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Respiratory Failure Quiz
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Which arterial blood gas pattern best describes type 2 respiratory failure in an untreated patient who is breathing room air?
- A Reduced tissue oxygen delivery despite normal blood oxygen content You picked ✕
- B Normal arterial oxygen with low arterial carbon dioxide You picked ✕
- C Low arterial oxygen with high arterial carbon dioxide You picked ✓
- D Normal arterial oxygen with high arterial carbon dioxide You picked ✕
- E Low arterial oxygen with normal or low arterial carbon dioxide You picked ✕
Correct!
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The answer is C: Low arterial oxygen with high arterial carbon dioxide
Explanation
Why C is correct
See Respiratory Failure. Option C is correct. Respiratory failure means that gas exchange is so impaired that the arterial oxygen is low (hypoxaemia), with or without a raised carbon dioxide (hypercapnia). It is usually defined as an arterial pO2 below 8 kPa when breathing air at sea level. It is divided into two types. In type 1 the oxygen is low and the carbon dioxide is normal or low. In type 2 the oxygen is low and the carbon dioxide is high. The cause of type 2 is inadequate ventilation of the alveoli (hypoventilation), a failure of the respiratory pump, as in a severe exacerbation of COPD, chest wall deformity, muscle weakness or drugs that suppress breathing. Too little fresh air reaches the alveoli, so carbon dioxide builds up and, on room air, oxygen falls as well. A useful reminder is that type 1 is one problem (oxygen) and type 2 is two problems (oxygen and carbon dioxide). Note too that hypoxia (low oxygen supply to the tissues) is not the same as hypoxaemia (low oxygen in arterial blood).
- Respiratory failure: gas exchange is so impaired that arterial oxygen is low, with or without raised carbon dioxide.
- Type 1: low oxygen with normal or low carbon dioxide.
- Type 2: low oxygen with high carbon dioxide, caused by hypoventilation.
Why the other options are incorrect
- A. Reduced tissue oxygen delivery despite normal blood oxygen content: This describes hypoxia, a lack of oxygen supply to the tissues, which can happen with normal blood oxygen, for example after a heart attack when blood flow is poor. Respiratory failure is defined by the gases in the arterial blood. Hypoxia and hypoxaemia are not the same thing. Your answer
- B. Normal arterial oxygen with low arterial carbon dioxide: A normal oxygen level excludes respiratory failure on its own definition. Low carbon dioxide with normal oxygen simply means the patient is overbreathing, for example from anxiety or pain. It is not a form of respiratory failure. Your answer
- D. Normal arterial oxygen with high arterial carbon dioxide: Failing ventilation lowers the oxygen in the alveoli as well as raising carbon dioxide, so on room air a high carbon dioxide level is usually accompanied by low oxygen. Normal oxygen with raised carbon dioxide can be seen if the patient is being given extra oxygen, but the stem says room air. It is not the standard definition. Your answer
- E. Low arterial oxygen with normal or low arterial carbon dioxide: This is the pattern of type 1 respiratory failure, in which oxygen transfer is the problem and breathing is often fast enough to clear carbon dioxide. It is seen in pneumonia, pulmonary embolism and fibrosis. The key difference is that the carbon dioxide is not raised. Your answer
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More questions from this set. Start the quiz to answer them and see each explanation.
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1
Which physiological change most directly results from hypoventilation?
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2
A previously healthy 24-year-old woman presents with fever, cough and breathlessness. An arterial blood gas on room air shows marked hypoxaemia with a normal carbon dioxide level. Which condition best explains her presentation and this type 1 respiratory failure?
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