Acute Respiratory Distress Syndrome (ARDS) is characterized by diffuse inflammatory alveolar-capillary membrane injury, non-cardiogenic pulmonary edema, and severe shunt-mediated hypoxemia.
Key Physiological Equations & Formulas
The Berlin ARDS Definition:
$$P/F \le 300 \text{ on PEEP } \ge 5\text{ cmH}_2\text{O within 7 days of known insult}$$Mild: 200 < P/F <= 300. Moderate: 100 < P/F <= 200. Severe: P/F <= 100.
ARDSNet Low Tidal Volume Protocol:
$$V_T = 6\text{ mL/kg PBW (titrate } 4-8\text{ mL/kg)}$$Strictly scale to Predicted Body Weight ($PBW_{male} = 50 + 0.91[Ht - 152.4]$, $PBW_{female} = 45.5 + 0.91[Ht - 152.4]$).
Driving Pressure Target:
$$\Delta P = P_{plat} - PEEP < 14-15\text{ cmH}_2\text{O}$$Strongest mechanical predictor of survival in ARDS.
Practical Clinical Pearls & Bedside Rules
- Plateau Pressure Ceiling: Maintain $P_{plat} \le 30\text{ cmH}_2\text{O}$ (or $\le 32$ if chest wall compliance is markedly reduced).
- High vs Low PEEP Strategy: Moderate-to-severe ARDS ($P/F < 200$) benefits from higher PEEP ($12-18\text{ cmH}_2\text{O}$) combined with prone positioning.
Initial Settings & Clinical Titration Protocol
| Parameter | Recommended Initial Setting | Titration Goal / Safety Threshold |
|---|---|---|
| Initial Tidal Volume | 6 mL/kg PBW (reduce to 5 or 4 mL/kg if Pplat > 30) | Maintain strict lung-protective ventilation. |
| PEEP Titration | 10 - 16 cmH2O based on ARDSNet High PEEP table | Maintain FRC and reduce cyclical shear atelectrauma. |
Bedside Troubleshooting & Red Flags
Warning
Refractory Hypoxemia ($P/F < 80$ on PEEP 15): Initiate early prone positioning ($>16\text{ h/day}$), neuromuscular blockade ($<48\text{ h}$), and evaluate for VV-ECMO.
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.