How to plot RR interval?

Hello everyone!

I have an RR interval (attached) extracted from an ECG signal that was sampled at 500 Hz. The R peaks and their indices were extracted using findpeaks(), then the diff() function was used on the R indices (expressed in seconds) to calculate the RR interval. The RR interval was converted in milliseconds following the standard procedure. Below an extract from the code:

[qrspeaks, locs] = findpeaks(iwt, time, "MinPeakHeight", 0.5, ...
    "MinPeakDistance", 0.42);
% RR interval
RR = diff(Rindex);
RR = RR'.*1000;  % conversion in ms

Now I should plot this signal on the y-axis, while on the x-axis I should have the time (in seconds) at which the heart beat occurs.

I need the time in seconds of the RR interval also for the spline interpolation of the signal. I would like to start interpolating from the first value of the signal, so to avoid the extra values between 0 and the first value of the RRI. I attached a plot as an example to make myself more clear. Below the code for the interpolation:

x = 1:length(RR);
xq = 0:0.25:length(RR);  % Instead of 0 I would like the first index value of the RRI
s = spline(x, RR, xq);
timespline = (1:length(xq))/4;

If you have different suggestions feel free to share, they're always welcome!

Thank you in advance for your reply!

[Bhupati kumar]

how do i get rr indices?

Answer 1

[Altaïr]

Hey @Luca Merolla,

I noticed you’ve successfully extracted the RR intervals and shared them in the RRI.mat file. Since the corresponding time array is not provided, a dummy time vector spanning 1s to 24s in 1s increments has been assumed for answering the query. The time array is constructed as follows:

t = 1:length(RR);

To perform the interpolation starting from the first valid RRI value (while avoiding near-zero values), define the query points beginning at t(1) (which is 1s in this case):

xq = t(1) : 0.25 : t(end);

Below is the full code for reference:

load('RRI.mat')
t = 1:length(RR);
% Define query points (e.g., 4 Hz resampling: step = 0.25s)
xq = t(1) : 0.25 : t(end);  
% Spline interpolation
s = spline(t, RR, xq);  
% Plot results
figure;
hold on;
plot(t, RR, 'o', 'DisplayName', 'Original RR');
plot(xq, s, '-', 'DisplayName', 'Spline Interpolation');
xlabel('Time (s)');
ylabel('RR Interval (ms)');
title('Interpolated RR Intervals');
legend
grid on;
hold off;

For more details on the spline function, you can check the MATLAB documentation by running:

web(fullfile(docroot, 'matlab/ref/spline.html'))