Time constant: What do we need to know to use it?

Mechanical ventilation is an important strategy to manage patients with respiratory ailments in the acute as well as chronic setups. To be able to do so, we need a good understanding of the physiology of the respiratory system and the disease pattern. Two major components of the lung physiology are compliance and resistance and these define time constants (TCs). Compliance is the measure of distensibility of lungs. It is the change in pressure by the change in volume. Resistance is the opposing force that is created when air flows in the airway. Compliance and resistance together produce an entity called TC. TC defines the amount of time taken in seconds for the lung unit to fill or empty. TCs normally vary within the lungs and do more so in the diseased lungs. This article gives an overview of TCs and its components for the better understanding of the reader.