Lightheadedness at the start of a breath-hold is NOT necessarily an indication of hyperventilating.
It can be caused by both low CO2 and/or low blood pressure and/or being excessively alkaline (from diet or other reasons). Low blood pressure can be caused by being too hot (vasodilated), dehydration, eating vasodilating foods such as garlic, onions or pineapple; or it can even be somewhat genetic; it can also be caused by large amounts of cardio training.
In some people, taking a single very deep breath can lead to a blackout (it nearly happened to me once).
The problem will be worsened if you attempt to inhale while standing or sitting, rather than floating face down.
The problem can be reduced by:
- Cooling the body (vasoconstriction increases BP)
- Avoiding vasodilating foods
- Eating more salt and drinking more water (together!)
- Storing more water in the body by eating lots of starchy carbohydrates and drinking lots of water along with it
- Breathing less before the dive
- Pausing for 10-30 seconds with full lungs, right before the last breath
- Inhaling while floating face down, or better yet, float with your feet at a depth of 5 feet, with your ears barely above the water; the water pressure gradient forces blood from your legs to your head, improving the BP in your brain
- Eat more acid forming foods (meat, eggs, etc.), not necessarily the best solution
- Packing less or taking less air (not a good idea, not the best solution)
I would say I'm a specialist in this area since I blacked out at the start of my dive at the AIDA world cup in Switerland in August 2000. After that I spent years trying to figure out why it happened (back then there was no one to ask!)
Either way, if you sense lightheadedness as you start the dive, the probability of a blackout or samba at the end of the dive is GREATLY INCREASED.