Table of Contents
Types of NIV
CPAP = continuous positive airway pressure
- Tight fitting mask which delivers fixed positive air pressure to keep the airways open
- Indications: sleep apnoea; type 1 respiratory failure (e.g. acute pulmonary oedema)
BiPAP = bi-level positive airway pressure
- Similar to CPAP, but the air pressure increases during the patient’s inspirations
- ePAP = pressure during expiration
- iPAP = pressure during inspiration
- Pressure support = difference in pressure between ePAP and iPAP (i.e. the amount of ‘help’ given on inspiration)
- Indication: type 2 respiratory failure (e.g. COPD exacerbation) with acidosis (pH<7.35) or exhaustion despite optimal medical therapy
NB. The oxygen concentration in the air can be adjusted because tubing from an oxygen supply can plug directly into the machine
Background physiology
Settings
CPAP
- Start at 4cmH2O and gradually increase to reduce hypoxia
- Maximum of 12cmH2O
- Start with high FiO2 and titrate down
BiPAP
- Start at 15/3cmH2O (i.e. iPAP of 15cmH2O, ePAP of 3cmH2O) and gradually increase iPAP to 20-30cmH2O as tolerated to optimise respiratory rate, chest expansion and tidal volumes (6-8ml/kg ideal body weight)
- Maximum of 30/8cmH2O
- Increase iPAP to reduce hypercapnia and achieve normal pH
- Titrate FiO2 to reach target oxygen saturations (88-92% or PaO2~8)
Complications
- Too high CPAP or ePAP: reduced venous return which can cause hypotension
- Too high iPAP: mask leak; stomach inflation leading which can cause aspiration (oesophageal opening pressure is 25cmH2O)
- Both: patient discomfort; claustrophobia; pressure sores; dryness; pneumothorax
Contraindications
Absolute
- Severe facial deformity
- Facial burns/trauma
- Fixed upper airway obstruction
- Inability to protect airway
- Severe epistaxis
- Vomiting
- Apnoea
NB. NIV should not be used for asthma (just delays inevitable intubation) or pneumonia (unless patient is not for intubation).
Relative
- pH<7.15
- Undrained pneumothorax
- Aspiration risk
- GCS <8
- Copious respiratory secretions
- Poor respiratory drive
- Need for continuous or near-continuous ventilatory assistance
- Confusion/agitation
- Facial/oesophageal/gastric surgery
Monitoring required
- Oxygen saturation monitoring (aim 88-92% in COPD patients, 94-98% in non-COPD patients)
- Regular arterial blood gasses (~30mins after each change to monitor pCO2 and subsequent pH)
- In COPD, aim PaO2~8mmHg and improving pH
- Blood pressure (check not becoming hypotensive)
- Respiratory rate
- Cardiac monitoring
Weaning off NIV
- Once the medications have had time to work and patients physiology has been corrected, NIV can be gradually weaned
- Options:
- Give patient time off NIV in day and gradually extend time off (leave overnight until last as respiratory drive naturally decreases)
- Gradually reduce pressures
