How do patients on Bilevel Positive Airway Pressure (BiPAP) blow off carbon dioxide?

How Patients Blow Off CO2 on BiPAP

Patients eliminate CO2 on BiPAP through the pressure differential between inspiratory (IPAP) and expiratory (EPAP) pressures, which increases tidal volume and minute ventilation, while EPAP simultaneously flushes exhaled CO2 from the mask and distal ventilator tubing through the exhalation port. 1, 2

Mechanism of CO2 Elimination

Pressure Support Creates Increased Ventilation

• The difference between IPAP and EPAP (called pressure support) directly increases tidal volume, which is the primary mechanism for improving alveolar ventilation and CO2 clearance 2
• IPAP provides ventilatory support during inspiration by generating higher pressure that overcomes the impedance of the lungs and chest wall, delivering a larger breath than the patient could achieve spontaneously 1
• The volume of air delivered is a function of the inspiratory pressure level, the inspiratory time duration, and the patient's lung/chest wall mechanics 1

EPAP Prevents CO2 Rebreathing

• EPAP maintains positive pressure throughout expiration, which actively flushes exhaled CO2 from the mask and distal ventilator tubing, preventing rebreathing of CO2-rich air 1
• A minimum EPAP of 3-4 cm H2O is required to adequately vent exhaled air through the exhalation port in single-limb circuits 1
• Without adequate EPAP, significant CO2 rebreathing can occur—studies show that up to 55% of expired tidal volume can remain in the inspiratory circuit at end-expiration with inadequate expiratory pressure 3

Titration to Normalize CO2

Target-Driven Approach

• The American Thoracic Society suggests targeting normalization of PaCO2 in patients with chronic hypercapnic COPD on long-term NIV, using higher inspiratory pressures and backup respiratory rates (high-intensity NIV) 1
• Studies demonstrate that NIV specifically targeting CO2 clearance reduces PaCO2 by approximately 4.9 mm Hg compared to less aggressive settings 1
• However, aggressive overnight titration to achieve normocapnia in a single night may not be safe and can result in glottic closure rather than increased ventilation 1

Monitoring CO2 Clearance

• Transcutaneous CO2 monitoring during titration allows real-time adjustment of pressures to optimize ventilation 1
• Daytime arterial blood gases can serve as surrogates for nocturnal CO2 changes when overnight monitoring is unavailable 1
• The addition of EPAP in neuromuscular patients reduces maximum overnight transcutaneous CO2 (from 8.1 to 7.3 kPa) and improves minimum oxygen saturation 4

Critical Technical Considerations

Circuit Design Matters

• BiPAP systems typically use single-limb circuits with an integral exhalation port on the mask or an exhalation valve inserted near the mask 1
• The exhalation port must allow adequate clearance of exhaled air—inadequate venting leads to CO2 rebreathing regardless of pressure settings 1, 3
• Adding a non-rebreathing valve can prevent CO2 rebreathing but increases expiratory resistance and work of breathing, potentially offsetting benefits 3

Pressure Settings Impact CO2 Clearance

• Initial settings typically start at IPAP 8-10 cm H2O and EPAP 4-5 cm H2O, with pressure support (IPAP-EPAP difference) of 4-6 cm H2O 1, 2, 5
• Higher pressure support increases tidal volume and minute ventilation, directly improving CO2 elimination 2, 6
• In COPD patients, BiPAP may paradoxically increase work of breathing and intrinsic PEEP compared to pressure support ventilation, potentially limiting CO2 clearance 7

Common Pitfalls

• Inadequate EPAP (<3 cm H2O) allows CO2 rebreathing in the circuit, negating the ventilatory benefits of higher IPAP 1, 3
• Excessive leak around the mask interface reduces effective pressure delivery and tidal volume, impairing CO2 elimination 1
• In COPD patients with dynamic hyperinflation, BiPAP can increase intrinsic PEEP and work of breathing, potentially worsening rather than improving ventilation 7
• Overly aggressive titration attempting to normalize CO2 rapidly can trigger glottic closure and patient-ventilator asynchrony 1