2-dimensional echocardiograms were performed on 150 standing adults in the parasternal, apical, and subcostal views. From each view, the shortest linear skin-to-heart distance was measured.
parasternal. : adjacent to the sternum.
Average skin-to-heart distances were: parasternal 32.1 +/- 7.9 mm (1.26378 inches).
So it`s less than 2 inches from the surface of the skin to the outer surface of the heart.
1 mm = 0.0393701 inches