In a constant IV infusion, how does the concentration change over time?

• A. Starts at zero, rises, then plateaus
• B. Starts high and declines
• C. Stays constant from time zero
• D. Fluctuates unpredictably

Answer: (A)

When a constant IV infusion starts, there is no drug in the plasma at the very beginning, so the concentration begins at zero and rises as drug is added. Because the infusion rate is constant and clearance is constant, the body eventually eliminates as fast as it is being delivered, reaching a steady-state concentration. At this steady state, the rate in equals the rate out, giving a plateau where the concentration no longer rises. The steady-state level is determined by the infusion rate divided by clearance (C_ss = infusion rate / clearance). The rise toward that plateau is typically exponential and takes several half-lives to approach. This pattern (rise from zero to a plateau) is what distinguishes a constant infusion from a bolus or fluctuating pattern.