Low inspired oxygen at altitude: blood gas pattern
In people suddenly exposed to high altitude, the lower inspired (breathed-in) oxygen pressure causes hypoxaemia (low oxygen in the blood), and the faster breathing it drives lowers carbon dioxide.
Remember
At altitude the oxygen in arterial blood falls, and the extra breathing this provokes (hyperventilation, meaning faster and deeper breaths) lowers carbon dioxide as well, so both gases are low: the type 1 respiratory failure pattern (low oxygen without high carbon dioxide).
The air at altitude holds the same proportion of oxygen as at sea level, but the total air pressure is lower. Oxygen therefore exerts less pressure in each breath: its partial pressure (pO2) is the share of the pressure that is due to oxygen.
Less oxygen reaches the alveoli (the tiny air sacs of the lungs), so the arterial pO2 falls. This is hypoxaemia, low oxygen in the arterial blood. The fall is picked up by the peripheral chemoreceptors, sensors that monitor blood oxygen, such as those in the carotid bodies in the neck. They increase the drive to breathe, so ventilation increases: this is hyperventilation.
Breathing faster and deeper removes more carbon dioxide, so the arterial pCO2 (the pressure of carbon dioxide) falls too. A low pCO2 is called hypocapnia, and here it shows that the breathing is responding, not failing. The end result is a low pO2 with a low pCO2, the gas pattern of type 1 respiratory failure.
Further adaptations take longer and are seen in people who live at altitude, including polycythaemia (more red cells), more 2,3-DPG (a molecule in red cells that helps them release oxygen) and more capillaries in the tissues. The rise in ventilation comes first, within minutes to hours of arrival.
Key facts
- Why oxygen falls
- the percentage of oxygen in the air is unchanged with height, but the lower air pressure lowers its partial pressure
- Gas pattern
- low arterial pO2 with low arterial pCO2
- Trigger
- hypoxaemia stimulates the peripheral chemoreceptors, which drive hyperventilation
- Why carbon dioxide falls
- hyperventilation increases carbon dioxide removal
- Arterial pH
- raised in people suddenly exposed to altitude, because hyperventilation produces a respiratory alkalosis (a rise in pH)
- Slower changes
- polycythaemia, more 2,3-DPG and more capillaries develop later, while hyperventilation is the immediate respiratory response
Exam traps
- Low oxygen with high carbon dioxide is the type 2 respiratory failure pattern. It does not follow from low inspired oxygen, because a raised carbon dioxide level needs hypoventilation (too little breathing), but low oxygen makes breathing faster and carbon dioxide is washed out.
- A normal pH does not go with low oxygen and low carbon dioxide in people suddenly exposed to altitude, because hyperventilation produces a respiratory alkalosis (a raised pH), so the arterial pH is raised.
- Arterial oxygen cannot be high when the inspired oxygen is reduced. A high oxygen with a low carbon dioxide would be seen with over-breathing of oxygen-enriched gas, not at altitude.
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