Respiratory mechanics: compliance, resistance and the work of breathing

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• Compliance is change in volume over change in pressure, and the total respiratory compliance of about 100 mL per cmH2O is LESS than either the lung or the chest wall alone (each about 200), because the two springs are in series.
• Airway resistance is proportional to one over radius to the fourth power (Poiseuille law) for laminar flow, so halving airway radius multiplies resistance sixteenfold; this is why bronchospasm, mucosal oedema and a small endotracheal tube so sharply raise the work of breathing.
• Most airway resistance lies in the medium-sized bronchi (generations 2 to 8), NOT in the small airways, because the summed cross-sectional area of the peripheral airways is enormous; small-airway disease is therefore silent until it is advanced.
• The lung pressure-volume curve is sigmoidal with highest compliance at FRC and low compliance at both extremes; ventilation at low volumes causes atelectasis and at high volumes causes overdistension and volutrauma, which is why protective ventilation aims for the compliant middle.
• By Laplace law (pressure equals two times surface tension over radius) small alveoli would collapse into large ones; surfactant lowers surface tension more as the alveolus shrinks, stabilising alveoli, raising compliance and preventing atelectasis; its loss (prematurity, ARDS) stiffens the lung.
• The time constant tau equals resistance times compliance (about 0.3 s normally); three time constants are needed for 95 per cent of a volume change, so a high respiratory rate with short expiration traps gas in obstructive disease (COPD, asthma).