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)

Results strongly suggestive of respiratory insufficiency and the need for NIV are:

SNIP/MIP less than 40 cmH₂O


SNIP/MIP less than 65 cmH₂O for men or 55 cmH₂O for women plus any symptoms or signs of respiratory function impairment, particularly orthopnoea


SNIP/MIP rate of decrease of more than 10cmH₂O over three months.

Peak cough flow (PCF)

Spirometry (forced vital capacity and slow vital capacity)

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)


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