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.
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.
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.
To understand the definition of NIV and CPAP, an awareness of the terminology is required.
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.
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.
The British Thoracic Society/International Consensus (BTS/ICS) guidelines outline logical steps to take before starting NIV or CPAP for acute hypercapnic respiratory failure.
Indications for NIV include:
These indications assume optimal medical management is already in place.
Indications for CPAP include:
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:
*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.
Key points regarding masks and settings include:
The following should be continuously monitored after commencement of CPAP/NIV:
The following complications can occur in the context of CPAP/NIV:
If NIV provides a therapeutic benefit initially, it should be worn as much as possible during the first 24 hours.
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.
