ECG Signal Analysis - ECG Acquisition and Automated Remote Processing - page 37

Fig. 2.14 Fiducial point detection in ECG wave

Figure 2.14 represents the starting points for search of different fiducial points.
The following sections represent search criteria for different fiducial points.

2.4.3.1 Search for Q-peak and Q-onset Point:

The criteria for a valid Q peak are laid down as follows:

1. The voltage level should be equal or below the baseline voltage;
2. The magnitude of voltage should be lowest in a window of 50 ms downside
from R peak;
3. There should be a slope sign reversal around of Q peak. This is ascertained by
average slope computation at ±4-ms positions from Q peak.

If either condition is not satisfied, it is concluded that a valid Q peak does not
exist and Q and Q-onset points coincide with each other. If a valid Q peak is found,
the Q-offset point is determined by a window search of 50 ms downside of Q peak.
The sample with minimum average slope within the window is considered as Q-
onset. The algorithm steps are given in the end of the chapter Appendix 4.

2.4.3.2 Search for P, P-onset, and P-offset Point

The search is initiated from Q-onset point downside along the array to gradually
reveal P, P-offset, and P-onset points. The curvature (positive or negate) of the P
wave is determined by computing the 20-point average slope at 40 samples
downside of Q-offset point. If the slope is found positive (negative), the P wave is
positive (negative), and hence, it is found as the sample with highest (lowest)