Breathing: It's in the blood but don't overdo it

How does hyperventilating help you hold your breath longer on a dive?Breathing, at its simplest, accomplishes two things. It puts oxygen in your blood, and takes carbon dioxide out.
Ever wonder what, exactly, drives us to breathe? It's not our oxygen levels dropping - they stay high for a couple of minutes or more in between breaths. What drives breathing is our blood's carbon dioxide level, precisely monitored by the brain.
When we hyperventilate, we don't increase our blood oxygen level one bit (it's already at 100 per cent), but we do lower our carbon dioxide levels massively. We defeat our built-in safety mechanism.
It might take minutes for those low levels to rise again to the point where we feel the need to breathe. For the hyperventilating diver this means a much longer breath-hold. Sounds great, until you consider this scenario: you blow off all your carbon dioxide, so your urge to breathe is suppressed for at least a few minutes.
Meanwhile, your body is consuming oxygen. Your oxygen level is dropping, dropping, dropping, but you don't notice it because your carbon dioxide level is still so low. You don't feel any increased urge to breathe, then suddenly you're unconscious. No warning symptoms.
If you don't have a witness who can resuscitate you - immediately - you're dead.
This is called shallow-water blackout, and odds are it will take the lives of some Kiwis this summer. To avoid it, don't hyperventilate more than a breath or two before diving, and don't breath-hold or free-dive without a vigilant buddy.
Gary Payinda MD is an emergency medicine consultant in Whangarei.
Have a science, health topic or question you'd like addressed? Email: drpayinda@gmail.com
(This column provides general information and is not a substitute for the medical advice of your personal doctor.)