LUNG TIME CONSTANTS (τ)

τ = R × C

A lung time constant is the product of airway resistance and lung compliance, representing the time required for a lung unit to fill or empty by approximately 63%. In clinical practice, it takes about three to five time constants for a breath to be fully inhaled or exhaled

Time Constant Milestones:

1 τ 63.2% Volume Exchanged
3 τ 95.0% Volume Exchanged — Clinical Goal
5 τ 99.3% Volume Exchanged — Steady State

Controls & Flow Profile

Resistance (R, cmH₂O/L/s) Compliance (C, L/cmH₂O) Respiratory Rate (RR, bpm) I:E Ratio (E component)1 :

Normal Asthma ARDS Emphysema Pseudonormal

Select a pathology to see hint

Time Constant (τ): sec

FLOW–TIME SCALAR (VCV Mode)

τ: sec

3τ: sec

Exp Time: sec

0s1s2s3s4s5s

Quiz: Identify the Lung Condition

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Flow Waveform Profile	Lung Condition	Clinical Intervention
Prolonged decay: Flow fails to hit zero	Increased Resistance (Asthma/COPD)	Lower Respiratory Rate / Increase Expiratory Time
Rapid decay: Hits zero almost instantly	Decreased Compliance (ARDS/Fibrosis)	Increase PEEP / Recruitment