Increasing accuracy of pulse transit time measurements by automated elimination of distorted photoplethysmography waves

Figures

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  Fig. 1. Graphical explanation of PTT calculation when using the PW foot as the landmark that indicates arrival of the PW. PTT pulse transit time, ECG electrocardiogram, PPG photoplethysmography

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  Fig. 2. Graphical representation of the seven PW elimination criteria of the ‘7Step PW-Filter’. ECG electrocardiogram, PPG photoplethysmography. S1 the detected PPGfoot should precede the detected PPGpeak in time, S2 the detected PPGpeak should be in the same heartbeat as the ECG R-peak, S3 the detected PPGfoot should be in the same heartbeat as the ECG R-peak, S4 the detected PPGfoot should have a lower magnitude than the detected PPG peak, S5 the detected PPGfoot must be in an upward slope of the valley of the PW, S6 the PW should be complete, the detected PPGpeak should be at a convex maximum of the PW, S7 the steepest rising part of the PW (maximum of its first derivative) must be situated between the detected PPGfoot and the detected PPGpeak

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  Fig. 3. Examples of PPG signals and their corresponding ECG signal for all ten subjects. Each example shows the first 10 s of the baseline measurements on volunteers under tranquil conditions. Subjects 6 and 12 have a PPG signal without any noticeable artefacts. Subjects 5 and 9 have many artefacts, rendering it difficult to identify the suitable PWs

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  Fig. 4. Examples of unsuitable PWs. Section 1 shows a detected PW foot that is not on the beginning of the PW but at the beginning of the clipped dataset. This is not correct because the foot has to be at the start of the PW itself and not at the start of the clipped dataset. Section 2 shows a detected PW peak being lower than the detected foot of the same PW. This is not correct because the peak should always be higher than the foot. Section 3 shows a PW that is nonrecognizable at all, causing the foot and peak to be incorrectly placed at the extreme values the of the clipped dataset. Section 4 shows a detected PW foot occurring later than the detected peak of the same PW. This is not correct, because the foot of the PW has to precede the peak of the PW

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  Fig. 5. Mean PTT values of the dataset of subject number five; no filtering (‘no filtering’), after manual elimination of unsuitable PWs by the first author (‘manual filter’) and after applying the ‘7Step PW-Filter’ (‘7Step PW-Filter’). The given PTT values are means over the entire dataset in which the individual PTT values were taken as averages per 60, 30 and 5 heartbeats or taken for each individual heartbeat. The whiskers indicate the standard errors of the means

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  Fig. 6. Comparison of the ‘7Step PW-Filter’ with the Gil method. The figure shows PTT values for the dataset without no filtering (‘no filtering’), the boundaries set by the Gil method (‘Gil filter range’) and the PTT values remaining in the dataset after eliminating unsuitable PWs with the ‘7Step PW-Filter’ (‘7Step PW-Filter’)