What is the difference between Continuous Positive Airway Pressure (CPAP) and Bilevel Positive Airway Pressure (BiPAP) in non-invasive ventilation?

Difference Between CPAP and BiPAP in Non-invasive Ventilation

BiPAP provides two pressure levels (inspiratory and expiratory) while CPAP delivers a single continuous pressure, making BiPAP more effective for patients with hypercapnic respiratory failure and those who cannot tolerate high CPAP pressures. 1

Basic Differences

• CPAP (Continuous Positive Airway Pressure) delivers a single constant pressure throughout the respiratory cycle, primarily used to correct hypoxemia by recruiting underventilated lung areas and improving oxygenation 1
• BiPAP (Bilevel Positive Airway Pressure) provides two distinct pressure levels:
  • IPAP (Inspiratory Positive Airway Pressure) - higher pressure during inspiration
  • EPAP (Expiratory Positive Airway Pressure) - lower pressure during expiration 1

Mechanisms of Action

CPAP

• Maintains a constant positive pressure throughout the breathing cycle 1
• Recruits collapsed alveoli (similar to PEEP in invasive ventilation) 1
• Unloads inspiratory muscles, reducing work of breathing 1
• Offsets intrinsic PEEP in COPD patients, potentially improving ventilation 1
• Main indication is to correct hypoxemia rather than provide ventilatory support 1

BiPAP

• Combines pressure support with CPAP principles 1
• IPAP provides ventilatory assistance during inspiration 1
• EPAP recruits underventilated lung and offsets intrinsic PEEP 1
• The pressure difference between IPAP and EPAP creates ventilation 1
• EPAP also serves to vent exhaled gas through the exhaust port 1

Clinical Applications

CPAP is preferred for:

• Cardiogenic pulmonary edema patients who remain hypoxic despite maximal medical treatment 1
• Chest wall trauma patients who remain hypoxic despite adequate regional anesthesia and high flow oxygen 1
• Diffuse pneumonia with hypoxemia 1
• Obstructive sleep apnea without respiratory acidosis 2

BiPAP is preferred for:

• Acute exacerbations of COPD with respiratory acidosis (pH <7.35) 1
• Patients who fail CPAP therapy due to intolerance of high pressures 3
• Acute or acute-on-chronic hypercapnic respiratory failure due to chest wall deformity or neuromuscular disease 1
• Decompensated obstructive sleep apnea with respiratory acidosis 1
• Patients who develop hypercapnia during CPAP treatment for pneumonia 1
• Weaning patients from invasive ventilation when conventional strategies fail 1

Practical Considerations

• BiPAP ventilators are simpler to use, cheaper, and more flexible than other ventilator types 1
• BiPAP has been used in the majority of randomized controlled trials of NIV 1
• BiPAP allows lower expiratory pressures, which may improve comfort for patients who cannot tolerate high CPAP pressures 3
• Recent evidence shows that patients who fail CPAP therapy due to low adherence achieve better compliance with BiPAP (7.0 vs 2.5 hours/night) 3

Potential Pitfalls and Caveats

• BiPAP may paradoxically increase work of breathing in some COPD patients compared to pressure support ventilation 4
• Rebreathing can occur with BiPAP if exhaust ports become occluded (e.g., by sputum) 1
• Normal EPAP levels (3-5 cm H₂O) may not completely eliminate rebreathing during BiPAP, especially with increased respiratory frequency 1
• For patients with complex sleep apnea, adaptive servoventilation may be more effective than BiPAP 5
• Both CPAP and BiPAP are contraindicated in patients after recent facial or upper airway surgery, with facial abnormalities, fixed upper airway obstruction, or who are vomiting 1