What is the difference between CPAP and BiPAP for a patient with respiratory distress?

CPAP vs BiPAP: Key Differences

CPAP delivers a single, fixed pressure throughout the entire respiratory cycle, while BiPAP delivers two distinct pressure levels—a higher pressure during inspiration (IPAP) and a lower pressure during expiration (EPAP)—making BiPAP more comfortable for patients who struggle with exhaling against high pressures. 1

Fundamental Mechanical Differences

CPAP (Continuous Positive Airway Pressure)

• Delivers constant, unchanging pressure throughout both inspiration and expiration, typically ranging from 4-20 cm H₂O 1
• Functions primarily to maintain upper airway patency by preventing collapse during sleep 1
• Recruits underventilated lung areas similar to PEEP in mechanically ventilated patients 1
• Corrects hypoxemia by increasing mean airway pressure and improving ventilation to collapsed lung regions 1

BiPAP (Bilevel Positive Airway Pressure)

• Delivers two separate pressure levels: higher inspiratory pressure (IPAP) and lower expiratory pressure (EPAP) 1
• The pressure differential between IPAP and EPAP provides ventilatory assistance, not just airway support 1
• Standard starting pressures are IPAP 8 cm H₂O and EPAP 4 cm H₂O, with typical pressure differentials of 4-6 cm H₂O 1, 2
• Can augment ventilation in patients with inadequate respiratory drive or muscle weakness, unlike CPAP 1, 3

Clinical Indications: When to Use Each

CPAP is First-Line For:

• Obstructive sleep apnea without hypoventilation—CPAP or auto-titrating PAP (APAP) are equally effective first-line therapies with no clinically significant differences in adherence, sleepiness reduction, or quality of life 4
• Acute cardiogenic pulmonary edema—CPAP is preferred over BiPAP due to lower myocardial infarction risk (31% vs 71% in one study) 2
• Type 1 respiratory failure (hypoxemic without hypercapnia)—BiPAP has 2.6 times higher failure rates compared to CPAP in this setting 2

BiPAP is Indicated For:

• OSA patients intolerant of high CPAP pressures: Switch to BiPAP when patients cannot tolerate CPAP >15 cm H₂O or when obstructive events persist at 15 cm H₂O despite adequate trial 1, 2
• Type 2 respiratory failure with hypercapnia (elevated PaCO₂)—BiPAP provides ventilatory support to reduce carbon dioxide retention 2
• Obesity hypoventilation syndrome (BMI >30 kg/m² with daytime hypercapnia) 2
• COPD with chronic hypercapnic respiratory failure and elevated baseline PaCO₂ 2
• Neuromuscular disorders affecting respiratory function, particularly when backup respiratory rate support is needed 2
• OSA with concomitant hypoventilation syndromes 2, 4

Practical Implementation Differences

Pressure Titration Approach

• CPAP titration: Start at 4 cm H₂O and increase incrementally until apneas, hypopneas, RERAs, and snoring are eliminated 1
• BiPAP titration: Start with IPAP 8 cm H₂O and EPAP 4 cm H₂O; increase IPAP to eliminate obstructive events while maintaining adequate pressure differential 1, 2
• Manual titration during attended polysomnography remains the gold standard for determining optimal pressures for both devices 1

Patient Tolerance Considerations

• BiPAP reduces work of exhalation by lowering expiratory pressure, improving comfort for patients who find CPAP's constant pressure difficult to exhale against 2, 3
• However, BiPAP does not improve overall adherence compared to CPAP in general OSA populations when both are tolerated 4
• BiPAP is more expensive than CPAP, which should factor into decision-making when clinical outcomes are equivalent 4

Critical Clinical Caveats

When BiPAP May Cause Harm

• In hypotensive patients: BiPAP can further reduce blood pressure and should be used cautiously 2
• In acute heart failure: BiPAP may increase myocardial infarction risk compared to CPAP and has not shown clear superiority 2
• In COPD patients with strong spontaneous breathing: BiPAP can paradoxically increase work of breathing and PEEPi (intrinsic PEEP) during the low-pressure phase compared to CPAP alone 5

Monitoring Requirements

• For acute respiratory failure: Do not delay intubation if BiPAP fails to improve clinical status within 1-2 hours 2
• Target oxygen saturation: Aim for SpO₂ 90-96% during titration, with some guidelines recommending ≥92-94% depending on respiratory drive 2
• Monitor for treatment failure: Approximately 29% of COPD patients in acute respiratory failure do not tolerate BiPAP 6

Common Pitfalls to Avoid

• Do not use BiPAP as first-line for simple OSA—it offers no adherence or outcome advantage over CPAP and costs more 4
• Ensure proper mask fitting before attributing failure to the device type—leaks increase work of breathing and aerophagia regardless of CPAP vs BiPAP 2
• Watch for rebreathing with BiPAP systems using single-limb circuits and passive exhalation ports, especially in tachypneic patients where EPAP 3-5 cm H₂O may not completely eliminate CO₂ rebreathing 1