From the Guidelines
To calculate driving pressure in Airway Pressure Release Ventilation (APRV), subtract the low pressure (P-low) from the high pressure (P-high), using the formula: Driving Pressure = P-high - P-low, as this directly correlates with tidal volume and lung stress 1. The driving pressure is a critical parameter in APRV, as it reflects the pressure differential that drives gas exchange during the brief releases to P-low. Key points to consider when calculating driving pressure include:
- The formula for driving pressure is based on the difference between P-high and P-low, which is analogous to the difference between plateau pressure (Pplat) and positive end-expiratory pressure (PEEP) in other ventilation modes 1.
- Driving pressure is the ratio of tidal volume to compliance, indicating the "functional" size of the lung, and values exceeding 15 cmH2O are of particular concern 1.
- Monitoring driving pressure helps clinicians assess lung compliance and adjust ventilator settings to minimize ventilator-induced lung injury, with the goal of keeping driving pressure below 15 cmH2O when possible 1.
- Specific targets for driving pressure should be individualized based on the patient's condition and lung mechanics, taking into account factors such as severe acidosis, high PEEP requirements, and chest wall stiffness 1.
From the Research
Calculating Driving Pressure for APRV Ventilation
To calculate driving pressure for Airway Pressure Release Ventilation (APRV), several methods can be employed as discussed in various studies 2, 3, 4, 5, 6.
- Formula-based estimation: One approach is to use a formula that estimates the intrinsic positive end-expiratory pressure (PEEP) present at the end of the APRV release. This method has been shown to provide accurate estimates of driving pressure 2.
- Exponential decay assumption: Another method assumes that expiratory flow, volume, and pressure decay mono-exponentially. This approach also provides accurate estimates of driving pressure 2.
- Volume-controlled ventilation: A third method involves temporarily switching to volume-controlled ventilation to estimate driving pressure. This method has been found to be accurate as well 2.
- Time-controlled adaptive ventilation (TCAV) method: The TCAV method uses an extended inspiratory time and brief expiratory release to stabilize and recruit collapsed lung. The precise timing of each release is based on analysis of expiratory flow and is set to achieve termination of expiratory flow at 75% of the peak expiratory flow 3.
Key Considerations
When calculating driving pressure for APRV ventilation, it is essential to consider the following:
- Monitoring: Careful monitoring of driving pressure is crucial to prevent ventilator-induced lung injury (VILI) 2, 3, 4, 5, 6.
- Protective ventilation strategies: Driving pressure levels should be kept within safe limits to prevent lung injury 2, 3, 4, 5, 6.
- Individual patient pathophysiology: The TCAV method is personalized and adaptive, conforming to individual patient pathophysiology and responding to changes in elastance over time 3.