Question on units

Question

Hi,

I am working with a heart-in-a-torso setup and the end goal is to compute the ECG at the electrodes on the torso. I have done the openCARP simulation on the heart domain. Next, I need to now compute the Geselowitz integral to compute the ECG, outside of the openCARP setup. So, here is my question on units. Internally, openCARP converts units consistently.  But outside, I am converting as follows, but the ECG magnitude seems orders off.

1) Vm in vm.igb is in mV
2) vm_grad (meshtool extract gradient) is in mV/ micron - converted to SI (i.e mV/m)
3) lead field gradient (lf_grad) in 1/micron - converted to SI
4) Volume is converted to m^3 (as openCARP mesh is in microns)
5) Conductivities are already in S/m - so no conversion needed

With these changes, my computed ECG looks like off by a factor of 1000s.. The morphology of the signal seems fine. My question is the following - is my understanding on the units correct?

And a follow-up question on that Does openCARP support Laplace solve where a current is injected at a site as a BC? It was giving 0 output, but it works with voltage source.

Thanks and regards

~anoop

Answer 1

Hey Anoop,

I am not sure if it is just a typo on your end because you say it correctly. But, if you want everything in SI units, you have to convert vm_grad to V/m not mV/m. That alone would explain the 1000x magnitude error in your ECGs. Otherwise, it looks correct to me.

The Laplace solver currently does not support current injection. Only dirichlet boundary conditions in the form of an extracellular potential stimulus.

Best,

Tobias

Comments

Hello Tobias,

yes the "mV" in SI units was a typo and thanks for confirming that current injection is not supported.

With minor variation and tweaks in the mesh geometry, I get significant variations in the ECG amplitude (after the units corrections), so I guess I can attribute that to the voltage source based solution which will always have a scaling factor related to the unknown "impedance" between the electrodes.. so maybe rather than focusing on the amplitude, a better way would be to focus on the morphology and only look at standardized ECG (scaled within a range)?

Thanks again, and best regards

~anoop