Before starting NIV

Blood gas sampling and additional testing

Arterial or capillary blood gas analyses provide an assessment of both daytime blood gas levels (PaO₂, PaCO₂, pH) and respiratory function through the day and night (bicarbonate).

Blood gas measurement

Transcutaneous carbon dioxide monitoring has been found to be a useful clinical tool for detecting respiratory failure in experienced centres because it can provide a continuous record of patient’s CO2 and O2. However it requires equipment not usually found in standard respiratory centres and some expertise to use and interpret the findings.


Pulse oximetry

In the following clip, Rosie Whitehead, a Respiratory Physiotherapist, describes these tests.

Video transcript

Describing blood gases

Rosie Whitehead

“We also will look at doing a capillary blood gas, which is a small amount of blood taken from your ear, similar to having a blood sugar done from your finger. That can look at your oxygen levels but also your carbon dioxide levels, which are what we are wanting to know if they’re starting to go high.

Often when you first start to notice symptoms with your sleep, they’re starting to go high overnight but will be normal during the day. But your bicarbonate level will have compensated for that, so we can see that that is higher, which can give us a sign that at night-time your carbon dioxide levels are going higher, alongside your symptoms that you might have noticed changing.

We can also look at doing your oxygen saturation, which can give us a good sign - more so if you are having an infection or you are acutely unwell - it can give us an idea if your oxygen levels aren’t as good and you aren’t ventilating as well. But it’s not as sensitive as if we had a CO₂ level, which would give us an idea if your respiratory muscles aren’t working as well.”

Additional tests

Additional testing should be available without delay if there are concerns about respiratory deterioration. However, complex testing should be used if the diagnosis of respiratory failure can be made using basic assessments as this will delay the initiation of NIV.

Nocturnal oximetry has been found to be useful for determining the need for NIV and has been shown to be more sensitive than forced vital capacity. The benefit is that it is widely available, easy to use and can be conducted remotely in the patient’s home thus avoiding delays.

Overnight transcutaneous capnometry, respiratory polygraphy and polysomnography can be useful in experienced centres when spirometry cannot be performed, when tests are inconclusive or when patients have bulbar impairment.

Which tests to use?

Arterial and capillary blood gases are sensitive measures of respiratory failure. Pulse oximetry is easy to use but can be falsely reassuring. Overnight testing using oximetry or polysomnography can be very sensitive and helpful particularly when there is uncertainty about the diagnosis but takes time to organise and interpret which can lead to delays in starting NIV.

The tests you might use in your centre will depend on the equipment and expertise you have available but for most people the best tests are the ones that can be done quickly, to ensure the information is available to clinicians and decisions are not delayed.

Good practice points