Why did you put electrodes on xyphoid area? Do you need 3D axis?

If you need only 2D then: http://en.wikipedia.org/wiki/Einthoven's_triangle

If you need 3D, then you can place a few electrodes on the chest to get the "frontal plane" data.

Using the Einthoven triangle simply pick the two longest vectors to determine axis. Polarity is not a real issue, if you swap exploring and reference electrodes, you will see the mirror vector pointing to opposite direction but amplitude is correct.

I obtain electrocardiograms (ECGs) using a Nihon Kohden ECG-6851 single-channel electrocardiograph (Nihon Kohden, Tokyo, Japan), with electrodes that were modified to maximize contact with the shaven skin of the rat (elbows and knees (shaved and alcohol 70)) . Registers were taken in the three bipolar derivations (I, II and III) and in the unipolar limb derivations (aVR, aVL and aVF). Paper speed is

50 mm/s and sensitivity is 1 mV/cm.

I calculate the heart axis using two methods. Both of them are commonly used:

a) Tilley´s table in I and III derivations

b) Mathematical formule: The formula used in this website http://www.my-ekg.com/formulas-calculos-ekg.html is based on the calculation of the frontal QRS axis using the I and AVF derivations and trigonometric calculation by magnitude of the vectors.

If you need something more only you need contact with me by mail.

Mucha suerte !!!