During prolonged fentanyl infusion, what happens when inactive sites saturate?

During prolonged fentanyl infusion, what happens when inactive sites saturate?

• A. Enhanced clearance reduces duration
• B. Metabolites activate drug prolonging DOA
• C. Renal excretion increases DOA
• D. Inactive sites saturate leading to extended DOA

Answer: (D)

When a lipophilic drug like fentanyl is given by prolonged infusion, it moves into tissue storage compartments that act as inactive binding sites, effectively serving as a reservoir that buffers plasma levels and delays the onset of effect. As the infusion continues, these sites fill up. Once they are saturated, additional fentanyl cannot be sequestered in those reservoirs, so more drug remains in circulation and at receptor sites, which can prolong the drug’s effects. After the infusion stops, the stored drug is slowly released back into the bloodstream, extending the duration of action. This mechanism explains the extended DOA with sustained infusions. It’s not about faster clearance, active metabolites, or increased renal excretion.

When a lipophilic drug like fentanyl is given by prolonged infusion, it moves into tissue storage compartments that act as inactive binding sites, effectively serving as a reservoir that buffers plasma levels and delays the onset of effect. As the infusion continues, these sites fill up. Once they are saturated, additional fentanyl cannot be sequestered in those reservoirs, so more drug remains in circulation and at receptor sites, which can prolong the drug’s effects. After the infusion stops, the stored drug is slowly released back into the bloodstream, extending the duration of action. This mechanism explains the extended DOA with sustained infusions. It’s not about faster clearance, active metabolites, or increased renal excretion.