Time Constants in the Lung

Expansion

Time constant = resistance x compliance, normally about 0.5 seconds in a healthy lung unit.

Filling and emptying are exponential:

• 1 time constant: 63 per cent complete
• 2: 86 per cent
• 3: 95 per cent
• 4: 98 per cent

Fast units have low resistance or low compliance and fill and empty quickly. Slow units have high resistance (asthma, chronic obstructive pulmonary disease) or high compliance (emphysema) and need longer.

Consequences:

• At fast respiratory rates slow units do not empty fully, so gas is trapped and auto-PEEP develops; this is why ventilating an asthmatic requires a long expiratory time and a slow rate
• Units with different time constants fill out of phase, so gas can move between units, a phenomenon called pendelluft
• Regional variation in time constants is a mechanism of V/Q mismatch, and it worsens as respiratory rate rises