For tachanun, al cheit, twice in selach lanu etc, we bang on our heart. My question is: if someone has Situs inversus, a condition where the heart is on the right side of the body instead of the left, would they bang on their right side (because that's where their heart is), or their left (because since nearly all Jews have it on their left, we don't look at his body personally, he just follows the Jewish custom to bang on the left)?
In SA OC 607 the Magen Avraham brings two reasons for this custom:
1) The Ari Z"L says that one should pound on their chest
2) A Medrish Kohelet that says "We bang on our hearts to show that that it (the heart) is what lead us astray"
Now I think both sources are expressing the same idea BUT in this case perhaps we can say that -
According to the ARI Z"L then someone with Situs Inversus can bang on the left side like everyone else.
According to the Medrish then one should bang on the side the heart is on.
My wife heard a drasha from a chasidic source that may apply. The left side is considered din and the right side is considered Rachamim. By banging on the left side of the chest we are "suppressing" din and asking for rachamim.
I checked the Art Scroll machzor, the Art Scroll daily (Hebrew only and with translation), and the Koren (Rabbi Sacks) daily siddurim. They all say
"Bang the left side of the chest with the right hand."
Apparently they hold that it does not involve where the heart is located.
Here is what comes to mind: Someone who is a lefty puts his Tefillin on his right arm, even though his Tefillin will not face the heart. Moreover, Nishmas Avraham Orach Chaim 27:1 brings down that someone who is right handed and who has dextocardia (heart on right side of the chest) still puts his Tefillin on the left arm, and the fact that his Tefillin doesn't face his heart does not preclude this person from putting it on his left hand.
Since we see that by Tefillin, the placement of the heart does not effect the outcome of where one puts on Tefillin, we can possibly extrapolate that by the Al Cheit's and matters of the like, the heart placement does not matter and should bang like someone with a normal heart position.