What is the difference between BiPAP (Bilevel Positive Airway Pressure) and CPAP (Continuous Positive Airway Pressure) for a patient with obstructive sleep apnea (OSA) and a history of obesity hypoventilation syndrome (OHS)?

BiPAP vs. CPAP: Key Differences and Clinical Applications

CPAP delivers a single, fixed pressure throughout the respiratory cycle, while BiPAP delivers a higher pressure during inspiration (IPAP) and a lower pressure during expiration (EPAP), making exhalation more comfortable against high pressures. 1

Mechanism of Action

CPAP (Continuous Positive Airway Pressure):

• Delivers one constant pressure level throughout the entire breathing cycle 1
• Maintains upper airway patency by acting as a pneumatic splint 1
• Standard starting pressure is 4 cm H₂O, with titration upward as needed 1

BiPAP (Bilevel Positive Airway Pressure):

• Delivers two distinct pressure levels: higher inspiratory pressure (IPAP) and lower expiratory pressure (EPAP) 1
• Reduces the work of exhalation by lowering pressure when the patient breathes out 1
• Standard starting pressures are IPAP 8 cm H₂O and EPAP 4 cm H₂O 1, 2
• Can provide ventilatory support through pressure differential (pressure support) 3

First-Line Therapy for OSA

For patients with uncomplicated OSA, CPAP or auto-adjusting CPAP (APAP) should be used as first-line therapy, NOT BiPAP. 1, 4

• The American Academy of Sleep Medicine recommends CPAP or APAP over BiPAP for routine OSA treatment (conditional recommendation) 1
• Meta-analyses demonstrate no clinically significant differences between BiPAP and CPAP in adherence, sleepiness reduction, quality of life, or residual apnea-hypopnea index 1
• BiPAP is more expensive than CPAP without providing superior outcomes in uncomplicated OSA 1, 4
• Modern CPAP devices with pressure relief technology (C-Flex, EPR) have reduced the historical comfort advantages of BiPAP 1

Specific Indications for BiPAP in OSA Patients

BiPAP should be reserved for specific clinical scenarios where CPAP is inadequate or not tolerated: 1, 2, 4

High Pressure Intolerance

• Switch to BiPAP when CPAP pressures exceed 15 cm H₂O and the patient cannot tolerate the pressure 1, 2
• BiPAP allows lower mean airway pressure while maintaining upper airway patency 3
• One study showed 23% of CPAP failures were due to intolerant pressures, and these patients achieved better adherence with BiPAP (7.0 vs 2.5 hours/night) 5

Pressure Requirements Exceeding Device Capacity

• BiPAP devices can deliver pressures higher than 20 cm H₂O, which standard CPAP units typically cannot provide 1, 4
• This applies to a small subset of patients requiring very high therapeutic pressures 1

Critical Indication: OSA with Obesity Hypoventilation Syndrome

For patients with OSA AND obesity hypoventilation syndrome (OHS), the treatment algorithm differs significantly based on OSA severity:

OHS with Severe OSA (AHI ≥30 events/hour)

• Start with CPAP rather than BiPAP as first-line therapy 2, 6
• The American Thoracic Society recommends CPAP for stable ambulatory patients with OHS and concurrent severe OSA 2
• CPAP and BiPAP show similar effectiveness in improving gas exchange, daytime sleepiness, sleep quality, and adherence in this population 6
• CPAP is less costly and requires fewer resources than BiPAP 6

OHS without Severe OSA (AHI <30 events/hour)

• BiPAP may be preferred over CPAP 2
• BiPAP offers greater benefits in patients with OHS who have sleep hypoventilation without severe OSA 2
• The guideline panel acknowledged less certainty about CPAP effectiveness in this subgroup 2

OHS with Acute-on-Chronic Hypercapnic Respiratory Failure

• Start BiPAP (NIV) before hospital discharge 2
• Mortality at 3 months was significantly lower in patients discharged on PAP therapy (2.3%) versus without PAP (16.8%, P<0.0001) 2

Additional BiPAP Indications Beyond OSA

BiPAP has established roles in conditions requiring ventilatory support: 2

• COPD with chronic type 2 respiratory failure and elevated baseline PaCO₂ 2
• Neuromuscular disorders affecting respiratory function, particularly those requiring backup rate support 2
• Acute hypercapnic respiratory failure with elevated PaCO₂ and respiratory acidosis 2
• Historical data shows higher BIPAP prescription rates in OSAS patients with COPD (9 of 16 patients) or OHS (11 of 17 patients) 7

Clinical Decision Algorithm for Your Patient

For a patient with OSA and OHS, follow this approach:

1. Determine OSA severity from diagnostic sleep study 2

   • If AHI ≥30 events/hour: Start with CPAP 2, 6
   • If AHI <30 events/hour: Consider BiPAP as initial therapy 2

2. Assess for acute hypercapnic respiratory failure 2

   • If hospitalized with acute-on-chronic hypercapnia: Start BiPAP before discharge 2
   • If stable outpatient: Follow algorithm based on AHI 2

3. If starting with CPAP, monitor for treatment failure indicators: 1

   • Continued obstructive events at CPAP ≥15 cm H₂O during titration 1
   • Patient discomfort or intolerance of high CPAP pressures 1
   • Inadequate symptom control despite optimal CPAP titration 5

4. Switch to BiPAP if CPAP fails 1, 5

   • Lower expiratory pressures with BiPAP improve tolerance 5
   • Better adherence and symptom control achieved in CPAP-intolerant patients 5

Important Clinical Caveats

Avoid common pitfalls when considering BiPAP:

• Do not use BiPAP routinely for uncomplicated OSA – it provides no outcome benefit over CPAP and increases cost 1, 4
• Ensure adequate PAP education, mask fitting, and acclimatization before assuming CPAP failure 1
• BiPAP with inadequate expiratory pressure (EPAP) can fail to prevent obstructive events 1
• In heart failure with reduced ejection fraction, adaptive servo-ventilation (ASV) is absolutely contraindicated due to increased cardiovascular mortality 8
• BiPAP may increase myocardial infarction risk in acute heart failure compared to CPAP 2
• Monitor for aerophagia (air swallowing) with BiPAP, which can cause gastric distention 2
• Proper mask fitting is essential regardless of device type to prevent air leaks that worsen outcomes 2

Pressure Titration Guidelines

When titrating BiPAP for your patient with OSA and OHS:

• Start with IPAP 8 cm H₂O and EPAP 4 cm H₂O 1, 2
• Maintain a pressure differential of 4-6 cm H₂O typically 2
• Increase IPAP until obstructive events (apneas, hypopneas, RERAs, snoring) are eliminated 1
• Manual titration during attended polysomnography remains the gold standard for determining optimal settings 1, 2
• Target SpO₂ 90-96% during titration, with some guidelines recommending ≥92% 2