L43 - IV Infusion 1

• Describe what happens in IV infusion

  - Drug added to large volume of parenteral fluid (up to 1L) and administered into vein.

  
  

  - Injected directly into blood stream so no absorption step necessary.

  
• List some advantages of IV infusion

  - Easily control drug plasma levels by changing infusion rate.

  
  

  - Can achieve constant drug plasma levels.

  
  

  - Less irritation/toxicity issues.

  
• Give some disadvantages of IV Infusion.

  - Needs constant monitoring and HCPs.

  
  

  - Can't administer high volumes to fluid restricted pts.

  
  

  - Solubility and stability of some drugs is an issue: May precipitate in bag.

  
• What does the One Compartment Method assume?

  - Drug rapidly distributes into homogeneous fluid-filled volume in body.

  
  

  - Elimination is 1st order process.

  
  

  - Linear kinetics: enzymes/transporters are not saturated.

  
• What is the infusion rate, R?

  R = constant amount of drug entering the body per unit time

  
• The balance between the input rate and the output rate determines how much drug is in the body.

  
  

  What eqn describes Cp?

  Cp = R / k x V (1 - e^-kt)

  
  

  = R / Cl ( 1 - e^-kt).

  
• Where else can we apply the IV infusion model?

  Where drug input corresponds to a 0 order process.

  Eg transdermal patcheswhere drug is absorbed with a constant rate.

  
• How can we predict drug conc at any time in an IV infusion?


  C = R / k x V (1 - e^-kt)

  
• How can we predict [drug] conc at time 0 as infusion is started?


  C0 = R / k x V (1 - e^-kt0) = 0

  
• How can we work out concentration at steady state?

  
  

  When can this eqn be used?

  Css = R / k x V.

  Css = R / Cl.

  - This eqn is only valid at the steady state of theplateau.

  - At long times, conc becomes constant and equal to R/Cl.

  
• Describe the accumulation phase.

  
  

  What's the input rate?

  
  

  What's the elimination rate? How do they compare at first?

  Input rate: R = constant.

  Elimination rate = A x k = Cl x C.

  - Because A is small in beginning, k is also small, so in accumulation phase elimination rate < input rate.

  - Drug accumulates in body and [plasma] increases according to eqnC = R/ kxV (1-e^-kt) = Css(1-e^-kt)

  
• Describe the plateau/steady state of IV infusion.

  As infusion proceeds, R remains constant.

  
  

  Elimination rate = A x k = Cl x C.

  Increases with time as A increases (1st order!).

  
  

  When elimination rate = input rate, Css has been achieved.
• How can we estimate elimination rate with time?

  Elimination rate = C x Cl OR = A x k = C x V x k.

  
  

  When steady state is reached, elimination rate = input rate of drug.

  
• How can we estimate infusion rate (R) required to achieve Css for a drug of known clearance?

  R = Css x Cl

  
• Describe the plasma levels when IV infusion has been stopped.

  Input rate = 0.

  Elimination rate = A x k = Cl x C.

  - Only elimination occurs, so [drug] declines exponentially with time.

  - Equivalent to IV Bolus administration, soC = C0 x e^-kt.

  lnC = lnC0 - kt.