Respiratory Disorders and Mechanical Ventilation
Core concepts
ARDS is an acute (≤1 week) syndrome of bilateral opacities not explained by effusion, collapse, or nodules, with respiratory failure not fully attributable to cardiac failure or volume overload, and PaO2/FiO2 ≤300 on PEEP ≥5 cmH2O. Severity bands: mild 201–300, moderate 101–200, severe ≤100. The only interventions with consistent mortality benefit are lung-protective ventilation and prone positioning in moderate-severe disease.
Lung-protective settings: tidal volume 6 mL/kg predicted body weight (PBW), plateau pressure ≤30 cmH2O, driving pressure (Pplat − PEEP) ≤15 cmH2O, permissive hypercapnia, and PEEP titrated to oxygenation and compliance rather than to a fixed table.
PBW: male = 50 + 0.91 × (height in cm − 152.4); female = 45.5 + 0.91 × (height − 152.4).
Distinguishing resistance from elastance
Peak airway pressure minus plateau pressure reflects resistive load (tube size and kinks, secretions, bronchospasm). Plateau pressure minus PEEP reflects elastic load (pulmonary edema, consolidation, pneumothorax, abdominal distension, mainstem intubation, chest wall stiffness).
Auto-PEEP is measured with an end-expiratory hold and is treated by reducing respiratory rate, shortening inspiratory time, accepting hypercapnia, and giving bronchodilators. In an obstructed, hypotensive patient, auto-PEEP is a likelier culprit than a pneumothorax.
Exam traps
- Setting tidal volume by actual body weight in an obese patient.
- Blaming high peak pressures on "stiff lungs" without checking plateau.
- Applying high PEEP to a patient with severe airflow obstruction.
- Using recruitment maneuvers routinely rather than as a rescue.
- Forgetting that prone positioning needs 12–24 hours per day to help.
Worked example
A 175 cm man: PBW = 50 + 0.91 × 22.6 ≈ 70.6 kg. At 6 mL/kg the target tidal volume is ≈424 mL, regardless of his actual 110 kg weight.
Rule of thumb: isolated peak rise means resistance; paired peak and plateau rise means compliance.