Anesthesia Student Survival Guide.pdf - Index of

Table 3.1 Drug remaining at multiples of the half-time.
Context Sensitive Half-Time
PhArmAcoloGy PrinciPles ● 33
Number of half-times Percent of drug remaining Percent of drug removed
0 100 0
1 50 50
2 25 75
3 12.5 87.5
4 6.2 93.8
5 3.1 96.9
Duration of Infusion
Fentanyl
Thiopental
Midazolam
Propofol
Figure 3.2 Context sensitive half times as a function of duration of infusion (Image Courtesy
J. Ehrenfeld).
● Context-sensitive half-time: As discussed above, some drugs are eliminated
from the plasma by redistribution to adipose tissue. As the adipose tissue
acquires more drug, the diffusion gradient from plasma to tissue decreases,
and the rate of redistribution decreases. This leads to the phenomenon of
context-sensitive half-times, in which the time to 50% reduction in drug concentration
increases with increasing total doses of the drug or duration of
infusion. Drugs that are highly redistributed but metabolized relatively slowly,
such as thiopental, are affected more than drugs with rapid metabolism, such
as sufentanil. The context sensitive half-times as a function of duration of
drug infusion are shown in Fig. 3.2.