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Hypercapnia is excess carbon dioxide in the cardiorespiratory system, which in scuba diving can be caused by overexertion or not being able to breathe slowly and deeply.


A short respiratory cycle (short, rapid breaths), decreased carbon dioxide elimination leading to a high concentration of dead air (air contained in the airway spaces) and an increase in carbon dioxide production can create a situation where that carbon dioxide production is higher than your respiratory system can eliminate.


Your respiratory reflex center in turn encourages you to breathe faster to try to eliminate this excess carbon dioxide. Because air is denser at depth, your diaphragm and other breathing muscles have to work harder, which further increases carbon dioxide production.


This vicious circle will only be broken if you stop and calm down, to give your respiratory system a chance to perform gas exchange properly. Which means returning to a slow, deep breathing pattern.


As carbon dioxide levels increase in the lung alveoli and circulation you may experience headaches, confusion, rapid breathing and even shortness of breath and these are  symptoms that could bring attention to this situation.


The deeper you dive, the denser the gas you breathe. The deeper the greater the pressure, therefore the smaller the volume and the denser the air you breathe. The more easily you can get overtired and the longer it takes you to recover.


If the high level of carbon dioxide is not controlled, this can eventually cause you to lose consciousness, which of course can lead a diver to lose the mouthpiece of his regulator and even drown.

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