other appropriate locations. This has the advantage that the neck normally is not covered by clothes since the laser signal penetrates only just a thin cover of cloth. The new approach is finally evaluated under the aspect of clinical applicability and the need for further investigations.
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