Hyperventilation question

[greengreg]

Hi everyone!

I'm new to this board, and I have a question on hyperventilation.

I'm taking a preparatory summer course in anatomy before starting dentistry school in a few months, and every student has to do a presentation so I thought it would be a good idea to do mine on the danger of hyperventilating in dives, because I like diving and because people should know about it. I've already collected some information but one thing I'm not really clear on is whether there's a limit on how far you can reduce blood CO2 levels by hyperventilating. That is, if a diver keeps hyperventilating, will they eventually remove almost all the CO2 from their blood? And if so, how much time will that take?

I'd really appreciate any help.

 

[knotical]

A quick thought experiment says you’ll be unable to remove almost all the CO2 from your blood.
CO2 is produced by cells throughout your body then carried to the lungs where it is removed. So your blood will continuously pick up CO2 on each circuit and there will always be a large “backlog” of CO2 (mostly bicarbonate) throughout your veins, waiting its turn at the lungs.

Additionally, the transfer of CO2 at the alveoli walls is nowhere nearly as efficient as with O2. Arterial and venous concentrations of CO2 typically only differ by about 10%.

 

[DivemasterDennis]

I would suggest that hyperventilation on scuba is not remarkably different from hyperventilation not on scuba. Anxiety, stress, adrenaline rush, etc. are causes, and the air intake and expelled is made up of the same percentage of gasses as at 1 atm (assuming no special mix like nitrox or heliox), and metabolized in the same manner. Beyond that I have nothing of further use to add.
DivemasterDennis

 

[fisheater]

You may have a more productive presentation if you take a look at how hyperventilation can lead to blackouts while freediving.

Check out things like Shallow water blackout - Wikipedia, the free encyclopedia

 

[greengreg]

Thanks! Actually, blackouts are precisely the main topic of the presentation, but I haven't seen anything that says whether prolonged hyperventilation will just continue reducing CO2 levels for as long as the person keeps hyperventilating. Suppose one diver hyperventilates for 20 minutes (I know it's excessive, it's just hypothetical) and another hyperventilates for 40 minutes, will the second diver have less CO2 in their blood or is there a "plateau" of CO2 removal that is reached after several minutes of hyperventilation?

 

[Steve_C]

I could be totally wrong but I thought it was mostly gas exchange with the exception of metabolizing O2 and producing CO2 within the body.. Stuff flowing across membranes from higher to lower partial pressures. If that is the case then you cannot get the CO2 level below that of the Atmosphere if you are at the surface. Of course that is pretty small %.

 

[TSandM]

CO2 is determined by minute ventilation; that is, the volume of gas that passes through the air exchange chambers (alveoli) determines the CO2. If you are breathing 20 liters per minute (a lot) it doesn't matter if you do this for five minutes or fifty, your CO2 will be the same. (It does take a couple of minutes for the CO2 to equilibrate, when you change your minute ventilation, though.)

Hyperventilation on scuba, though, rarely results in a LOW CO2 level. The reason for this is that the work of breathing is higher on scuba than on land; because of the resistance, most people don't truly hyperventilate, in the sense of running too MUCH air through their alveoli. Instead, what they do is use too high a breathing RATE, with relatively small volumes. The result is that they spend a lot of gas ventilating the parts of the lung structure that don't participate in gas exchange (dead space) and don't really get a lot of air down where it's effective in eliminating CO2. For this reason, a fast breathing rate on scuba usually results in an ELEVATED CO2, rather than a low one.

 

[PsyWulf]

Deep and shallow water blackouts are actually a pretty interesting topic,with things like fast ascents linked to rapid changes in oxygen partial pressures causing blackouts and drowning

 

[greengreg]

Thanks again, everyone.

That's pretty interesting, TSandM. I didn't think about minute volume as a factor here.

 

[Pin83]

This may get a little heavy on physiology, sorry...
The other thing to consider is the effect of hyperventilation with regard to respiratory alkalosis, which shifts the oxygen-haemoglobin disassociation curve to the left, and causes an increased affinity for oxygen, and can decrease oxygen offloading in the tissues. I believe the same can occur in the brain (effectively cerebral hypoxia), although I think it's more theoretical than proven...
If your muscles are working hard, the acidic environment of your muscles negates this tendency, and the oxygen offloads ok, but if you are static it may be more noticeable.

There are other (more significant?) factors, such as cerebral vasoconstriction and effective hypocalcaemia (from albumin binding it up), but they are easier to find information on...