Cardiopulmonary Anatomy, Physiology, and Pathophysiology
Lung volumes and capacities
Spirometry measures VT, IRV, ERV, and VC; RV requires helium dilution, nitrogen washout, or plethysmography. FRC = ERV + RV; TLC = VC + RV. Predicted VT for ventilation is 6–8 mL/kg predicted body weight, not actual weight.
Mechanics and gas exchange
Static compliance = ΔV/ΔP; normal adult ventilated values are about 50–100 mL/cmH2O. Airway resistance = ΔP/flow; normal roughly 0.5–2 cmH2O/L/s. Laplace's law (P = 2T/r) explains why small alveoli collapse without surfactant. Surfactant lowers surface tension and increases compliance; deficiency produces atelectasis and RDS. V/Q mismatch responds to oxygen; true shunt does not. CaO2 = (1.34 × Hb × SaO2) + (0.003 × PaO2). CO2 travels mainly as bicarbonate, not dissolved gas.
Worked example
A 70-kg man has VT 500 mL, plateau 25 cmH2O, PEEP 5 cmH2O. Static compliance = 500/(25−5) = 25 mL/cmH2O. This is low, pointing to ARDS, pneumonia, or pulmonary edema. If peak pressure is 45 cmH2O, the peak–plateau gradient is 20 cmH2O, indicating added airway resistance (secretions, bronchospasm, small ETT).
Traps
Do not use peak pressure to calculate static compliance. Do not label all hypoxemia as shunt. Remember that RV is not measured by office spirometry.