How is the diagnosis of respiratory failure made?
The diagnosis of respiratory failure requires an arterial blood gas which provides information on the levels of the blood oxygen and carbon dioxide levels. An arterial blood gas simply involves a needle connected to a syringe, which is then inserted at the wrist directly into the point where the pulse can be felt. Sometimes the pulse at the wrist is weakened and so a different site has to be used and this is usually the groin and, less commonly, the elbow.
Finding the underlying cause of respiratory failure
Once the diagnosis has been made, further investigations will be required to find the underlying cause. This may include:
- Chest X-ray: this may show infection, fluid or tumours of the lung.
- Blood tests: these may include full blood count, kidney tests and liver function tests. They may help to work out the cause and also to detect any factors that may be worsening the respiratory failure, such as a low blood haemoglobin level (anaemia).
- Troponin blood tests: these are used to determine if there has been recent heart injury - for example, a heart attack which may have caused the respiratory failure.
- Thyroid function tests: an underactive thyroid gland, when a long-term (chronic) condition, may cause respiratory failure with a raised carbon dioxide level.
- Spirometry: this is used to measure the lung volumes and capacity and is useful in the evaluation of chronic cases.
- A heart ultrasound scan (echocardiography): this can look for cardiac causes, such as a leaking heart valve or heart failure.
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