lecture 42: IV Infusion part 2

Review and Consolidation

  • Importance of reviewing part one of IV infusion lectures and completing exercises to solidify understanding.
  • Lectures designed to be interactive: listening is combined with practical exercises.

Plasma Profiles During IV Infusion

  • Initial Phase: Starts with zero concentration, which increases over time until reaching a plateau (steady state).
  • Key Equations:
    • Accumulation phase: C=Rclearance(1−e−kt)C = \frac{R}{\text{clearance}}\left(1 - e^{-kt}\right)
    • Steady-state concentration on plateau: Css=RclearanceC_{ss} = \frac{R}{\text{clearance}}
  • After infusion ceases, elimination follows the same principles as IV bolus administration.

Time to Steady State

  • Steady state is crucial for patient or animal stabilization with constant drug concentration.
  • Ability to predict time to reach steady state is valuable for clinical practice.
  • General Equation for Steady State Predictions:
    • C=Css(1−e−kt)C = C_{ss}\left(1 - e^{-kt}\right)
  • Fraction of Equilibrium (F):
    • F=C<em>tC</em>ssF = \frac{C<em>{t}}{C</em>{ss}}
    • Example:
    • If F=0.5F = 0.5, then concentration at time tt is 50% of steady state.
Rearranging the Equation
  • To find time to reach steady state:
    • ln⁡(1−F)=−kt\ln(1 - F) = -kt
    • Time only depends on drug's half-life or elimination rate constant (k), not on infusion rate.
Practical Timeframes for Steady State
  • Half-Life Analysis:
    • After 1 half-life, reach 50% of steady state.
    • 2 half-lives: 75%
    • 4 half-lives: 94%
    • Practically, 4 half-lives are used to predict when steady state is essentially reached (often considered sufficient).

Effect of Infusion Rate

  • Different infusion rates lead to different peak concentrations but do not affect time to steady state:
    • Concentration profile shown increases with higher infusion rates; steady state concentration correlates directly with rate.
    • Example: If infusion rate is doubled, steady state concentration also doubles.
  • Considerations: Time to reach steady state always remains the same across varying infusion rates, solely dictated by clearance and volume of distribution.

Comparison of IV Bolus vs. IV Infusion

  • IV Bolus:
    • Rapid achievement of high initial concentration, followed by decline due to elimination.
  • IV Infusion:
    • Slow buildup of concentration toward steady state, allowing for maintained constant drug levels post-reach.
  • Combination Approach: Administer a bolus dose to achieve needed concentration quickly followed by IV infusion to maintain drug levels during therapy.

Predicting Concentration with Both Administered Methods

  • Total concentration in the body considers contributions from both IV bolus and ongoing infusion:
    • Model total concentration as:
      C=C<em>0e−kt+C</em>ss(1−e−kt)C = C<em>0 e^{-kt} + C</em>{ss}\left(1 - e^{-kt}\right)

Changing Infusion Rates

  • Adjustment of infusion rates during therapy can be necessary (toxicity management, therapy optimization, etc.).
  • When changing rate, time to reach the new steady state again relies on half-life, taking approximately four half-lives.
  • Modeling Post-Rate Adjustment:
    • Conceptualize initial infusion stopped and new infusion started to predict overall body drug concentration at any given time.
    • Use prior rate's model to determine remaining concentration and adjust for new infusion rate to establish a total concentration expected in the body.