CPAP vs BiPAP - Non-Invasive Positive Pressure Ventilation

(Fischer's Mastery of Surgery)

Definitions

CPAP - Continuous Positive Airway Pressure

• Delivers a single, constant pressure throughout the entire respiratory cycle (both inspiration and expiration)
• The pressure level is equivalent to PEEP (Positive End-Expiratory Pressure)
• Patient-driven rate - completely relies on the patient's own respiratory effort and drive
• No extra pressure boost during inhalation
• Delivered via: helmet, nasal mask, or full facemask

BiPAP - Bilevel Positive Airway Pressure

• Delivers two different pressure levels:
  • IPAP (Inspiratory Positive Airway Pressure) - higher pressure during inhalation
  • EPAP (Expiratory Positive Airway Pressure) - lower pressure during exhalation (equivalent to PEEP)
  • The IPAP-EPAP difference = pressure support (analogous to PS in invasive ventilation)
• Patient-driven rate with a backup rate - newer devices can be set to cycle between pressures at a minimum rate even if the patient fails to initiate a breath
• Delivered via: facemask only

Key Physiological Effects

Indications

CPAP Indications

• Obstructive Sleep Apnea (OSA) - primary chronic use
• Acute cardiogenic pulmonary edema - well-evidenced reduction in intubation rate
• Mild-moderate hypoxemic respiratory failure
• Post-operative atelectasis
• Bridge therapy in selected hypoxic patients

BiPAP Indications

• Acute exacerbation of COPD - strongest evidence; reduces intubation, length of stay, and in-hospital mortality (should be started early, not after medical therapy failure)
• Acute cardiogenic pulmonary edema
• Neuromuscular disease with respiratory failure (e.g., ALS, myasthenia gravis)
• Obesity Hypoventilation Syndrome (OHS) - preferred when patient cannot tolerate high CPAP levels
• Hypercapnic respiratory failure of any cause
• Patients who fail CPAP
• Bridge to intubation in impending respiratory failure
• DNR/DNI patients - as ceiling-of-care respiratory support

Starting Settings

Contraindications (Both)

• Altered mental status / inability to protect airway - risk of aspiration if vomiting occurs
• Apnea (CPAP specifically, as it has no backup rate)
• Neuromuscular blockade - both modes require an open glottis and intact respiratory drive
• Copious secretions (patient must be able to cough and clear)
• Facial fractures or trauma preventing mask seal
• Recent esophageal, gastric, or duodenal anastomoses (positive pressure can insufflate the GI tract and stress anastomoses)
• Hemodynamic instability
• Inability to cooperate (delirium)
• Bacterial pneumonia is a relative contraindication for BiPAP - the high mask pressures make it very difficult to expectorate sputum and reduce bacterial bioburden

Advantages Over Invasive Mechanical Ventilation

• Avoids ETT-related complications (VAP, airway/lung injury)
• Preserves upper airway defense mechanisms
• Patient can speak, eat, clear secretions when mask removed
• Less sedation required
• Shorter hospital stay
• Reduced mortality in appropriate populations (especially COPD exacerbation)

Important Practical Notes

• Tightness of mask seal is critical for effectiveness - leaks reduce delivered pressure
• Do NOT use physical restraints on patients wearing NIV masks - they must be able to remove the mask if they vomit
• Standard ventilators set to NIPPV mode have additive IPAP + EPAP (total delivered = sum). Standalone BiPAP units have independent IPAP/EPAP (set IPAP = delivered IPAP)
• CPAP is appropriate in the right patient for COVID-19 (including helmet CPAP)
• Early initiation in COPD exacerbation is key - late initiation after failed medical therapy eliminates the mortality benefit

Summary Comparison