APRV Time Settings: Thigh and Tlow
For APRV mode, set Thigh to 4-6 seconds and Tlow to 0.5-0.8 seconds, with the release phase terminating when expiratory flow reaches 50-75% of peak expiratory flow on the flow-time waveform. 1
Thigh (High Time) Settings
- Set Thigh to 4-6 seconds to maintain prolonged alveolar recruitment and optimize oxygenation 1
- The prolonged high-pressure phase creates an inverse ratio ventilation pattern (inspiratory time > expiratory time) that increases mean airway pressure and promotes alveolar stability 1
- This extended inspiratory duration allows unrestricted spontaneous breathing throughout most of the respiratory cycle 2, 3
Tlow (Low Time/Release Phase) Settings
- Set Tlow to 0.5-0.8 seconds as recommended by the American Thoracic Society and Society of Critical Care Medicine 1
- The release phase should terminate when expiratory flow reaches approximately 50-75% of peak expiratory flow on the flow-time waveform 1
- This brief release duration allows partial exhalation to generate tidal volumes for CO2 clearance while creating auto-PEEP that maintains alveolar recruitment between release phases 1
Critical Pitfall: Personalized vs. Fixed Settings
The evidence strongly distinguishes between two APRV approaches with dramatically different outcomes 4:
- Personalized-APRV (P-APRV): Sets the release phase based on changes in lung mechanics using the slope of the expiratory flow curve, which has shown promise in stabilizing alveoli and reducing ARDS incidence 4
- Fixed-setting APRV (F-APRV): Uses constant release phase duration regardless of patient physiology 4
The Tlow of 0.5-0.8 seconds should be adjusted based on the individual patient's expiratory flow curve, not left as a fixed value 1, 5. Monitor the flow-time scalar and terminate the release when flow reaches 50-75% of peak, which personalizes ventilation to the patient's lung mechanics 1.
Pressure Settings Context
While you asked specifically about time settings, the pressure parameters are interdependent 1:
- Phigh: 20-30 cmH2O to promote alveolar recruitment 1
- Plow: typically 0-5 cmH2O (though not explicitly stated in highest-quality evidence, this is derived from the release pressure of 6 ± 5 cmH2O reported in clinical trials) 6
- Avoid Phigh >30 cmH2O due to increased risk of barotrauma and hemodynamic compromise 1
Monitoring Requirements
- Continuously monitor the flow-time waveform to ensure Tlow terminates at 50-75% of peak expiratory flow 1
- Watch for excessive tidal volumes during spontaneous breaths at Phigh, as patient-triggered breaths can generate much larger volumes than intended, potentially causing alveolar overdistension 3
- The performance of APRV is highly dependent on precise operator-selected settings, where small variations in the very short expiratory time can lead to either de-recruitment or excessive tidal volumes 2
Special Consideration for ARDS
When using APRV in patients with concurrent ARDS and the risk of delayed cerebral ischemia (as in subarachnoid hemorrhage), spontaneous modes like APRV may lower sedation requirements while meeting cerebral perfusion goals, but these patients require intracranial pressure monitoring 7.