Are there any official rules for sharing a held breath?

Question

In many movies, there are dramatic scenes where one victim is trapped underwater and another is responsible for giving them air.

Are there any official rules for those dramatic "rescue breath" moments?

We are writing an adventure where PCs are escaping a collapsing cavern underwater. PCs can get trapped underwater by boulders as they try to escape. PCs can help each other while keeping track of their oxygen reserves.

The challenge we have discovered there is a large gap between characters with high Constitution who can last for dozens of rounds underwater - and the weakest PCs who die in just 6 rounds: an almost 10x difference. Additionally, there seems nothing the long-lasting PCs can do to help the short-lasting PCs. Ideally, we are hoping to find a rules mechanic where one PC can help another PC last longer underwater - such as sharing some of their breath (i.e. like we see in movies) and triaging who they can save while balancing it against their own chances of survival as well.

Here is the table of breath holding we created from the rules:

PCs can hold their breath for a number of minutes equal to 1 + their Constitution modifier (minimum of 30 seconds). Once choking, a PC survives a number of rounds equal to their Constitution modifier again then drop to zero hit points the next round.

$$ \textbf{Breath Holding Table} \\ \begin{array}{r|l|l} \text{Con modifier} & \text{Seconds of breath} & \text{Rounds of breath until 0 hit points} \\ \hline \text{-4 to -1} & \text{36 secs} & \text{6} \\ \text{0} & \text{66 secs} & \text{11} \\ \text{+1} & \text{132 secs} & \text{22} \\ \text{+2} & \text{198 secs} & \text{33} \\ \text{+3} & \text{264 secs} & \text{44} \\ \text{+4} & \text{330 secs} & \text{55} \\ \end{array} $$

From this, it is clear that a PC with high Constitution has substantial extra time from a single breath to survive. Are there any rules for PCs blowing air into the mouth of another PC to prevent them from drowning? For example, could a PC give half of their breath to another PC - thus perhaps extending their time by half again?

Note regarding medical viability from comments: This is, of course, how CPR works as well. The air we breath in is 20% oxygen. The air we breath out is 15% oxygen. Hemoglobin is unusually effective at extracting additional oxygen.

Answer 1

No Official Rules Exist, however...

The answer is probably "No." While D&D Science! doesn't exist in the same way that Real World Science! exists, some things inherently have to stay the same in order for there to be any basis in reality for players to find a foothold in the game.

For example, the maximum time you can hold your breath in D&D without starting to suffocate is 1 + your Constitution modifier, which is capped at +5. This means you can hold your breath for up to 6 minutes.

A creature can hold its breath for a number of minutes equal to 1 + its Constitution modifier (minimum of 30 seconds).

After that, you have a number of rounds equal to your CON modifier, still capped at +5, meaning the maximum time you have remaining (excluding Death Saves) is 30 seconds before it is officially dying.

$$5 rounds * 6 seconds/round = 30 seconds$$

When a creature runs out of breath or is choking, it can survive for a number of rounds equal to its Constitution modifier (minimum of 1 round). At the start of its next turn, it drops to 0 hit points and is dying, and it can't regain hit points or be stabilized until it can breathe again.

This means that, total, one can hold one's breath for 6.5 minutes before dying.

Now, "coincidentally", the longest possible time one can go without oxygen before the brain begins to irreparably damage is ~5-6 minutes. In other words, D&D is actually pretty faithful to science here.

Now, how does this tie-in to the question?

A Typical Breath consumes about 5% of the total oxygen in the volume of air inhaled. This is why one can breath into and out of a bag, even a plastic one, and not immediately die (well, unless one gets the thing lodged in one's throat).

However, when you hold your breath, your body starts using much more oxygen out of that breath simply because the body is attempting to maximize the available oxygen out of that volume.

In fact, a measurable increase in used oxygen is found even when you only slow your breathing down and don't completely hold it. While I can't find any official figures for the amount of oxygen used, that NCBI paper found close-to a 10% increase in blood-oxygenation levels when breathing was slowed to 6 breaths per minute, or one breath every ten seconds. And, since matter can't be created nor destroyed (aside from magic quantum physics, but even then, energy is conserved), that means that that extra 10% Sp₀₂ is coming out of the air being inhaled.

Even without this, two creatures (assuming both of the same, size, stats, and metabolic requirements) sharing a breath does not increase the effective amount of oxygen.

• At best, the ratio of consumption increases exactly how the volume increases, so twice the air is available, but consumed at twice the rate. Therefore, no change.
• In the ideal realistic scenario, some air is lost as the two creatures attempt to align their respiratory orifices to each other to share this breath, meaning there is less than twice the oxygen but still twice the consumption.
• In the worst realistic scenario, one of the pair (or both) loses their hold on their breath and vents pretty-much all of it, meaning that there is roughly the same amount of air (or none) and still twice the consumption, meaning you have about half (or none) of the survival time.

So, overall, no positive change can result.

Answer 2

Well, as there are no official rules let's see.

The maximum length of time a character can hold there breath in DnD is 6 minutes, which is 1/4 the max in the real world (24 minutes, 3.45 seconds without passing out), but that is with them hyperventilating on oxygen first, so does not really count. Free divers can hold their breath for about 11-15 minutes without hyperventilating in pure oxygen. They also have a larger lung capacity between 1 and 4.5 liters larger (the average adult male has a lung capacity of 6 liters). How does this apply to dnd? Not sure in the real world you don't suffer brain damage until 5 minutes after you pass out, and even then it is possible to be resuscitated long after that depending on the temperature of the water.

This all tells us that real world science does not apply to dnd, which we all probably knew, so if cannot help us, however math can.

If you (a low con character) can hold your breath for 30 seconds and I (a high con character) can hold mine for 6 minutes, if you exhale and then we share a kiss of life (my attempt at humor, aka share a breath of air), and I give you half a lungful, you will only have 13.75 seconds of oxygen. While I only have 2 minutes and 45 seconds of oxygen.

The math behind this: 6 min -30 s = 5.5 min 5.5 / 2 = 2.75 min (sharing half a lungful of air)

2.75 min/6 min ~= 0.45 (the actual amount of good air I have given you)

0.45 * 30 s = 14.75 s (the number of seconds of air you now have).

Answer 3

So as a couple of answers have already stated, there is nothing in the existing RAW to guide a DM on this.

If you’re anticipating that it will be a major element of a campaign and that your players are likely to think of trying it, it may be worth building a guide into the campaign or simply homebrewing a way to resolve it.

Doing this well requires three things: plausibility, playability and reasonability.

Plausibility

Is it plausible that the way you rule is how it would work in the real world (or at least in a fantasy world with magic but which still adheres to most of the same physical rules)?

I feel there are two factors which determine how long a person can hold their breath; their lung capacity and the rate at which they consume oxygen (each a part of what a character’s constitution score represents.)

It would be implausible to say a barbarian with con 20 could empty his lungs into the relatively tiny lungs of a rogue with con 10, or to say the air in his lungs after 3 minutes is just as useful to him (or the rogue) as freshly breathed air.

Playability

Science is often complicated. DnD 5e is certainly less complicated than 4.5, but it’s still complicated enough without trying to be entirely faithful to every element of physics etc. When devising a system that deals with a version of real world mechanics, it still has to be simple enough to avoid holding up play with oodles of rule calculations.

If we go with exact percentages and ratios it starts getting too complex. Something like lung capacity of 3 + con modifier litres might work, and having useable oxygen in a lungful of air decend by quarters based on how much of total time has been consumed might work. (Rounded down, of course.)

Reasonability

Obviously we don’t want to break the game, and set precedents for really dumb unbalanced things. With a rule for something as incredibly specific as this I don’t immediately see how it could be a problem, but after making your best guess, you really just have to play test to see how it goes.

In the end, all three considerations may need to be revised on the fly as you play test and find the bugs in it.

Example situation

Let’s assume a spherical cow... I mean, let’s assume a Barbarian of CON 20 and a Rogue of CON 10.

+——————————+——————————+——————————+
|          |Barbarian |Rogue     |
+——————————+——————————+——————————+
|minutes   |         6|         1|
+——————————+——————————+——————————+
|capacity  |        8L|        3L|
+——————————+——————————+——————————+

After 1 minute, the rogue is about to start choking. If the barbarian gives him some of his breath, by the above guidelines the barbarian gives 3 of his 8 L to the rogue, which rounded down leaves him with half his capacity. The 5 minutes he had left becomes 2.5.

He has already used some of the oxygen in that air (1 minute's worth out of 6), so rounded down to the quarter, the 3 L the rogue gets (a lung full for him) is only 3/4 full of oxygen, good for 45 seconds.

That’s far from trivial, but doing it again would take 3 of the barbarian’s 5 L - he’s not gonna be able to sustain that.