Before starting NIV
Respiratory muscle testing
This section describes the different respiratory muscle tests, their benefits and drawbacks and how they might be used in practice.
When to use respiratory muscle testing
Respiratory muscle testing can be a helpful marker of deteriorating lung function or ability to cough. They should always be combined with other assessments such as a patient’s symptoms and blood gas sampling and additional testing. A single test on its own might show a clear problem but a deteriorating trend in results may be important to identify patients who are likely to develop respiratory failure.
There is limited evidence to tell us which is the best testing regime but current evidence suggests SNIP/MIP and peak cough flow are likely to be the most helpful in detecting respiratory dysfunction.
The downsides of respiratory muscle testing
These tests rely on good technique and effort which can be difficult, particularly for patients with bulbar or cognitive dysfunction. They can be tiring and stressful for patients.
Normal tests do not exclude respiratory muscle weakness.
Tests may be aerosol generating which means they may not be practical due to the risk of infection, particularly during the COVID-19 pandemic (eg forced vital capacity and cough capacity).
In the following clip, Rosie Whitehead, a Respiratory Physiotherapist, describes the different tests used to assess respiratory function.
Video transcript
Describing the different respiratory function tests
Rosie Whitehead
“There’s a few different types of test that we can do, and assessment we can do, of patients in clinic. One of the tests we can do is a forced vital capacity, that’s looking at the lung function of the patient - how much air they can get in and blow out in one breath. We do that through a machine that will work out the litres and the predicted percent for that patient. We can then look at trends, if there’s any changes with that - but also if it starts to get below 80%. Alongside the other tests, we might think that they’re starting to have some respiratory muscle weakness.
We can also do a SNIP test, which is a little probe that goes into your nose and if you take a quick breath in through your nose, so a sniff in through your nose, then it looks at how strong your muscles are again. It can look at it in a slightly different way than the FVC test can because it’s less impacted by your throat weakness. So it’s another test you can look at alongside the other test to see whether someone has got respiratory weakness.
We can also look at peak cough flow which will look at the strength of the person’s cough. It can give us an idea if they are able to adequately clear their secretions. So they might have reported that they are struggling to clear their chest, but that can give you an objective marker as to if they aren’t effectively able to clear, which is especially important if it’s someone who’s got impaired swallowing, if things are going down the wrong way. There is a much higher risk of a chest infection, not being able to clear that properly.”
Tests currently used in motor neurone disease
Sniff nasal inspiratory pressure (SNIP) and maximal inspiratory pressure (MIP)
Evidence suggests that these are the earliest predictor of respiratory insufficiency.
If both SNIP and MIP are performed, staff should base the assessment on the better respiratory function reading.
Results strongly suggestive of respiratory insufficiency and the need for NIV are:
SNIP/MIP less than 40 cmH₂O
OR
SNIP/MIP less than 65 cmH₂O for men or 55 cmH₂O for women plus any symptoms or signs of respiratory function impairment, particularly orthopnoea
OR
SNIP/MIP rate of decrease of more than 10cmH₂O over three months.
Peak cough flow (PCF)
Nasal Inspiratory Pressures combined with peak cough flow has also been shown to be a sensitive predictor for the need for NIV.
This test can also indicate whether cough augmentation strategies should be considered.
A PCF equal to or less than 270l/min in stable patients suggests a higher likelihood of respiratory complications and indicates the need for assisted airway clearance strategies.
A PCF less than 160l/min suggests a high risk of pneumonia because respiratory secretions cannot be cleared.
Spirometry (forced vital capacity and slow vital capacity)
Vital capacity can be measured either as a forced exhalation (forced vital capacity) or as a slow exhalation (slow vital capacity).
It is reported as a percentage of the normal value expected for a person of the same height and gender.
If spirometry is abnormal it can indicate respiratory failure.
Spirometry is insensitive to early respiratory failure where vital capacity can be normal despite abnormal SNIP/MIP and therefore, this test should be combined with SNIP/MIP and peak cough flow.
Poor spirometry may be caused by poor technique, particularly in those with bulbar dysfunction. An oronasal mask should be available for respiratory testing as it may be more successful than mouthpieces in some patients. Slow vital capacity may be easier to perform.
Whichever technique is used, it should be used consistently each time in order to assess trends in the measurements.
Results strongly suggestive of respiratory insufficiency and the need for NIV are:
Less than 80% of predicted value plus any symptoms or signs of respiratory impairment (particularly orthopnoea)
OR
FVC/VC less than 50% of predicted value.
Oronasal mask
Vital capacity test
“I find the most effective way to monitor patients is to have all the information available to me when I'm seeing a patient: their tests and previous tests, their current symptoms and how they are progressing. Then we can have a really informed discussion about what's going on. This is why it's so helpful having these tests available in the clinic and having our respiratory physiotherapist and nurse to discuss and get feedback straight away.”
Esther Hobson , Consultant Neurologist, Sheffield MND Care and Research Centre
Good practice points
Respiratory muscle testing should involve a measure of inspiratory pressure AND peak cough flow combined with a history and other measurements for a thorough assessment.
Previous results should be looked at to detect likely deterioration.
An oronasal mask should be available for respiratory testing.