Differentiate between non-invasive ventilation and CPAP machine

Non-invasive ventilation (NIV) and continuous positive airway pressure (CPAP) are forms of ventilator support used in acute respiratory failure when a patient remains hypoxic despite optimization 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. 

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 under ventilated or collapsed alveoli for gas exchange and allows for the removal of the exhaled gas. 

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 apnea (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

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.

 So when NIV/CPAP should 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 hypercapnia respiratory failure. 


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

Hyper capnic 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. 
  • Common contraindications
  • Vomiting/excess secretions (aspiration risk)
  • Confusion/agitation
  • Impaired consciousness
  • Bowel obstruction
  • Facial burns/trauma
  • Recent facial/upper GI/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. 

These were some points about Non-invasive ventilation and it’s comparison with CPAP.

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