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Non-Invasive Ventilation

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Non-invasive ventilation is a way of providing ventilatory support to patients in respiratory failure without using an invasive airway device (e.g. ET tube or tracheostomy tube).

The devices are becoming increasingly commonplace in medical practice and can be set up by trained professionals in the Emergency Department, on the wards, in ICU or even at home. As an F1 you may commonly come across NIV for patients with severe exacerbations of respiratory diseases in the Emergency Department, or, in patients with obstructive sleep apnoea who use it overnight.

Generally speaking, if you are considering starting a patient on NIV, a senior clinician must be involved!

Types
As a foundation doctor, the two main types of NIV that you should know about are:
  • Continuous positive airway pressure (CPAP)
  • Bi-level positive airway pressure (BiPAP)
CPAP provides a single level of pressure throughout respiration. During inspiration, the additional pressure recruits more alveoli and so surface area across which gas exchange can occur increases. On expiration, the patient has to breathe against the same pressure so those alveoli remain open at the end expiration. This again prolongs gas exchange time (i.e. by increasing functional residual capacity).

BiPAP uses two levels of pressure instead of one; one pressure for inspiration (IPAP) and one for expiration (EPAP). The IPAP level is higher than the EPAP level and works in the same way that CPAP does on inspiration. It also gives additional respiratory support to reduce the patients work required to breathe; thus preventing fatigue of the respiratory muscles. Where BiPAP differs, is that the EPAP pressure is lower than the expiratory pressure in CPAP – the patient therefore finds it easier to breathe out but still gets the benefit of more recruited alveoli being open at the end of expiration. The difference between the IPAP and EPAP pressure is known as the pressure support.

When to use it?
This is not a decision to be made by an FY1. However, for information purposes there’s no hard and fast rule about when to use NIV but generally, it is used in patients who are not ventilating in spite of full medical management, i.e.:
  • CPAP in acute pulmonary oedema where diuresis has failed
  • BiPAP in type 2 respiratory failure (for example in COPD exacerbations), where nebulisers and IV treatments have failed to ensure good oxygenation
Most hospitals will have a guideline detailing at what degree of acidosis or hypercapnia NIV should be considered

How to start it?
Lots of hospitals now have specialist nurses or physiotherapists that set patients up on NIV but the principles are helpful as an F1, especially if out of hours you need to make changes to the set up! Changes should be driven by seniors.

CPAP:
  • Ensure well-fitting mask and counsel the patient that it may be claustrophobic and an unpleasant sensation (some people describe it as having your head stuck out of the window of a speeding car)
  • Choose an appropriate level of supplemental oxygen 
  • Start low – usual settings are 5, 7.5 or 10 cmH2O, so start at 5 cmH2O and titrate to response
  • Titration should be by 2-3 cmH2O every 5 minutes until a satisfactory response achieved 
  • Avoid going over pressures of 15-17 cmH2O!
BiPAP:
  • Again, ensure a well-fitting mask and counsel the patient 
  • Supplemental oxygen
  • Again start low:
    • EPAP usually starts at 4-5 cmH2O
    • IPAP is the pressure you change. Start at 10 cmH2O then titrate by 2-5 cmH2O increments every 10 minutes until therapeutic response or patient tolerability reached. 
    • IPAP can reach a setting of 20 cmH2O – definitely have a senior or an experienced NIV nurse if your patient is requiring any higher. Consider ITU involvement!
  • Monitoring should occur with serial ABGs (or arterial line samples). The BTS guidelines state that if NIV has failed by 4 hours, intubation should be considered. 
  • The weaning process takes about 4 days, gradually increasing the amount of time that the patient spends off the ventilator. 
Contraindications
There are few absolute contraindications to NIV:
  • Respiratory or cardiac arrest
  • Lack of consent or uncooperative patients (they will pull it off)
  • Facial trauma/burns
  • Reduced GCS (it relies on spontaneous breaths)
  • Untreated pneumothorax
  • Skull base fracture (can cause iatrogenic pneumocephalus)
  • Persistent vomiting 
Lack of experienced staff is also a contraindication to consider.

Relative contraindications include:
  • Extreme anxiety
  • Morbid obesity 
  • Copious respiratory secretions
  • Poor respiratory drive
If one of the relative contraindications are present, your senior should be making the decision about NIV.

Counselling points and potential complications
Be sure to go through the following when explaining NIV to your patient so they can make informed consent. The most common complications are:
  • Anxiety/claustrophobia (it is helpful to start by placing it on their face intermittently, then ask them to hold it & increasing pressures slowly)
  • Stomach distention, vomiting and/or aspiration
  • Irritation to the eyes and raised intraocular pressure
  • Pressure sores around the fit of the mask (talk to the nurses if the patient is complaining of discomfort, to ensure a better fit & use gauze to cushion if necessary)
  • Congestion of nasal sinuses
  • Raised intracranial pressure
  • Can cause hypotension if the patient is hypovolaemic
Final point...
NIV can be a very useful tool to ventilate patients without invasive procedures. Always involve a senior (even if over the phone) and finding a nurse who is comfortable setting up NIV can help too. Have a very low threshold to escalate and remember that extensive explanation and reassurance can dramatically help patient tolerance!

Dr Emily Cramond-Wong, Trust grade ST1

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