Understanding pulmonary mechanics begins with the fundamental equation of motion, which describes the balance of forces required to inflate the lungs during positive pressure ventilation.
Key Physiological Equations & Formulas
The Equation of Motion:
$$P_{vent} + P_{mus} = (Flow \times R_{aw}) + \frac{V_T}{C_{rs}} + PEEP_{tot}$$Total pressure delivered equals resistive pressure + elastic recoil pressure + baseline positive end-expiratory pressure.
Static Respiratory System Compliance:
$$C_{stat} = \frac{V_T}{P_{plat} - PEEP}$$Normal static compliance in healthy adults: 50-100 mL/cmH2O; in ARDS: <30 mL/cmH2O.
Airway Resistance:
$$R_{aw} = \frac{P_{peak} - P_{plat}}{\dot{V}}$$Normal resistance: 2-5 cmH2O/L/s in non-intubated, 5-10 cmH2O/L/s with ETT.
Respiratory Time Constant:
$$\tau = R_{aw} \times C_{rs}$$1 tau = 63% volume change, 3 tau = 95%, 5 tau = 99% complete alveolar filling/emptying.
Practical Clinical Pearls & Bedside Rules
- Resistive vs Elastic Work: A sudden rise in Ppeak with stable Pplat indicates airway resistance increase (secretions, bronchospasm, kinking). A rise in both Ppeak and Pplat indicates compliance reduction (atelectasis, pneumothorax, pulmonary edema).
- Time Constant Rule: Allow at least 3-4 time constants for full exhalation to prevent dynamic hyperinflation (intrinsic PEEP).
Initial Settings & Clinical Titration Protocol
| Parameter | Recommended Initial Setting | Titration Goal / Safety Threshold |
|---|---|---|
| Tidal Volume (Vt) | 6 mL/kg PBW (4-8 mL/kg) | Prevent volutrauma; adjust by predicted body weight, not actual body weight. |
| PEEP | 5 cmH2O | Maintain FRC and prevent end-expiratory alveolar derecruitment. |
| Inspiratory Flow | 60 L/min (Adult) / 10-20 L/min (Peds) | Adjust to meet patient flow demand and avoid flow starvation. |
Bedside Troubleshooting & Red Flags
High Peak Pressure Alarm: Check Pplat immediately via inspiratory hold. If Pplat is low, suction and check ETT. If Pplat is high, rule out pneumothorax, pulmonary edema, or worsening ARDS.
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.