Non-invasive ventilation (NIV) [advanced]

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

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