What is Bias Flow in a Ventilator?
Bias flow is a continuous stream of fresh gas delivered through the ventilator circuit that serves two critical functions: it maintains circuit pressure during spontaneous breathing and actively flushes exhaled CO₂ from the system to prevent rebreathing. 1
Primary Functions of Bias Flow
Gas Delivery and Pressure Maintenance
Bias flow provides a constant stream of gas (typically 2-3 times the patient's minute ventilation) that prevents pressure drops in the circuit during inspiration. 2 This is particularly important in non-invasive ventilation (NIV) systems using single-limb circuits where the same tubing carries both inspired and expired gas. 1
In bi-level pressure support ventilation, the expiratory positive airway pressure (EPAP) is maintained by continuous bias flow from the ventilator, which simultaneously serves to vent exhaled gas through the exhaust port. 1 Without adequate bias flow, the circuit cannot maintain the set EPAP level.
CO₂ Elimination and Prevention of Rebreathing
Bias flow actively flushes exhaled CO₂ from the mask and distal ventilator tubing during the expiratory phase, preventing rebreathing of CO₂-rich air. 3 This is especially critical in single-limb NIV circuits where there is no separate expiratory limb.
A minimum EPAP of 3-5 cm H₂O (maintained by bias flow) is required to adequately vent exhaled air through the exhalation port and prevent CO₂ rebreathing. 1, 3 When EPAP levels fall below this threshold or when respiratory frequency increases significantly, rebreathing can occur and exacerbate hypercapnia. 1
Clinical Implications and Settings
Flow Requirements Based on Patient Condition
In acutely distressed COPD patients with increased minute ventilation, high respiratory frequency, and short inspiratory time, peak inspiratory flow rates may exceed 60 L/min, requiring high bias flow rates to prevent a fall in applied pressure. 1
In contrast, during domiciliary use (such as for obstructive sleep apnea), lower bias flow rates are sufficient because minute ventilation and peak inspiratory flows are much lower. 1
Triggering Mechanism
- Bi-level ventilators employ flow sensors that detect changes in the machine-produced bias flow to identify when the patient initiates a breath. 1 When the patient begins to inhale, the flow sensor detects the disruption in bias flow and triggers the ventilator to deliver the set inspiratory pressure.
Important Caveats and Pitfalls
Potential Adverse Effects
Increasing bias flow during pressure support ventilation can paradoxically increase both triggering delay and work of breathing required to trigger the ventilator (WOBtr). 4 Higher bias flow rates at a given pressure support level result in decreased pressure support time and reduced tidal volume. 4
The bias flow system may not be desirable during pressure support ventilation because it can increase the work required to trigger breaths, particularly in patients with high airway resistance or auto-PEEP. 4
Circuit Obstruction
- Occlusion of the exhaust port (for example, by sputum) can exacerbate hypercapnia through rebreathing, as the bias flow cannot adequately flush CO₂ from the circuit. 1 It is critical to ensure that exhalation ports or valves are fitted and functioning properly.
Patient-Specific Considerations
- In tachypneic, anxious patients who fail to improve or develop worsening hypercapnia on NIV, inadequate CO₂ clearance due to insufficient bias flow/EPAP should be considered. 1 Normally used EPAP levels (3-5 cm H₂O) do not completely eliminate rebreathing when respiratory frequency increases significantly. 1
High-Frequency Oscillatory Ventilation Context
- In high-frequency oscillatory ventilation (HFOV), bias flow serves a different but related purpose: it provides fresh gas to replenish oxygen and remove CO₂ from the patient circuit. 5, 6 The bias flow rate in HFOV affects CO₂ removal when it falls below a threshold that causes substantial increases in bias flow CO₂ concentration. 5