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).
Arterial blood gases are usually done by taking a sample of blood from the radial artery whereas capillary blood gases sample from the ear lobe which is less painful and easier to perform and likely to be just as accurate.
An arterial partial pressure of carbon dioxide (PaCO₂) is greater than 6 kPa suggests respiratory failure and therefore a high risk of mortality.
Patients with an PaCO₂ of less than or equal to 6 kPa should be also considered for NIV if they have any symptoms or signs of respiratory impairment.
The bicarbonate levels of over 26mmol/l indicate respiratory failure and the level may rise before other measures of respiratory function decline. This is because it rises as a way of compensating for nocturnal raised CO₂ so will be abnormal at any time of day, not just at night.
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.
However, daytime hypoxia is a late sign of respiratory failure and so normal daytime oximetry can be falsely reassuring.
In the following clip, Rosie Whitehead, a Respiratory Physiotherapist, describes these tests.
Describing blood gases
“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 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
Daytime hypoxia is a late sign of respiratory failure and so daytime oximetry can be falsely reassuring.
Additional testing can be used in patients if there are uncertainties about respiratory deterioration. NIV should not be delayed whilst waiting for these tests if the diagnosis can be made on basic tests alone.