ECG Signal Analysis - ECG Acquisition and Automated Remote Processing - page 38

absolute magnitude with respect to baseline amplitude within a window of 70
samples downside the Q-onset.
The P-onset (P-offset) is found from the P peak by a window-based search
along downside (upside). Both points are taken as the sample with minimum
average slope computed over ±7 points on either side of P peak. The algorithm
steps are given in the end of the chapter Appendix 5.

2.4.3.3 Search for S-peak and S-offset Point

The search is similar to finding Q and Q-onset, with the difference that it is initiated
upside along the sample array starting from the R peak of the respective beat.

2.4.3.4 Search for T, T-onset, and T-offset Point

In principle, the search technique is similar to finding P, P-onset, and P-offset. At
first, the T peak is found after determining its curvature, followed by T-onset and
T-offset point detection. After determining all the characteristic points in the
dataset, the following time-plane features are computed:
The characteristic points are detected in all beats over the dataset and the values
of the different wave durations and intervals are computed. Figure 2.15 shows
some qualitative results of accurate detection of characteristic points with 30,000
samples. For visual clarity, only few betas are shown. In the figure, the onset,
offset, and wave peaks are indicated as separate colored lines along with the
baseline index and baseline voltage. The determined and single baseline voltage is
also shown as horizontal line. The following time-plane features are computed as
follows;

QRS interval = S-offset index - Q-onset index
RR interval = R(i ? 1) index - R(i) index
ST duration = T-onset index - S-offset index
QT duration = T-offset index - Q-onset index.

QT interva ð Þ
R R interval

p

Q ð c ¼

P height = P-peak amplitude - Baseline voltage
T height = T-peak amplitude - Baseline voltage
QRS amplitude = R-peak amplitude - Q-peak amplitude
P-wave duration = P-offset-P-onset
T-wave duration = T-offset-T-onset