Clinical meaning
Respiratory mechanics describes the physical properties of the respiratory system that determine ventilation. Two key concepts: compliance (distensibility — the change in volume per unit change in pressure, C = ΔV/ΔP) and resistance (opposition to airflow, R = ΔP/flow). Static compliance (Cstat = Vt / [Pplat - PEEP]) reflects lung and chest wall elastance without flow effects. Normal Cstat is 50-80 mL/cmH2O. Reduced compliance occurs in ARDS, pulmonary fibrosis, pneumonia, pleural effusion, chest wall rigidity, and abdominal hypertension. Airway resistance (Raw = [PIP - Pplat] / flow) reflects opposition to gas flow through airways. Normal Raw is 5-10 cmH2O/L/s. Elevated resistance occurs in bronchospasm, secretions, ETT obstruction/kinking, and small ETT diameter. Driving pressure (ΔP = Pplat - PEEP) correlates with mortality in ARDS — target <15 cmH2O. Transpulmonary pressure (Ptp = Pairway - Ppleural) is the true distending pressure of the lung.
Diagnosis & workup
Diagnostics & workup: - Static compliance: perform inspiratory hold (Pplat measurement) — Cstat = Vt / (Pplat - PEEP) - Airway resistance: Raw = (PIP - Pplat) / inspiratory flow rate - Driving pressure: Pplat - PEEP (target <15 cmH2O in ARDS — strongest individual predictor of ARDS mortality) - Auto-PEEP: perform expiratory hold — measure trapped pressure above set PEEP - Stress index: analysis of pressure-time curve shape during constant-flow ventilation — concave up = overdistension, concave down = recruitment potential, straight = optimal - Esophageal manometry: measures pleural pressure to calculate transpulmonary pressure (guides PEEP setting in severe ARDS) - Point-of-care ultrasound: assess for pleural effusion, pneumothorax, atelectasis affecting compliance