Why Can’t You Hold Your Breath Until You’re Dead?
I thought a blog post at PLoS by Greg Downey was interesting, particularly because it intersects with a book that I would also like to recommend, The Disappearing Spoon by Sam Kean, which is about the history of the periodic table.
The post is about holding one’s breath, and, while I’m not recommending working through the entire post, the video that Downy posted is pretty neat.
But mostly I wanted to talk a little bit about why you can’t hold your breath until you die. When you hold your breath, eventually the urge to breathe becomes irresistible, and the question is why this is. The exact reason is, it seems, very complicated (as this paper by MJ Parkes illustrates), but a little is known about how it works. This following is a little rough, but it gives the sense.
The partial pressure of oxygen and carbon dioxide in blood changes as you hold your breath. Oxygen is used up by metabolic processes, reducing its partial pressure, and carbon dioxide is produced, raising its partial pressure. One might think that there are mechanisms to detect these concentrations, and give rise to the impulse to breathe if oxygen gets too low and/or carbon dioxide gets too high.
Early research pointed to the partial pressure of CO2 being the crucial variable. Back in the day before intrusive institutional review boards, studies such as the following (Schneider, 1930) could take place. Hook some people up to a tube, and, without their knowledge, switch the gas they receive from the tube to pure nitrogen. When such subjects inhale from the tube and exhale into the room, their oxygen is going down but carbon dioxide levels are staying relatively constant. Do people experience the panic of imminent asphyxiation? As Schneider (1930) writes: “The sensations experienced during an acute anoxemia, such as result from nitrogen breathing, are not distinctly unpleasant; in fact they are quite like those of nitrous-oxide anesthesia…” Subjects in this study had to be observed and monitored for cues that they were about to pass out – “cyanosis, mask-like facial expression, pupil dilation, eye convergence, falling systolic pressure.” Results such as these point to the idea that it’s the buildup of CO2, rather than the lack of oxygen, that gives rise to the impulse to breathe.
Having said that, Parkes (2006) reviews evidence that shows that it’s more complicated than this, and there could be multiple systems at work here. The fact that the system seems to work on CO2 levels rather than O2 is interesting, of course, but it seems reasonable to think that under normal circumstances – in worlds without tanks of pure nitrogen about – these two are likely to vary systematically with one another, so either could work.
In modern environments, all things are, of course, not always equal, and here enters the vignette by Kean. He discusses how NASA decided to use nitrogen in simulators subsequent to the horrible fire during training for the Apollo missions in 1967 in which three astronauts were killed. Using nitrogen rather than pure oxygen on the ground is useful insofar as it reduces the chance of fires, but its use had a side effect. In one case, five men entered a compartment filled with nitrogen and collapsed. There was no oxygen to breathe, but, like the subjects in Schneider’s study, they simply passed out; two of them died. As Downey remarks, linking this to his interest, diving, the fact that the urge to breathe seems to be triggered by high CO2 levels “may also be the reason that free diving participants pass out with some frequency; they run low on oxygen before carbon dioxide levels get high enough to prompt breathing.” In short, again quoting Downy: “When you breath hold, you are not so much ‘running out of air’ as you are fighting powerful impulses to breathe when you don’t really need the oxygen yet.”
So, anyway, this is all pretty neat. There’s an adaptive problem, which is to replenish oxygen, which is necessary for normal metabolic processes. One part of the solution to the problem seems to be – but, again, only a part, as Parkes indicates – a sensory system designed to measure CO2 and motivate breathing when this level gets high, which is, under normal circumstances, related to when levels of O2 are low. As this level rises, the phenomenological urge to breathe is produced, motivating appropriate action. This urge can be offset, for a time, but only for a time, and this limit seems to have to do with the increasing chance of damage due to lack of oxygen. Again, it’s complex, but still, we seem to be building toward a satisfying account of these systems.
An alternative view is that because breathing is “automatic,” and “overriding” automatic processes “depletes” the “self’s” “resources,” it gets harder and harder over time. This theoretical approach has the virtue of not “resorting” to any sort of functional explanation/storytelling.