Expansion
Rising carbon dioxide is the main stimulus, with falling oxygen contributing later
Expansion
The breaking point is reached at an arterial carbon dioxide of about 6.5 to 7 kPa (50 mmHg) and an oxygen tension of about 8 kPa (60 mmHg).
Contributors:
- Rising carbon dioxide and falling pH, the dominant stimulus
- Falling oxygen, contributing later
- Lack of lung movement: afferents from chest wall and lung stretch receptors contribute, which is why taking a breath of an inert gas relieves the urge without changing blood gases
- Voluntary cortical override
Prolonged by prior hyperventilation, oxygen breathing, and starting at a large lung volume.
The danger of prior hyperventilation is the important point. Lowering carbon dioxide delays the breaking point but does nothing to increase oxygen stores. Oxygen can therefore fall to unconsciousness before carbon dioxide rises enough to force a breath, causing shallow water blackout and drowning. On ascent, falling ambient pressure lowers alveolar oxygen tension further, precipitating the event in the final metres.