Hysteresis Effect Site — FRCA Primary MCQ
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Correct answer: A — It describes a time lag between plasma concentration and effect site concentration
Hysteresis describes the clockwise or anticlockwise loop seen when plotting plasma concentration against effect over time. It reflects the time lag for drug to equilibrate between plasma and the effect site (biophase). The ke0 (effect site equilibration rate constant) quantifies this lag. Drugs with a small ke0 have slow equilibration and more pronounced hysteresis.
Reference: Peck Hill Williams 2014 Pharmacology for Anaesthesia and Intensive Care