APRV applies continuous high CPAP ($P_{high}$) for prolonged duration ($T_{high}$) with brief cyclic pressure releases ($T_{low}$) to $P_{low}$, facilitating spontaneous breathing at any point in the respiratory cycle.
Key Physiological Equations & Formulas
APRV Time Settings:
$$T_{high} \approx 4.0-6.0\text{ s}, \quad T_{low} \approx 0.4-0.8\text{ s}$$Ratio $T_{high}:T_{low} \approx 8:1$ to $10:1$ provides continuous open-lung recruitment while brief releases clear CO2.
Expiratory Flow Termination Rule:
$$\text{Terminate } T_{low} \text{ at } 75\% \text{ of Peak Expiratory Flow}$$Creates intrinsic PEEP in alveolar units, preventing end-expiratory alveolar collapse during release.
Practical Clinical Pearls & Bedside Rules
- Spontaneous Breathing Advantage: Unrestricted spontaneous breathing in APRV improves dependent dorsal ventilation and lowers sedation requirements.
- Contraindications: Severe obstructive lung disease (COPD/Asthma) with high airway resistance where brief releases worsen gas trapping.
Initial Settings & Clinical Titration Protocol
| Parameter | Recommended Initial Setting | Titration Goal / Safety Threshold |
|---|---|---|
| Initial Phigh | Set equal to conventional Pplat (typically 24-30 cmH2O) | Limit Phigh <= 30-35 cmH2O. |
| Initial Plow | 0 cmH2O | Allows rapid peak expiratory flow acceleration. |
Bedside Troubleshooting & Red Flags
Warning
Alveolar Derecruitment from Excessive Tlow: If Tlow is too long (terminating at <50% peak expiratory flow), alveoli collapse during the release phase. Set Tlow precisely by flow-time curve.
Key Takeaways & Summary
- Always evaluate patient synchrony and physiological response before changing ventilator parameters.
- Maintain lung-protective strategies to minimize driving pressure ($\Delta P$) and mechanical power.
- Continuously reassess liberation and extubation readiness on daily morning rounds.