What are ‘flow triggering’ and ‘bias flow’?

Flow triggering is a popular method for allowing patients to initiate breaths during mechanical ventilation. It works by setting a continuous “background flow” of gas through the ventilator circuit during expiration called Bias Flow. During spontaneous breathing inspiratory effort diverts some of this background flow away from the circuit to the patient. This resulting decrease in background flow exiting the ventilator is sensed and triggers an assisted breath.

Settings: The clinician sets the flow trigger sensitivity as they would pressure sensitivity. This is typically set at 1-2 L/min below the bias flow setting which may be programmed into the ventilator or can be adjusted by the clinician (e.g. 5-20 L/min).  Bias flow can be a significant source of oxygen consumption.  

Not all ventilators use bias flow and some have different methods of achieving flow triggering. Always refer to the product manual for device specifications.

Issues Related to Flow Triggering

Circuit Leak: If a leak develops in the circuit gas flow will be diverted away from the expiratory flow sensor causing ventilator malfunction by continuous triggering (“chattering” ) analogous to an excessive sensitivity that will impede ventilation and set-off an alarm condition.

Inappropriate Sensitivity Setting: An insensitive flow trigger level (e.g. > 5 L/min below bias flow) will increase the time delay between the onset of patient effort and the onset of the mechanical breath that often results in substantial increases in patient work of breathing and respiratory drive and patient-ventilator asynchrony. As described above, a too sensitive ventilator causes excessive triggering that can interfere with ventilation and can increase the risk of hyperinflation, barotrauma, hemodynamic instability and hyperventilation during assist-control or high levels of pressure support ventilation.

Artifacts during Indirect Calorimetry: Bias flow through the circuit interferes with the measurement of VO2 and VCO2 as it increases the total ventilation measurement by the calorimeter above the patients minute ventilation. Pressure-trigger should be used during calorimetry measurements.

Inadvertent PEEP: In some of the older ventilators that allow a high bias flow (e.g. 15-20 L/min), these flows may interact with the resistance of the expiratory block (pneumotachographs and valving) to create circuit backpressure. This would be more prominent when higher levels of PEEP are used in tandem with high bias flow. This can be easily detected when PEEP measured within the circuit exceeds the set PEEP (e.g. set PEEP = 10 and measured PEEP = 14 cmH2O).

Example Bias flow O2 consumption calculation:

For the following scenario, total oxygen consumption and oxygen consumption due to bias flow are 16.7 LPM and 6.7 LPM respectively. RR 20, TV 500, bias flow 10, leak zero, FiO2 1.0, expiratory time 2.

[Device O2 consumption rate in LPM = (Minute ventilation + (bias flow x RR x expiratory time/60) + leak) x (FiO2 – 0.21)/0.79]

Additional resources:

Ventilator Bias Flow Value Database (Coming soon)

Calculator to estimate impact of bias flow on O2 consumption