Finding length of time in between peaks.

I am currently working on a project involving signal processing of ECG and PPG data to calculate Pulse Transit Time (PTT). I need to determine the time between corresponding peaks in ECG and PPG signals with precision down to fractions of a millisecond. However, I'm concerned that my current method might not be providing accurate results. I would appreciate any guidance or suggestions on how to improve my calculations.

This is my code at the minute:

clc;
clear;
close all;

All_Data = {};  % set up cell array for all workbook data
Plotted_Data_ECG_PPG = {}; % set up cell array for ECG and PPG plotted data
PTT_Data = {}; % set up cell array for PTT data
BP_Calculation = {}; % set up cell array for BP calculation

SYS_Data_single = {}; % Initialize the cell array for storing single systolic data values
DIA_Data_single = {}; % Initialize the cell array for storing single diastolic data values

collected_BP_Data = [];  

windowSize = 2;  % window size for filtering data
ecgThreshold = 0.375; % Threshold for ECG peaks
ppgThreshold = 0.1; % Threshold for PPG peaks

for i = 1:11  % set up loop to run for 11 iterations (number of workbooks)
    filename = sprintf('%03d.xlsx', i); % set up each filename (001-011) to be read on each iteration
    opt = detectImportOptions(filename); % import data from filename
    sheets = sheetnames(filename);  % retrieve names of sheets
    
    for j = 1:numel(sheets)  % iterate over each sheet within a workbook 
        tBC = readtable(filename, opt, 'Sheet', sheets{j});  % extract sheets from workbooks
        Time_Data = tBC{:, 'Var4'};  % extract time data from column 4
        
        % Remove invalid cells from time data column
        rows2delete = isnan(Time_Data);
        tBC(rows2delete,:) = [];
        
        Time_Data = tBC{:, 'Var4'};
        % Fit polynomial line to smooth out time data
        xx = (1:numel(Time_Data))';
        p = polyfit(xx, Time_Data, 1);
        Time_Data_new = polyval(p, xx);
                
        ECG_Data = tBC{:, 'Var6'};  % extract ECG data from column 6
        PPG_Data = tBC{:, 'Var8'};  % extract PPG data from column 8
        SYS_Data = tBC{:, 'Var10'};  % extract systolic pressure data from column 10
        DIA_Data = tBC{:, 'Var12'};  % extract diastolic pressure data from column 12

        % Select the first value as the single data value for SYS and DIA
        SYS_Data_single_value = SYS_Data(1);
        DIA_Data_single_value = DIA_Data(1);
        
        % Apply filter to ECG and PPG data
        ECG_Data_smoothed = movmean(ECG_Data, windowSize);
        PPG_Data_smoothed = movmean(PPG_Data, windowSize);
        
        % Normalize ECG and PPG data around 0 for accurate calculations
        ECG_Data_normalized = ECG_Data_smoothed - mean(ECG_Data_smoothed);
        PPG_Data_normalized = PPG_Data_smoothed - mean(PPG_Data_smoothed);
        
        % Find peaks for ECG and PPG data above thresholds
        [ecgPeaks, ecgPeakIdx] = findpeaks(ECG_Data_normalized, 'MinPeakHeight', ecgThreshold);
        [ppgPeaks, ppgPeakIdx] = findpeaks(PPG_Data_normalized, 'MinPeakHeight', ppgThreshold);
        
        % Calculate PTT for each pair of corresponding peaks
        numPeaks = min(length(ecgPeaks), length(ppgPeaks));
        PTT = zeros(numPeaks, 1);
        for k = 1:numPeaks
            PTT(k) = (Time_Data_new(ppgPeakIdx(k)) - Time_Data_new(ecgPeakIdx(k))) * 1000; % Convert measurement to milliseconds
        end
        
        % Make PTT values positive
        PTT = abs(PTT);
        
        % Place the PTT data in PTT_Data cell array
        PTT_Data{i, j} = PTT;
   end
end   

I would greatly appreciate any help. I have included one of the files that is being used.

[Rena Berman]

(Answers Dev) Restored edit

Answer 1

[Star Strider]

I already addressed this in my Answer to your previous question on exactly the same topic.

If you are having problems with it, address it there rather than opening another question.

My problem with your file is that the ‘TIME:’ values make no sense. I originally thought that they were fractions of a day, and so converted them to something useful using 'ConvertFrom','datenum' however those values do not make sense. I had to construct a millisecond vector to make the times come out even remotely realistic, however it is obvious that those are not correct. Your code does not at all address the ‘TIME:’ vector.

You need to provide information on the ‘TIME:’ column that permits the values to be useful. There is no current information on how to interpret those values.

EDIT — (12 Mar 2024 at 14:56)

I took another look at the times that I created (calculated from the ‘TIME:’ variable), and I believe they are correct, based on the calculated heart rate (that I just now added). So my interpreting them as fractions-of-a-day and converting them as ‘datenum’ representations appears to be correct.

.