Typically, TEM fixations are said to penitrate about 1/2 mm on average (Tissue type dependent). 1mm thick tissue is usually fixed for between 1-2 hours with the primary fixative. I can not guarantee that full penetration of the tissue will occur for tissues 2mm thick. It might, but the only way to know is to run a test, then section and check with TEM. A nice technique is to section thicker tissues w/ cryostat at 1mm thickness. Check w LM. If the sample looks good, then re-mount on a blank stub and thin section for TEM. This works nicely for immuno-labeled tissues. Result is that I did not waste my time thin sectioning “blank, non-labeled tissue.

Whole heart fixations are difficult to do well. Fixatives like what you have described are ”ok” for light microscopy level observation, but a doul fixation using a mix of PF with glutaraldehyde would produce a better looking specimen under the microscope. However, fixatives are generally thought of as shallow penetrators, perhaps 1/2 to 1 mm. Even with a blend of immersion and perfusion done at the same time, there will likely be internal regions of the tissue that are not well fixed, which will be obvious when sections of the heart are examined with microscopy.

It depends of which Kind of tissue