Btw: literature references are warmly welcomed.

I can't give a direct answer to your question, but here is a paper that might lead you in the right direction:

Article Forward Problem of Electrocardiography: Is It Solved?

Alexander Wilson thank you. If we consider formal fields and potentials the changes propagate at the speed of light, so in a way this is an answer.

The answer to 'how fast' is 'far slower that the speed of light' - because electric potentials propagate in the body via the flow of ions and via successive shifts/reversals in the cell membrane potentials (inside-to-outside) of adjacent cells. (This is completely different from how electric conduction happens in metallic conductors, or in semiconductors, which basically is at the speed of light.)

Certain (specialized) bodily tissues, e.g. nerves, & the cardiac conduction system, have the characteristic of faster electrical propagation compared to other types of tissues.

Thus, the answer is not a single number.

I'm sure there are many, more-scholarly answers with velocity measurments that are easily found in the literature. Perhaps check 'nerve conduction velocity testing'.

The body surface electrograms are primarily due to volume conduction which can be mainly extracellular rather than cell-to-cell conduction requiring crossing many cell membranes each with their associated capacitances and resistances. Thus although they are still subject to resistive and capacitance effect, they are much less that the multiple component going across cell membranes. The net effect is a much faster conduction, still likely not the speed of light, but very fast.

0.5-1.0 mt/sec in atria, 2.0-4.0 mt/sec in His-Purkinje system.

Article Frequency Content and Characteristics of Ventricular Conduction

The rate through the heart muscle, cell-to-cell is different than the question of propagation through the body.

Certainly. Nerve impulses are extreme slowly compared to the speed of electricity, since electric current can flow in the order of 50-99% of the speed of light; but they are also very fast compared to the speed of the blood flow. Some myelin neurons conduct up to 120 m / s (432 km / h).

Agreed for Nervous tissue. However, cardiac tissue is not nervous tissue. So the cell-to-cell speed is much slower. Even the Purkinje system is made up of specialized cardiac myocytes not nervous tissue.

How fast does the ECG signal propagate within the body?

The action potentials generated by the SA node spread throughout the atria primarily by cell-to-cell conduction at a velocity of about 0.5 m/sec.

The AV node is a highly specialized conducting tissue that slows the impulse conduction considerably (to about 0.05 m/sec) thereby allowing sufficient time for complete atrial depolarization and contraction (systole) prior to ventricular depolarization and contraction.

The left and right bundle branches conduct the impulses at a very rapid velocity (about 2 m/sec). The bundle branches then divide into an extensive system of Purkinje fibers that conduct the impulses at high velocity (about 4 m/sec) throughout the ventricles. This results in rapid depolarization of ventricular myocytes throughout both ventricles.

If there is heart block, impulse passes through the myocardium itself at a much slower rate.

Atrial activation is complete within about 0.09 sec (90 msec) following SA nodal firing. After a delay at the AV node, the septum becomes activated (0.16 sec). All the ventricular mass is activated by about 0.23 sec.

Dear @All, generally you are right that all the mechanisms which require active conductance (i.e. sustained on expense of the energy of the medium and not of the energy of the source) are order(s) of magnitude slower than the passive conductance. Of course conductance velocity depends on neural control, which in case of the heart appears under the name of a dromotropic arm of cardiac regulation, so this value is constrained. What we originally measured was passive conductance velocity.

This is a nice thread in that it adds good clarity to both types of conduction; which for this thread have been labeled either as "passive" or "active" conduction.

Since it has been clarified that the original question was aimed at a a greater understanding of the much faster "passive conduction velocity", I confess that my curiosity now has been spiked. Thus I boldly ask Dr. Buchner if he could share more regarding the research clinical target behind the original question, please (?). Thanks.

Dear Alan Brewer , Dear All,

this is rather basic science approach, which at the time being does not address any specific clinical target, however seems to have a potential. If you look at general, not only bioengineering approach, typically we analyze conductance and capacitance in the language of frequency domain. We identify (or derive from first principles or some simple modelling assumptions) model parameters and finally we get at certain transmittance, which, as we believe, represents the properties of the system. The generality of this approach allowed us to unify many different types of media: vacuum, air, continuous systems, crystals, liquids etc etc. We can address optical properties, dielectric properties, conductance and many other properties and determine the frequency response. The "velocity" of the formal field is related to dielectric and magnetic permeability - if you take wave equation, the inverse of velocity is square root of the product of both permeabilities. It was interesting to us how well does this model mimic the reality of the living tissue with all its structures etc. so we made quite a naive attempt to at least determine the lower bound for this velocity, which was what we actually did.