This viva is relevant to Section B(v) of the 2017 CICM Primary Syllabus, which expects the exam candidate to "describe the concepts of effect-site and context sensitive half time".
"Chemical alteration of an agent (drug) that occurs by virtue of the sojourn of the agent in a biological system".
- The Pharmacology and Experimental Therapeutics Department Glossary at Boston University School of Medicine
Characteristics of Phase I reactions:
Examples of Phase I reactions:
Characteristics of Phase II reactions:
Examples of Phase II reactions:
"First-order kinetics... is where a constant fraction of drug in the body is eliminated per unit of time"- college answer to Question 5(p.2) from the second paper of 2009.
When doubling the concentration of reagents also doubles the reaction rate, the increase in rate is by a factor of 2 (2 to the first power, or 21). That "first power" gives rise to the term "first order". In that fashion, one can have a "second order" reaction where doubling the concentration of reagents quadruples the reaction rate (i.e 2 to the second power, 22).
In chemistry, when doubling the concentration of reagents has no effect on the reaction rate, the increase in rate is by a factor of 0 (i.e. 20). This is zero-order kinetics. The relationship of concentration to reaction rate can therefore be plotted as a straight line.
Michaelis-Menten kinetics describes enzymatic reactions where a maximum rate of reaction is reached when drug concentration achieves 100% enzyme saturation.
Thus, when the patient is receiving regular doses of the drug, if the concentration is already high then relatively small changes in the dose will produce a disproportionately large change in drug concentration.
Non-linear elimination kinetics is the term which describes drug clearance by Michaelis-Menten processes, where a drug at low concentration is cleared by first-order kinetics and at high concentrations by zero order kinetics (eg. phenytoin or ethanol).