The concept described as the 'Time constant' in neonatal respiratory physiology corresponds to which of the following?

• A. Dynamic compliance
• B. Time constant
• C. Work of breathing
• D. Static compliance

Answer: (B)

Time constant describes how quickly the respiratory system responds to a pressure change, and in neonates it is the product of airway resistance and lung/chest wall compliance. It tells you how fast inflation and deflation occur: roughly 63% of the volume change happens after one time constant, and after about three time constants you approach the new equilibrium. In newborns, small airways and variable surfactant status can change resistance and compliance, so the time constant helps predict whether ventilation will fill and empty within the planned inspiratory and expiratory times. A longer time constant means slower filling/emptying, which can lead to inadequate ventilation if the inspiratory time is too short; a shorter time constant means quicker changes, with less risk of air trapping if the timing matches the ventilator cycle.

Dynamic compliance refers to volume change per pressure during flow (includes resistance effects), static compliance is the volume change per pressure with no flow (no resistance), and work of breathing is the energy required to overcome both elastic recoil and airway resistance. These describe related properties but do not define the rate of filling/emptying that the time constant captures.