Explain bi pap
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BiPAP bilevel positive airway pressure mechanism indications settings
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Here is a thorough explanation of BiPAP drawn from multiple authoritative textbooks.
BiPAP (Bilevel Positive Airway Pressure)
What Is It?
BiPAP (also written BPAP) is a form of non-invasive ventilation (NIV) that delivers air through a tightly fitting face mask - no endotracheal tube required. The term "bilevel" refers to its defining feature: it uses two separate, independently adjustable pressure levels during the breathing cycle, unlike CPAP which maintains a single constant pressure.
- IPAP (Inspiratory Positive Airway Pressure) - the higher pressure delivered when you breathe in
- EPAP (Expiratory Positive Airway Pressure) - the lower pressure maintained when you breathe out
The difference between IPAP and EPAP is called pressure support, and this is what actively assists breathing and drives CO2 elimination.
Figure: BiPAP compared to CPAP and high-flow nasal oxygen. Key distinction - BiPAP provides balanced CO2 clearance AND O2 support, whereas CPAP favors CO2 clearance and HFNCO2 favors oxygenation. BiPAP is delivered via facemask only. - Fischer's Mastery of Surgery
How It Works (Mechanism)
The machine senses the patient's inspiratory effort (via a drop in flow or pressure) and switches from EPAP to IPAP, then cycles back to EPAP when flow tapers at end-inspiration.
IPAP does two things:
- Offloads the respiratory muscles - reduces the work of breathing
- Overcomes intrinsic PEEP (iPEEP), especially important in COPD where air trapping increases end-expiratory lung volume
- Delivers larger tidal volumes, improving alveolar ventilation and CO2 clearance
EPAP (the expiratory pressure) acts like PEEP:
- Keeps alveoli open (prevents collapse/atelectasis)
- Recruits lung units, improving oxygenation
- Counterbalances iPEEP in obstructive disease
When NIV is initiated, the patient's respiratory rate decreases, allowing more effective lung emptying and larger tidal volumes. - ROSEN's Emergency Medicine
Newer BiPAP devices also include a backup rate (BPAP-ST mode) which guarantees cycling between IPAP and EPAP at a set minimum rate, in case the patient's drive is insufficient. This distinguishes it from older models that relied entirely on patient effort. - Murray & Nadel's Respiratory Medicine
BiPAP vs CPAP - Key Differences
| Feature | CPAP | BiPAP |
|---|---|---|
| Pressures | Single fixed pressure | IPAP (higher) + EPAP (lower) |
| CO2 clearance | Moderate | Strong |
| O2 support | Moderate | Balanced |
| Respiratory rate | Patient-driven | Patient-driven + optional backup rate |
| Respiratory muscle unloading | No | Yes |
| Interface | Helmet, nasal mask, or facemask | Facemask only |
Indications
Strongly recommended / first-line:
- COPD exacerbation with hypercapnic respiratory failure (pH ≤7.35, PaCO2 ≥45 mmHg) - this is the best-supported indication; BiPAP reduces mortality, intubation rates, and hospital length of stay
- Acute cardiogenic pulmonary edema - BiPAP reduces left ventricular afterload by lowering transmural pressure
- Asthma exacerbation - as a bridge while treating the underlying cause
Other uses:
- Type 2 (hypercapnic) respiratory failure from any cause
- Obstructive sleep apnea (OSA) with residual central events after CPAP
- Neuromuscular disease causing respiratory insufficiency (e.g., ALS, Duchenne muscular dystrophy)
- Obesity hypoventilation syndrome
- Bridge to intubation in impending acute respiratory failure
BiPAP is preferred over CPAP for type 2 (hypercapnic) respiratory failure because it actively assists ventilation and CO2 clearance, whereas CPAP does not.
Initial Settings
According to ROSEN's Emergency Medicine and Roberts & Hedges' Clinical Procedures in Emergency Medicine:
| Parameter | Starting Value | Notes |
|---|---|---|
| IPAP | 10-12 cm H2O | Titrate up to a max of ~20 cm H2O |
| EPAP | 5 cm H2O | Increase to 8 if obese or significant iPEEP |
| FiO2 | 100% initially | Titrate down to SpO2 88-92% (COPD), or ≥94% (others) |
Titration rules:
- Increase IPAP (increase pressure support = IPAP minus EPAP) if patient remains hypercapnic or has high work of breathing
- Increase EPAP if patient remains hypoxic
- Adjust by 1-2 cm H2O increments at a time
- IPAP >20 cm H2O should be avoided - causes discomfort and gastric insufflation
- Reassess at 30-60 minutes; check ABG at 1-2 hours
Contraindications
Absolute:
- Respiratory or cardiac arrest
- Active vomiting / high aspiration risk
- Inability to protect airway
- Facial trauma preventing mask fit
- Neuromuscular blockade (BiPAP requires an open glottis)
Relative:
- Depressed consciousness not due to hypercapnia (GCS <10)
- Copious secretions unable to be cleared
- Hemodynamic instability unresponsive to fluids/vasopressors
- Bacterial pneumonia (mask pressure makes sputum clearance difficult)
When BiPAP Fails
Failure rate is approximately 10-15%, rising with lower presenting pH and greater illness severity. Signs of NIV failure and indications to escalate to intubation include:
- Persistent tachypnea, accessory muscle use, or worsening acidosis after 30-60 minutes
- No improvement in pH or PaCO2 on repeat ABG
- Worsening mental status
- Hemodynamic instability
- Life-threatening hypoxemia
Patient-Ventilator Asynchrony
A key complication with BiPAP is asynchrony - when the ventilator's cycling does not match the patient's neural respiratory timing. This occurs in up to 43% of patients receiving NIV. The main cause is mask leaks: inspiratory leaks cause prolonged machine inspiration, while expiratory leaks cause auto-triggering. Modern NIV ventilators include leak-compensation algorithms to minimize this. - Murray & Nadel's Respiratory Medicine
Sources:
- ROSEN's Emergency Medicine, 9th ed. - Non-Invasive Ventilation (COPD chapter & General NIV chapter)
- Fischer's Mastery of Surgery, 8th ed. - Common Approaches to Noninvasive Ventilation
- Murray & Nadel's Textbook of Respiratory Medicine - BPAP with Backup Rate; Modes and Settings
- Roberts & Hedges' Clinical Procedures in Emergency Medicine - Initiation of NPPV
A useful video overview comparing CPAP and BiPAP:
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