@Sparksbet No not mouth-to-mouth resuscitation. That's after someone is already collapsed. This is swimming to someone underwater and delivering a breath to them. (We don't use all the oxygen in our breath.) The air you breath in is 20% oxygen. The air you breath out is 15% oxygen.

Are you asking as a player or as a DM? (PS, I learned buddy breathing during a scuba course over 40 years ago. That wasn't me exhaling into someone's lungs ... )

@HellSaint I'd say that if the modifier is 0 or less they last zero rounds. I think the table is wrong, with the text as written. "Once choking, you survive zero rounds" makes sense for a con modifier of zero, based on the text as written.

@HellSaint Thanks for the catch. I applied the "+1 round to the other rows now." Hopefully now right.

@KorvinStarmast Per your reference to (rpg.meta.stackexchange.com/questions/5270/…) this question falls into the #2 answer category of the accepted answer.

OK, I'll remove that comment. Your first two lines on the table still do not fit the rules text as presented. Also, are you asking as a DM or as a player? You are in a world filled with magic. Why isn't buddy breathing being addressed by magic?

@KorvinStarmast I am not following the math error. 30 seconds minimum (for any negative modifier) + 6 seconds (1 round) until you drown = 36 seconds. See HellSaint's comment as well. Is there something I am missing?

Six times 0 equals zero. That's the problem. Six times zero does not equal 1. You don't get a round of choking if you don't have a con modifier that results in a positive number.

And I ask you a third time, are you asking as a player, or as a DM? What problem are you trying to solve?

@KorvinStarmast Drowning = 30 seconds. Choking = 0 seconds. 0HP = 6 seconds later. Total = 36 seconds.

Not as I see it. 0 HP happened without delay because there is zero choking. I understand how you arrived at that and disagree that 0 = 1 round. You only get that grace if you have a modifier that is a positive number. I honestly think that this is a way better question for a forum, since it appears that you are trying to come up with a homebrew to replicate what you mention in your question about stuff in ER.

@KorvinStarmast - the text says "you drop to 0 hp the next round". Our column heading was time to 0 hp.

And I ask for the fourth time: are you asking as player or DM? What problem are you trying to solve? Are you trying to come up with a house rule that fits this idea of yours into the game?

@KorvinStarmast That's an interesting interpretation but that's not the question. We are just asking if official rules exist whether for player or for DM. Not homebrew.

@KorvinStarmast The question states that.

OK, hope it all works out.