Assessing respiratory function
This information is for health and social care professionals. You may also want to see our information for people living with or affected by MND on:
Motor neurone disease (MND) is a progressive and terminal disease that attacks the motor neurones, or nerves, in the brain and spinal cord. Respiratory muscle weakness occurs eventually in everyone with MND and respiratory function significantly predicts both survival and quality of life.
Monitoring respiratory function will help the health professional and the person with MND to reach decisions about management in a timely way.
It is also important because:
- effective management of respiratory function is likely to have a positive impact on quality of life for the person with MND
- it may prevent an acute respiratory emergency, or inappropriate action being taken in the event of an emergency. If someone presents in hospital with rapidly decompensating ventilatory failure, intubation or a tracheostomy may be carried out.
Therefore, when monitoring someone with MND, it is important to:
- ask about symptoms of respiratory involvement
- look for signs of weakness in the trunk and respiratory muscles.
As part of the initial assessment to diagnose MND, or soon after diagnosis, tests should be performed to establish the baseline respiratory function.
Tests may include:
- pulse oximetry (SpO2) – to measure the level of oxygen in the blood (at rest and breathing room air)
- forced vital capacity (FVC) or vital capacity (VC) – to find out the volume of the lungs from a full breath in to a full breath out
- sniff nasal inspiratory pressure (SNIP) or maximal inspiratory pressure (MIP) – a test of the strength of the muscles used to breathe in.
These tests should be carried out by a health professional who has the knowledge and experience to perform them. If the person with MND has severe bulbar impairment (weakness in the tongue, mouth, throat) or severe cognitive problems, it may be difficult to choose the right mask or mouthpiece to use for these tests. As a minimum, a pulse oximetry should be carried out.
Respiratory function tests should be performed every two to three months, but this may vary depending on:
- whether there are any signs or symptoms of respiratory impairment
- the rate of progression of an individual’s MND
- the person’s preference and circumstances.
Because respiratory function testing monitors progression, it may be frightening for someone with MND. However, it can promote discussion of the options and potential intervention.
Even mild respiratory symptoms or signs should highlight the need for early referral for respiratory medicine assessment. The specialist respiratory team may include a respiratory consultant, respiratory physiotherapist and specialist nurses. Referral should also be made to the specialist palliative care team, who can advise on psychological strategies, medication and advance care planning.
Referral should be made to the specialist respiratory team to perform an arterial or capillary blood gas analysis if the person with MND has percutaneous oxygen saturation (SpO2) equal to, or less than 94% (or 92% if they have known lung disease).
Referral to a specialist respiratory service is also indicated when:
- a person has sleep-related respiratory symptoms, despite the SpO2 being within normal limits
- the arterial partial pressure of carbon dioxide (PaCO2, measured from a blood sample) is greater than 6 kPa: in this case, an urgent referral is indicated
- they have symptoms or signs of respiratory impairment despite the PaCO2 equal to or less than 6 kPa
- they have symptoms or signs of respiratory impairment despite a normal overnight pulse oximetry.
Guidelines suggest non-invasive ventilation (NIV) should be considered once FVC falls to 50% of the predicted estimate for the individual. Yet many people with MND experience nocturnal hypoventilation well before this point. No one test is best for monitoring or indicating the ideal time to start NIV.