ECG Signal Analysis - ECG Acquisition and Automated Remote Processing - page 33

Fig. 2.11 Finding slope
inversion across maximum
and minimum point in a
96-ms window

p max

slp _ r 18+

slp _ r 20-

96 ms window

slp _ q 20-

slp _ q 18+

p max

match ATC, this window may contain either R or Q or S peak as its local maxima
or minima, or even tall T peak for some ECG records.
Figure 2.10 represents some of the possibilities of different QRS shapes which
may be captured by moving 96-ms window. Now, from this position, any of the R
or Q or S peak within this window is determined by a direction-based slope sign
reversal search initiated from the local maxima or minima, as illustrated with
Fig. 2.11 .
This is determined as follows:
Let p max and p min are the indexes of maximum and minimum points in a 96-ms
window. Then, 16-point average slope at 20 samples downside (i.e., decreasing
index) and 18 samples upside (i.e., index increasing) at p max and p min is calculated.
Let these values be:

for p max: slp r 20 ; slp r 18 þ ; for p min : slp q 20 and slp q 18 þ :

and

ð 2 : 12 Þ

if slp r 20 [ 0 and slp r 18 þ \ 0

we may conclude that there is a slope sign reversal near to p max

and if slp q 20 \ 0 and slp q 18 þ [ 0 ð 2 : 13 Þ

we conclude that there is a slope sign reversal near to p min.
Both Eqs. ( 2.12 ) and ( 2.13 ) may also be satisfied in a moving 96-ms window.
But it does not ensure a neighborhood of R peak, since a positive or negative T
peak may also satisfy Eqs. ( 2.12 ) and ( 2.13 ) respectively. To determine the correct
position of QRS, rule (2), i.e., STC criteria is used.
For Eqs ( 2.12 )or( 2.13 ) being satisfied, a new window of 96 ms is formed around
p max (or p min ) as midpoint. From the starting point of the new window formed, for
each samples (say position k), 8-point average slope is computed at k and (k ? 25)
position, to assess the position of upside and downside of positive R wave.

Condition 1 : ð slope Þ k [ ¼ slp th ð 2 : 14 Þ

Condition 2 : ð slope Þ k þ 25 \ 0 ð 2 : 15 Þ

where (slope) k is slope at k, and (slope) k+25 is the same at k ? 25 position.