Can you explain bilevel positive airway pressure (BiPAP) therapy and guide me on how to set the initial parameters for an adult with acute on chronic respiratory failure, such as during a COPD exacerbation?

BiPAP in Acute-on-Chronic Respiratory Failure: Initial Settings and Approach

For adults with acute-on-chronic respiratory failure from COPD exacerbation, start BiPAP when pH <7.35 with PCO₂ >6 kPa (45 mmHg) after 30 minutes of standard medical therapy, using initial settings of IPAP 10-15 cmH₂O and EPAP 4-5 cmH₂O, targeting oxygen saturation of 88-92%. 1

Understanding BiPAP (Bilevel Positive Airway Pressure)

BiPAP delivers two levels of positive airway pressure:

• IPAP (Inspiratory Positive Airway Pressure): Higher pressure during inspiration that augments tidal volume and reduces work of breathing 1
• EPAP (Expiratory Positive Airway Pressure): Lower pressure during expiration that maintains airway patency and counteracts intrinsic PEEP 1

The pressure differential (IPAP minus EPAP) provides ventilatory support, while EPAP acts similarly to CPAP in preventing alveolar collapse 1

When to Initiate BiPAP

Clear Indications for NIV/BiPAP:

Start BiPAP when hypercapnic (PCO₂ >6 kPa or 45 mmHg) AND acidotic (pH <7.35) if respiratory acidosis persists >30 minutes after initiating standard medical management (bronchodilators, steroids, controlled oxygen). 1

Severity-Based Approach:

• Mild-moderate acidosis (pH 7.25-7.35): BiPAP is highly effective at preventing intubation and reducing mortality 1, 2
• Severe acidosis (pH <7.25): BiPAP should be attempted before intubation, but must be delivered in ICU with immediate intubation capability 1, 2
• pH <7.20: Consider immediate intubation; BiPAP has higher failure rates 1, 2

Do NOT Use BiPAP Routinely When:

• pH >7.35 without acidosis (even if hypercapnic) - these patients have compensated chronic hypercapnia 1
• Normal or only mildly elevated PCO₂ without acidosis 1, 2

Initial BiPAP Settings for COPD Exacerbation

Starting Parameters:

IPAP: 10-15 cmH₂O 1, 2, 3 EPAP: 4-5 cmH₂O 1, 2, 3 Backup rate: 12-15 breaths/min (if using spontaneous-timed mode) 1 FiO₂: Titrate to maintain SpO₂ 88-92% 1

Titration Algorithm:

1. Increase IPAP by 2 cmH₂O every 5-10 minutes if:

   • Persistent tachypnea (RR >25-30)
   • Continued use of accessory muscles
   • Patient reports inadequate support
   • Persistent hypercapnia on repeat blood gases 1, 2

2. Maximum IPAP: 20-25 cmH₂O (higher pressures rarely tolerated and increase leak/discomfort) 1, 2

3. EPAP adjustments:

   • Increase EPAP by 1-2 cmH₂O if persistent obstructive apneas or if patient has significant intrinsic PEEP 1
   • Keep IPAP-EPAP differential at minimum 4 cmH₂O, maximum 10 cmH₂O 1

4. Typical effective settings: IPAP 14-18 cmH₂O, EPAP 4-6 cmH₂O 1, 3

Critical Oxygen Management

Target SpO₂ 88-92% in COPD patients at risk for hypercapnic respiratory failure - this is non-negotiable. 1

Oxygen Titration Steps:

• Start with 24-28% Venturi mask or 1-2 L/min nasal cannula before blood gases available 1
• Avoid excessive oxygen - PaO₂ >10 kPa increases risk of worsening respiratory acidosis 1
• Recheck blood gases 30-60 minutes after any oxygen or BiPAP adjustment 1
• If pH normal and PCO₂ normal on initial gases, can target SpO₂ 94-98% UNLESS history of prior hypercapnic respiratory failure requiring NIV 1

Critical Pitfall:

Never abruptly stop supplemental oxygen - this causes life-threatening rebound hypoxemia with rapid desaturation below baseline 1

Monitoring and Response Assessment

Immediate Assessment (within 1-2 hours):

Check for improvement in: 1, 2

• Respiratory rate (should decrease toward <25-30)
• Accessory muscle use (should diminish)
• Patient comfort and dyspnea (subjective improvement)
• Mental status (should improve, not worsen)
• Blood gases: pH trending toward normal, PCO₂ decreasing

Repeat ABG at 1-2 hours, then every 4-6 hours until stable 1, 2

Signs of BiPAP Failure Requiring Intubation:

• Worsening ABGs and/or pH after 1-2 hours 1
• No improvement in ABGs/pH after 4 hours 1
• Deteriorating mental status, inability to protect airway 1
• Hemodynamic instability 1
• Inability to tolerate interface despite adjustments 1, 2
• Severe acidosis pH <7.25 with worsening trend 1, 2

Interface Selection and Patient Tolerance

Oronasal (full-face) mask is preferred for acute respiratory failure over nasal mask alone, as mouth breathing is common during respiratory distress. 2

Improving Tolerance:

• Reassure patient, explain the therapy 2
• Start at lower pressures (IPAP 8-10) if patient very anxious, then rapidly titrate up 1
• Ensure proper mask fit to minimize leaks 1, 2
• Consider minimal sedation ONLY in ICU setting with extremely close monitoring (increases failure risk) 2

Special Considerations

For Neuromuscular Disease/Chest Wall Deformity:

• May need higher tidal volumes (6-8 mL/kg) requiring higher IPAP 1
• Often require controlled ventilation modes initially 1
• Target similar pH and PCO₂ goals 1

For Acute Heart Failure with Pulmonary Edema:

• Both CPAP and BiPAP are effective 1, 2
• Prefer BiPAP if hypercapnia present 2
• Monitor blood pressure closely - positive pressure can reduce preload and lower BP 1

Contraindications to BiPAP:

• Inability to protect airway, impaired consciousness (unless purely from hypercapnia) 1
• Hemodynamic instability requiring vasopressors 1
• Recent esophageal surgery 2
• Copious secretions unable to clear 1
• Facial trauma/burns preventing mask seal 1

Location of Care

• pH 7.25-7.35: Can initiate in high-dependency unit or step-down with appropriate monitoring 1
• pH <7.25: Must be in ICU with immediate intubation capability 1, 2
• Requires continuous pulse oximetry, frequent vital signs, and ability to obtain serial blood gases 1