I concur with others, that it makes little difference if the lungs are cut through high, low or dead center. In my 50+ years of white tail deer hunting, I have seen no significant differernce in how long it takes for the deer to expire, in the difficulty tracking, or the distance traveled after the shot, etc. As for heart shots ... no doubt as good a shot placement as there is. BP drops like a hammer and the deer will go down very quickly. But again, a small target area with little room for error.
Using a high power rifle, a hard hitting shot gun slug, or a big ol' 50 cal. muzzle loader bullet, etc. .... missing a few inches high-low or left-right more times than not makes little difference. These result in long, wide wound channels with terrific shock damage aalong the way.
Obviously a broadhead has to clean cut something vital. A heart shot ... miss 3" - 4" high you are likely hitting hard, tough bone and may not get the penetration to cause a fatal injury. I have tracked such shots for hours upon hours with no success in recovery. Or miss a few inches low, you end up slicing the brisket. If not high enough to clip a largeartery or vein, the deer will likely not bleed out quickly if at all. Last fall for example my bud made such a shot, though aiming for the heart of a large doe. It dropped like a hammer and lay there for over an hour. When my buddy came down and approached, she jumped and hauled a$$. He tracked for over an hiour and gave up for lack of blood trail. My tracking dog got to the trail about 4 hours later. She trailed the doe almost a mile and we found the doe lying down, unable to get up. We dispatched her quickly. No dog ... no deer. If that shot had been 2"-3" higher, it would have been a perfect heart shot.
Don't get me wrong Big Tuna. A heart shot is a 100%, drop dead, quickly killing shot. Make it if you can. I just go for the double lunger every chance I get, archery or firearm. And at my age, a big kill zone is welcomed !!