L8 - Constant rate infusions

Constant Rate IV Infusions

  • Overview: Focuses on various methods of drug administration, emphasizing constant rate IV infusions and leading to dosage regimen design.

  • Learning Outcomes:

    • Define constant IV infusion and compare it to other drug administration methods.

    • Understand effects of IV infusion on drug distribution in the body.

    • Evaluate the impact of drug clearance rate on maintenance dosing.

    • Analyze plasma concentration-time profiles for drugs via IV infusion, including calculating pharmacokinetic (PK) parameters.

    • Discuss clinical relevance and applications of IV infusions in practice.

    • Calculate infusion rates and loading doses.

Assumptions for IV Infusions

  • Constancy of infusion rate and zero-order input.

  • Short or long-term infusion relative to drug half-life.

  • Focus on long-term infusions.

Key Definitions

Therapeutic Index

  • Ratio indicating drug safety, defined as the toxic dose in 50% of the population over the effective dose in 50%.

Therapeutic Range

  • Range of drug concentrations that achieves desired therapeutic effects with minimal risk of toxicity.

Drug Administration Profiles

Single Dose Administration

  • Example: Paracetamol, exhibiting a characteristic plasma concentration-time profile with peaks and elimination phases.

  • T max: Time to maximum plasma concentration.

Multiple Dose Administration

  • Maintains steady state plasma concentration by administering a drug at regular intervals.

  • Steady state: Rate of input equals rate of elimination.

    • Optimal for achieving therapeutic efficacy while avoiding toxicity.

Therapeutic Range and Individual Variability

  • Therapeutic ranges are population averages; individual responses may vary due to genetic, metabolic, or elimination factors.

  • Therapeutic Drug Monitoring: Adjusting doses based on individual patient variability.

Continuous Drug Dosing Methods

  • Constant Rate Infusions and Repeat Oral Dosing.

Constant Rate Infusion

  • Administers drug directly into blood to bypass absorption barriers, ensuring rapid delivery and maintaining steady state.

  • Used in emergencies, anesthesia, chemotherapy, and when oral administration is not feasible.

Benefits and Limitations of IV Infusions

  • Benefits:

    • Precise drug control and avoidance of peak-trough fluctuations.

    • Immediate cessation in case of adverse effects.

  • Limitations:

    • Requires clinical supervision and is restricted to institutional settings.

Concentration-Time Relationships

Equations for Plasma Concentration

  • Plasma concentration at time t:

    • ( C(t) = \frac{R_{0}}{V_{d}} e^{-k_{el}t} ) (pre-infusion equation)

    • At steady state: ( C_{ss} = \frac{R_{0}}{CL} ) where CL is clearance.

  • Time to reach steady state typically takes 5-7 half-lives.

Loading Doses

  • Purpose: Quickly reach steady state in critical situations.

    • Loading dose equation: ( LD = C_{ss} \times V_{d} )

  • Example calculation provided for loading doses and steady state concentrations.

Post-Infusion Kinetics

  • Post-infusion plasma concentration falls according to exponential decay, governed by elimination half-life.

  • Equations used include pre-infusion and post-infusion relevant terms to calculate concentration at specified times.

Multi-Compartment Models

  • Steady state reached after distribution equilibrium, often following different half-lives compared to single compartment models.

Summary of Key Concepts

  • At steady state, input = output.

  • Dose rate influences steady state concentration, but not the time to reach it (determined by half-life).

  • Different equations apply pre-infusion, during infusion, and post-infusion phases.

Example: Procanamide Infusion

  • Detailed case study example demonstrating the practical application of IV infusion concepts, including calculations for steady state, concentration, and volume of distribution.