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Non-invasive ventilation (NIV) and continuous positive airway pressure (CPAP) are forms of ventilatory support used in acute respiratory failure when a patient remains hypoxic despite optimisation of medical management. Both have additional indications in the chronic setting.
These machines are commonly found on respiratory wards, high dependency units and intensive care. As a medical student or foundation doctor, you would not be expected to initiate or use a machine without senior input, but it is good to have an awareness of why they are used and how they work.
What is NIV?
NIV is a form of breathing support delivering air, usually with added oxygen, via a facemask by positive pressure, used in respiratory failure. The term NIV is often used interchangeably with the trade name BiPAP (Bi-level Positive Airway Pressure), which is the most commonly used machine in the UK. ¹
NIV delivers differing air pressure depending on inspiration and expiration. The inspiratory positive airways pressure (iPAP) is higher than the expiratory positive airways pressure (ePAP). Therefore, ventilation is provided mainly by iPAP, whereas ePAP recruits underventilated or collapsed alveoli for gas exchange and allows for the removal of the exhaled gas.
In the acute setting, NIV is used in type 2 respiratory failure with respiratory acidosis (for example in a COPD exacerbation).
What is CPAP?
CPAP supplies constant fixed positive pressure throughout inspiration and expiration. It, therefore, is not a form of ventilation, but splints the airways open. If delivered with oxygen, it can allow a higher degree of inspired oxygen than other oxygen masks. In the chronic setting it is used for severe obstructive sleep apnoea (splinting the upper airway) and in the acute setting for type 1 respiratory failure, for example in acute pulmonary oedema (recruiting collapsed alveoli). This article will focus on its use in respiratory failure.
A little physiology…
To understand the above definitions, you need an awareness of the terminology.
Positive airway pressure
Positive airway pressure refers to the pressure outside the lungs being greater than the pressure inside of the lungs. This results in air being forced into the lungs (down the pressure gradient), requiring less respiratory effort (offloading respiratory muscles to reduce the work of breathing). In addition, the amount of air remaining in the lungs after expiration (the ‘forced residual capacity’) is increased, expanding the chest and lungs.
Recruitment refers to a process where bronchioles and alveoli which would normally collapse at the end of expiration, are kept open (more lung volume is ‘recruited’). The phase of breathing that requires the most energy is the process of overcoming the pressure required to re-expand collapsed segments of the lungs. Therefore, by keeping these open, gas exchange efficiency is increased as less energy is required to breathe, and there is more surface area for gas exchange.
An additional effect is ‘stenting’ of the upper airways, keeping them open and reducing the work of breathing.
So 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 felt to be unwell enough to warrant NIV/CPAP senior input should be sought urgently.
Below is a brief guide to the logical steps that should be taken before starting NIV or CPAP, based on BTS/ICS guidelines for the management of acute hypercapnic respiratory failure.2
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 that 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
These indications assume that 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.
*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. 3
For a full list, refer to the BTS guidelines.2
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 4.4
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:5
Pulse oximetry: aiming for 94-98% (or 88-92% in CO2 retainers) using supplemental oxygen as required.
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. Treatment should be continued until the underlying pathology is treated/resolves and the patient begins to improve clinically.
If the patient is showing signs of clinical improvement, trials off NIV can take place to see how the patient manages. The period of time off NIV can then be gradually increased until it is felt the patient no longer requires it. Usually, even after NIV is no longer required during the day, a further night of NIV is recommended.
CPAP in an acute setting is often weaned more quickly than NIV is, once the patient is stable and no longer in respiratory distress. H2O can be reduced by around 2cm every 5-10 minutes. If H2O is reduced to 4cm and the patient remains stable at this setting, a trial off CPAP can be attempted alongside supplemental oxygen.
NIV (also known as BiPAP) and CPAP are used as breathing support delivered via a facemask for those with respiratory failure that has not responded to optimal medical management.
For further reading, the BTS guidelines give the most comprehensive, up to date information.
Dr Alexandra Dipper
Anaesthesia UK: Non-invasive ventilation in the ICU. Richard Beringer. Available from: [LINK].
Davidson AC, Banham S, Elliott M, et al. BTS/ICS guideline for the ventilatory management of acute hypercapnic respiratory failure in adults. Thorax 2016;71:ii1-ii35.
Non-invasive ventilation in acute respiratory failure. Thorax. 2002;57:192-211.
Life in the fast lane: Non-invasive ventilation and the critically ill. Kane Guthrie. Available from [LINK].
Begum Ergan et al. How should we monitor patients with acute respiratory failure treated with noninvasive ventilation? European Respiratory Review 2018 27: 170101; DOI: 10.1183/16000617.0101-2017. Available from: [LINK].