Shiken premium Upgrade Banner

Non-invasive Ventilation and CPAP Explained

Non-invasive ventilation (NIV) and continuous positive airway pressure (CPAP) are forms of ventilatory support often used in cases of acute respiratory failure when a patient remains hypoxic despite optimisation of medical management. Additionally, these machines are applicable to the chronic setting.

As a medical student or foundation doctor, you would not be expected to initiate or use a machine without senior input; however, it is beneficial to have an understanding of why and how they are employed.

What is NIV?

NIV is a form of breathing support that delivers air, usually with added oxygen, via a facemask by positive pressure. The most commonly used machine in the UK is BiPAP (Bi-level Positive Airway Pressure).

NIV differs in air pressure according to inhalation and exhalation. During inhalation, the inspiratory positive airway pressure (iPAP) is higher than the expiratory positive airway pressure (ePAP). Ventilation is therefore provided mainly by iPAP, while ePAP removes the exhaled gas and manages underventilated or collapsed alveoli for gas exchange.

In the acute setting, NIV is utilised for type 2 respiratory failure with respiratory acidosis, such as in COPD exacerbations.

What is CPAP?

CPAP supplies a fixed level of positive pressure throughout inhalation and exhalation. It is not a form of ventilation and instead splints the airways open. When used with oxygen, CPAP is capable of delivering a higher concentration than other oxygen masks. In the chronic setting, it is employed for severe sleep apnoea and in the acute setting for type 1 respiratory failure, such as in acute pulmonary oedema.

A Little Physiology...

To understand the definition of NIV and CPAP, an awareness of the terminology is required.

Positive Airway Pressure

Positive airway pressure is when the pressure outside the lungs is greater than the pressure inside. As a result, air is pushed into the lungs (down the pressure gradient), reducing the requirement for respiratory effort and increasing the amount of air remaining in the lungs after exhalation (the 'functional residual capacity').


Recruitment is when bronchioles and alveoli that typically collapse at the end of expiration remain open, allowing for more lung volume ('recruited'). This increases gas exchange efficiency as less energy is needed to breathe, and there is an additional surface area for gas exchange.

When Should NIV/CPAP be Started?

As a medical student or junior doctor, you would not be expected to set up or adjust settings by yourself. If a patient is deemed to require NIV/CPAP, senior input should be sought immediately.

Brief Guide to Steps Before NIV/CPAP

The British Thoracic Society/International Consensus (BTS/ICS) guidelines outline logical steps to take before starting NIV or CPAP for acute hypercapnic respiratory failure.

NIV/CPAP Indications


Indications for NIV include:

  • COPD with respiratory acidosis (pH <7.35)
  • Hypercapnic respiratory failure secondary to chest wall deformity (scoliosis, thoracoplasty) or neuromuscular disease
  • Weaning from tracheal intubation

These indications assume optimal medical management is already in place.


Indications for CPAP include:

  • Hypoxia in the context of chest wall trauma despite adequate anaesthesia and high flow oxygen (pneumothorax should be ruled out using a chest x-ray prior to commencing CPAP)
  • Cardiogenic pulmonary oedema
  • Pneumonia: as an interim measure before invasive ventilation or as a ceiling of treatment
  • Obstructive sleep apnoea

These indications assume optimal medical management is already in place.


If NIV/CPAP is indicated, it should be discussed with the patient if possible to gain their consent. If the patient is too unwell for this to happen the medical team need to determine if NIV/CPAP is in the patient's best interests.


Contraindications for CPAP/NIV include:

  • Vomiting/excess secretions (aspiration risk)
  • Confusion/agitation*
  • Impaired consciousness*
  • Bowel obstruction*
  • Facial burns/trauma
  • Recent facial/upper gastrointestinal/upper airway surgery*
  • Inability to protect airway*
  • Pneumothorax (undrained)*

*If NIV is the ceiling of care, it may be used in these cases, or if there is a plan in place for conversion to tracheal intubation.

Mask and Settings

Key points regarding masks and settings include:

  • A full-face mask should be trialled first.
  • CPAP is often started at 4cm H2O and gradually increased to reduce hypoxia.
  • NIV is often started at iPAP 10 and ePAP 4.
  • H2O is typically increased in 2-5cm intervals by approximately 5cms every 10 minutes until a therapeutic response is achieved.
  • Based on current evidence pressures should not exceed 25cm H2O at any point.
  • In order to maintain pressures, it is important to achieve a good seal with the NIV mask.


The following should be continuously monitored after commencement of CPAP/NIV:

  • Pulse oximetry: aiming for 94-98% (or 88-92% in CO2 retainers) using supplemental oxygen as required.
  • ECG
  • Blood pressure
  • Respiratory rate
  • Pulse
  • Consciousness level
  • Arterial blood gas: performed prior to commencement and 30-60 minutes after any change in settings until the patient is stable (minimum of 1, 4 and 12 hours after initiation).


The following complications can occur in the context of CPAP/NIV:

  • If ePAP is set too high venous return can be impaired leading to hypotension.
  • If iPAP is set too high it can impair venous return, cause the mask to leak, reduce patient tolerance and cause stomach inflation increasing the risk of aspiration.
  • NIV can cause pressure sores due to the tight-fitting mask, particularly over the bridge of the nose.



If NIV provides a therapeutic benefit initially, it should be worn as much as possible during the first 24 hours.

Non-Invasive Ventilation in the Acute Setting

Treatment should be continued until the underlying pathology is treated or resolved and the patient begins to show signs of clinical improvement.

If the patient is improving, trials off non-invasive ventilation (NIV) can be done to test how the patient fares. The length of time off NIV should then be gradually increased until it is determined that the patient no longer requires it. Even when NIV is no longer needed during the day, an additional night of NIV may be recommended.


CPAP in an acute setting is often weaned more quickly than NIV when the patient is stable and is no longer in respiratory distress. The water pressure (H2O) can be decreased by 2cm every 5-10 minutes. If the water pressure has been lowered to 4cm and the patient remains stable, a trial off CPAP with supplemental oxygen can be attempted.


NIV (BiPAP) and CPAP are breathing supports given via facemask for those who have experienced respiratory failure that has not been managed optimally with medical treatment.

For more information, the BTS guidelines have the most up-to-date and comprehensive information.

Join Shiken For FREE

Gumbo Study Buddy

Explore More Subject Explanations

Try Shiken Premium
for Free

14-day free trial. Cancel anytime.
Get Started
Join 10,000+ learners worldwide.
The first 14 days are on us
96% of learners report x2 faster learning
Free hands-on onboarding & support
Cancel Anytime