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Poincaré plot quantification
From: |
Samir Sharshar |
Subject: |
Poincaré plot quantification |
Date: |
Fri, 15 Oct 2004 21:49:17 +0200 |
Hello,
I know that the subject is not really relevant in this mailing list,
but I hope someone who's working on biomedical signals helps me.
I'm currently analyzing Heart Rate Variability (HRV). Study of HRV is
lot of interest because it may leads to early diagnosis of myocardical
infarction or sudden death. Methods of analysis call both to linear and
non linear tools.
The raw signal is formed by the RR intervals (ie the time between two
spikes R on the electrocardiogram) or tachogram.
Poincaré plot is drawn by plotting the RR at time i vs RR at time i-lag
(lag usually between 1 and 4). Typically a Poincare plot draw an
ellipse. Two indices are calculated from the graph.
The first (called in the medical litterature SD2) seems to take into
account the differences between the markings and the transversal axe.
The second (called SD1) idem with the longitudinal axe.
To compute SD1 and SD2 I write the following function :
function [SD1,SD2,ratio] = casado_poincare(x,lag)
if(size(x,1)<size(x,2))
x = x';
end
if(isempty(lag))
lag=1;
else
if(lag>4)
error('lag must be >= 1 and <= 4');
end
end
S1 = x(1+lag:size(x,1),1);
S2 = x(1:size(x,1)-lag,1);
# Longitudinal axis
p2 = polyfit(S1, S2, 1); # get the equation of a least square line
y2 = polyval(p2,[1:1:size(S1,1)]);
SD2 = std(y2'-S2); # compute the SD
# Transversal axis
p1 = polyfit(S1,flipud(S2),1);
y1 = polyval(p1,[1:1:size(S1,1)]);
SD1 = std(y1'-flipud(S2));
ratio = SD1/SD2;
I'm not sure that I'm computing SD1 and SD2 in the right way. I do not
found in papers the detailed algorith to do so.
Thanks in advance,
Samir Sharshar MD
EA 2689-Inserm, Lille France
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