The Heart is no lower than the mamilary line
Leveled with the upper half of the arm
(not to the lower half) see pretending to Tefilin
We know that the largest portion of the heart is located on the left side of the chest beneath the sternum and above the spine.
The heart doesn't sit flush in the chest, but is turned so that the left ventricle is more posterior and the right ventricle more anterior.
The apex, or bottom portion of the heart is located at the fifth intercostal space, mid-clavicular line and is also known as the point of maximal impulse or PMI. (see link about the 5th rib)
Knowing the correct location of the PMI can help you in evaluating heart size. The base, or top portion, of the heart is located at the second intercostal space and centered under the sternum with the edges extending out just beyond the sternal border on each side.
details about where "Level With The Heart" is actually located