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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 seen on respiratory wards, in high dependency and intensive care settings. 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 (for example in a COPD exacerbation), with respiratory acidosis (pH < 7.35).
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 part of breathing that requires the most energy is overcoming the pressures required to re-expand collapsed parts of the lungs. Therefore by keeping these open, gas exchange efficiency is increased (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 foundation junior doctor, you would not be expected to set up or adjust settings by yourself, and should always be getting a senior involved if a patient is unwell enough to warrant NIV/CPAP.
However, below is a brief guide as to the logical steps you should take before starting NIV or CPAP, taken from The BTS/ICS Guideline for the ventilator management of acute hypercapnic respiratory failure. 2
1) Is it INDICATED?
Is the patient in respiratory failure, as indicated by an ABG? (If you need a refresher of interpreting ABG’s, have a look at our ABG interpretation guide)
Have they given consent/is it in best interests?
Do I need CPAP or NIV?
The following scenarios may be indications for NIV (this is in the context of optimal medical management already being in place):
COPD with a respiratory acidosis pH <7.35
Hypercapnic respiratory failure secondary to chest wall deformity (scoliosis, thoracoplasty) or neuromuscular diseases
Weaning from tracheal intubation
The following scenarios may be indications for CPAP (this is in the context of optimal medical management already being in place):
Chest wall trauma who remain hypoxic despite adequate anaesthesia and high flow oxygen (ensure checked for pneumothorax prior to commencement)
Cardiogenic pulmonary oedema
Pneumonia: as an interim measure before invasive ventilation or as a ceiling of treatment
Obstructive sleep apnoea
2) Any CONTRAINDICATIONS?
Vomiting/excess secretions (aspiration risk)
Recent facial/upper GI/upper airway surgery*
Inability to protect airway*
*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
3) What SETTINGS?
A full-face mask should be trialled first.
CPAP: Often started at 4cmH2O and gradually increased to reduce hypoxia
NIV: Often started at iPAP 10, ePAP 4. 4
Increase 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 the pressures, it is important to achieve a good seal with the NIV mask.
In the first 24 hours, continuous pulse oximetry and ECG monitoring should be in place. In addition, close monitoring of other vital signs, consciousness level and arterial blood gases is required.
Blood pressure: Ensure not becoming hypotensive. If so, you will need a full A-E assessment, and if on NIV, consider reducing the iPAP.
Oxygen saturations: Aiming for 94-98% (or 88-92% in CO2 retainers). Can be supplemented with oxygen.
Respiratory rate: A sensitive measure of how hard someone is working.
Arterial blood gases: Prior to starting, and around 30 – 60 minutes after any change until stable. At a minimum of 1, 4 and 12 hours after initiation.
Too high CPAP/ePAP
Reduced venous return and consequently hypotension.
Too high iPAP
Reduced venous return and consequently hypotension. Can cause the mask to leak, reduces tolerance, and can lead to stomach inflation with the risk of vomiting and aspiration.
In general, NIV can be uncomfortable, and cause pressure sores, particularly over the bridge of the nose. Pressures should be gradually titrated to the desired level and different masks can be used to counteract pressure damage or vulnerable areas padded.
When to wean
Again, a decision made by a senior!
If NIV provides a therapeutic benefit initially, it should be worn as much as possible during the first 24 hours. Treatment should be given until therapeutic success (e.g. resolution of the acute underlying cause, with the patient stabilised, pH ≥ 7.35).
This usually takes place by trials off NIV during the day, for example when eating/drinking, increasing in length. Caution is taken at night, as respiratory effort naturally reduces. Usually, even after NIV is no longer required in the day, a further night of NIV is recommended.
CPAP in an acute setting will be weaned more quickly than NIV, when the patient is stable and no longer in respiratory distress. The settings can be reduced by around 2cm every 5-10 minutes. When they are down to 4cm and remain stable, they can be trialled on a nasal cannula.
NIV (also known as BiPAP) and CPAP are used as breathing support delivered via a facemask for those in respiratory failure which has not responded to medical management. An ABG is required prior to starting, and for monitoring. You are not expected to start NIV by yourself: A senior will always be involved in decision making.
NIV machines are not found in every ward, but are in respiratory high dependency bays, HDU and ICU. It is worth having a look at one if you get the chance!
For further reading, the BTS guidelines give the most comprehensive, up to date information.
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 https://err.ersjournals.com/content/27/148/170101 [Accessed 12/05/2019]