How to plot mean electrical axis using MATLAB?

I need to determine mean electrical magnitude and mean electrical axis for a data set using MATLAB. Could anyone suggest a general procedure to follow? I am having trouble plotting the perpendicular lines.

[Azzi Abdelmalek]

Provide some data, and what do you mean by mean electrical axis

[Kathy]

By Mean Electrical Axis I mean the summation of all the occurring vectors in a cardiac cycle. I have data from three leads taken in an ECG and I need to plot the normals to leads 1 and 3, which will determine the mean electrical axis.

Answer 1

[Star Strider]

It's easier to do what you want with leads I and aVF because they're orthogonal. Plot the voltages on I against the voltages on aVF and you pretty much have all the data you need. Since Lead III is defined in the direction of -120°, a line orthogonal to it would be -30°. This is illustrated in this code snippet:

LeadIII = @(x) x.*tand(-120);
LeadIIIOrth = @(x) x.*tand(-30);

x = linspace(0, 1, 10);
LIII = LeadIII(x);
LIIIO = LeadIIIOrth(x);

figure(1)
plot(x,LIII, '-b',  x,LIIIO,'-r')
axis equal
grid

Rename the anonymous functions to be anything you want. Bear in mind that owing to anatomical considerations, tissue characteristics, and lead placement, the angles aren't exactly multiples of 60°, and the measured EKG voltages in the leads don't exactly measure the fields generated by the heart.

A brief PubMed search pulled up a couple articles (free online) that look to me to be relevant to what you want to do, even if they don't specifically address your question:

‘Spatiotemporal representation of cardiac vectorcardiogram (VCG) signals’ Biomed Eng Online 11:16 2012

‘Linear affine transformations between 3-lead (Frank XYZ leads) vectorcardiogram and 12-lead electrocardiogram signals’ J Electrocardiology 42(6):622 2009

Yours sounds like an interesting project. Have fun with it!