QuestionSBA
QuestionChoice
Carbon monoxide poisoning is a type of insufficient oxygen-carrying capacity that presents with a low oxygen saturation and a normal PaO2 (it only affects the oxygen that is normally bound to haemoglobin).
10024354
e
Carbon monoxide poisoning
QuestionChoice
Insufficient ventilation means that carbon dioxide can not be exhaled: this increases PaCO2. As carbon dioxide is acidic, this will cause the arterial pH to decrease. As inhalation is also affected, there is less oxygen in the blood leading to a low PaO2. A lower total arterial oxygen content (CaO2) means that oxygen saturation is also low as there are fewer oxygen molecules to bind to haemoglobin. As the problem is not with oxygen diffusion, the Alveolar-arterial gradient is normal.
10024351
b
Insufficient ventilation e.g. COPD
QuestionChoice
In the presence of insufficient oxygen, hypoxaemia triggers hyperventilation which normalises carbon dioxide levels (which may have increased due to ventilation or perfusion abnormalities). This also normalises the pH. PaO2 is low as oxygen is unable to diffuse into the circulation, which also causes a low oxygen saturation. A high Alveolar-arterial gradient indicates that the partial pressure of oxygen in the alveoli is higher than in arterial blood as there is an issue with the oxygenation of the blood.
10024350
a
Insufficient oxygenation e.g. VQ mismatch
QuestionChoice
Insufficient oxygen-carrying capacity does not affect carbon dioxide levels, therefore PaCO2 is unaffected. PaO2 is also unaffected as this refers to the partial pressure of oxygen in the plasma rather than bound to haemoglobin. In anaemia, haemoglobin affinity is unchanged i.e. the percentage of haemoglobin binding sites that are occupied with oxygen remains the same. Therefore the oxygen saturation of haemoglobin remains unchanged. As oxygenation is not affected in anaemia, the Alveolar-arterial gradient is normal.
10024352
c
Insufficient oxygen-carrying capacity e.g. anaemia
QuestionChoice
Insufficient oxygen in the atmosphere triggers hyperventilation which decreases PaCO2. As carbon dioxide can release hydrogen ions and cause an acidic environment, the lack of CO2 causes pH to increase (becomes more alkaline). The PaO2 decreases due to a lack of available oxygen which therefore causes a low oxygen saturation. As the problem is not with oxygen diffusion, the Alveolar-arterial gradient is normal.
10024353
d
Insufficient oxygen in the atmosphere e.g. altitude
Concept
Chapter
2693
5770
Respiratory failure
Topic
150
Respiratory physiology
17268
Which mechanism of respiratory failure correlates to this clinical pattern with the presence of compensatory hyperventilation?

PaCO2 – normal

PaO2 – decreased

Arterial Ph – normal

%SaO2 – decreased

A-a gradient – high
a