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Time Constant 63.2% — FRCA Primary MCQ

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ModeratePharmacology - General PrinciplesTime Constant 63.2%FRCA Primary

Regarding exponential processes in pharmacokinetics the time constant (τ) is defined as:

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Correct answer: B — The time for the process to reach 63.2% completion (or for the variable to fall to 36.8% of its initial value)

The time constant (τ) = 1/rate constant (k). After one τ the process is 63.2% complete (or the variable has fallen to 36.8% of its initial value). After 3τ: 95% complete. After 5τ: 99.3% complete. The relationship between τ and half-life: t½ = 0.693 × τ (or τ = 1.44 × t½).

Reference: Peck Hill Williams 2014 Pharmacology for Anaesthesia and Intensive Care