<prompt> After Hyperventilating for some time holding the breath is dangerous becauseDecrease CO2 shift the O2 dissociation curve to the leftAlkalosis can lead to tetanyIt can lead to CO2 NarcosisDue to lack of stimulation by CO2, anoxia can go into dangerous levels  <response>: In our body its not O2, but CO2 that maintains the respiratory drive though main function of respiration is to maintain adequate oxygen level in the bodyThus its not hypoxia but hypercarbia that is required to inform the brain that respiration is inadequate.If normally a person holds his breath he will soon have decrease O2 level hypoxia and increase CO2 level hypercarbiaHypercarbia will stimulate the brain and increase the respiratory drive, forcing the person to take breath.Lets see what happens when a person hyperventilatesDecreased PaCO2 of bloodNormal O2 saturation despite hyperventilation arterial O2 saturation remains same as blood is almost fully saturated at normal ventilation rateLets see what happens when such a person holds his breath. Soon his blood O2, level will fall hypoxia but his blood CO2 level will be normal or below normal as he was hypocapnic due to hyperventilation His blood O2 level will continue to fall, but his CO2 level will still be not high enough to stimulate the brain for respiratory drive.The person will continue to hold his breath and his O2 level may fall to lethal levels without any respiratory drive his brain is still unaware that he is in danger of dying from hypoxia. 