Pathophysiology
Clinical meaning
Respiratory mechanics describes the physical properties of the respiratory system that determine ventilation. Two key concepts: compliance (distensibility โ the change in volume per unit change in pressure, C = ฮV/ฮP) and resistance (opposition to airflow, R = ฮP/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 (ฮP = Pplat - PEEP) correlates with mortality in ARDS โ target <15 cmH2O. Transpulmonary pressure (Ptp = Pairway - Ppleural) is the true distending pressure of the lung.