Anesthesia Student Survival Guide.pdf - Index of

32 ● AnesthesiA student survivAl Guide
● Zero-order pharmacokinetics: A few drugs are eliminated via processes
that obey zero-order kinetics, in which the drug is metabolized at a fixed
rate, regardless of its concentration. (see Fig. 3.1)
● First-order pharmacokinetics: Most drugs are metabolized via processes
that obey first-order kinetics, meaning that the rate of drug metabolism
is proportional to the concentration of the drug (see Fig. 3.1). The rate of
elimination is usually described in terms of the drug’s half time, which is
the time in which metabolism and excretion reduce the plasma concentration
of the drug to 50% of its starting value. As further time progresses, the
process continues as detailed in Table 3.1. Note that after 5 half-times have
passed, 96.9% of the drug has been eliminated, and for practical purposes,
the drug has been fully eliminated.
● Clearance: The clearance of a drug is defined as the theoretical volume
of blood that is completely cleared of drug per unit time. It is analogous to
the creatinine clearance rate of the kidneys. Different pathways of clearance
for a drug (i.e. renal and hepatic) are additive, and a decrease in a major
pathway of clearance will prolong the effect of drugs that use that pathway
for elimination (e.g., administration of a drug that is mainly cleared by the
kidneys to a patient with impaired renal function will result in a relatively
long duration of action).
Concentration
Zero Order Kinetics
First Order Kinetics
Time
Figure 3.1 Zero vs. first order kinetics (Image Courtesy J. Ehrenfeld).