How to find the infelction points from the data

Hello all can any one help me how to find the inflection point from the data I have. I am new to matlab and tried various methods to find but cannot help for my data.

Can anyone help me to find the inflection point. The data which I have provided is the medical data of patient with pulse waves. I want to find the inflection point at the point where the reflection is ocuuring. Please help me to find the point in each curve.

Thankyou

Answer 1

[Star Strider]

If you want to find the dicrotic notch in each arterial pulse wave, try this:

D = load('data_1.mat');
y = D.column1;                                                      % Data
x = linspace(0, numel(y), numel(y));                                % Create ‘x’ Vector

ix1 = islocalmin(y, 'MinProminence',1.1, 'MinSeparation',100);
ix2 = islocalmin(y, 'MinProminence',5);
ix = ix1 & ~ix2;

figure
plot(x, y)
hold on
plot(x(ix), y(ix), '+r')
hold off
grid

It gets most of them, however since your signal is a bit noisy, it is difficult to get all of them. Other functions, such as findpeaks, might be able to do better. I did not try findpeaks here.

[Alon nani]

thankyou for your fast repley. here in this i need to find the amout of refelction happening in the curve. that occurs in the first increment of every curves. i need to find the inflection point that is anacrotic notch. here I am finding the point after the curve is happening. from the photo i need to find P1.

[Star Strider]

As always, my pleasure!

With that clarification, change the code to:

D = load('data_1.mat');
y = D.column1;                                                      % Data
x = linspace(0, numel(y), numel(y));                                % Create ‘x’ Vector

ix1 = islocalmax(y, 'MinProminence',1.1, 'MinSeparation',75);
ix2 = islocalmin(y, 'MinProminence',25,  'MinSeparation',200);

figure
plot(x, y)
hold on
plot(x(ix1), y(ix1), '+r')
plot(x(ix2), y(ix2), '+m')
hold off
grid

That identifies most of them correctly. There is still a bit to do, however you should be able to take it from here.

[Alon nani]

HI thank you for the help. The code which you have provided is identifying the dicrotic notch not the inflection point(anacrotic notch), I wangt to identify the inflection point (Anacrotic notch).

[Steven Lord]

In addition to islocalmin and islocalmax I think the ischange function might be of interest to you.

[Alon nani]

Where to add ischange in the code. I am new to matlab and unable to figure it out please help me.

[Alon nani]

Please help me with these, please

I am very thankful to you.

[Star Strider]

This is as close as I can get to identifying them:

D = load('data_1.mat');
y = D.column1;                                                      % Data
x = linspace(0, numel(y), numel(y));                                % Create ‘x’ Vector
x = 1:numel(y);
x = x(:);

ix1 = islocalmax(y, 'MinProminence',1.1, 'MinSeparation',75);
ix2 = islocalmin(y, 'MinProminence',25,  'MinSeparation',200);
[ix3,s3,i3] = ischange(y, 'linear', 'Threshold',75);

ChgPts = [s3(ix3)  i3(ix3)  x(ix3) y(ix3)];
conds = ChgPts(:,1)>0 & ChgPts(:,1)<1;
ix3 = ChgPts(conds,3);

figure
plot(x, y)
hold on
plot(x(ix1), y(ix1), '+r')
plot(x(ix2), y(ix2), '+m')
plot(x(ix3), y(ix3), 'gp')
hold off
grid
% xlim([0 3E+3])

It also identifies a few others, so experiment with this.

Also, explore and experiment with the findchangepts function to get the result you want.

[Alon nani]

Thankyou it is working for most of the data.

[Star Strider]

As always, my pleasure!

Answer 2

[KSSV]

https://in.mathworks.com/matlabcentral/answers/383721-how-to-find-inflection-point-please-help

[Alon nani]

But I only have single row matrix

[KSSV]

You x will be

x = 1:length(column1) ; 
y = column1 ; 

[Alon nani]

I need only one point in the curve where the reflection is occuring, that point is called inflection point(Augmented pressure) i want to find that point.