Pharmacology IV Infusion Quiz

IV Infusion

• IV infusion involves administering drugs intravenously at a constant rate over a specific time interval.
• Intravenous refers to "into the vein".
• Infusion is a slow injection of a substance into a vein.

Learning Objectives

• Students will be able to define IV infusion.
• Students will be able to understand the history of IV infusion.
• Students will be able to explain the purpose for intravenous therapy.
• Students will be able to explain the different types of intravenous fluids (IV bolus and infusion).
• Students will be able to calculate the flow rate for an infusion.
• Students will be able to identify the advantages of IV infusion.

History of IV Infusion

• IV technology originated from cholera treatment studies in 1831.
• IV techniques further developed in the 1930s but became widely available in the 1950s.

Purpose of IV Infusion

• Maintain stable plasma concentration.
• Avoid periods of low drug concentration.
• Dosage adjustments
• Maintain or replace body stores.
• Restore acid-base balance.
• Administer medication.
• Provide nutrition.

IV Solution Administration Types

• Bolus dose.
• Infused slowly into the plasma at a constant or zero-order rate.

Intravenous Bolus Administration

• IV bolus drugs enter the bloodstream directly.
• Distribution of the drug occurs throughout the body tissues.
• Drug concentration in tissues depends on blood flow, molecular weight, lipophilicity, plasma protein binding, and binding affinity with the tissue.
• Drugs are mostly eliminated by the kidneys and/or liver after metabolism.

Pharmacokinetic Parameters

• Apparent volume of distribution (VD) is the first pharmacokinetic parameter.
• VD represents the volume where the drug is distributed within the body.
• Elimination rate constant (k) is the second pharmacokinetic parameter.
• k governs the rate at which drug concentration declines over time.

IV Infusion Calculation

• IV(dose)—-> DB, Vd----->K---------> Elimination

IV Infusion Considerations

• High precision is achieved by infusing drugs intravenously as a drip.
• The body is considered a single, kinetically homogeneous unit.
• This route is applicable only for drugs that rapidly distribute throughout the body.
• Drugs move dynamically within this compartment

Zero-order Input Process

• Drugs administered by constant IV infusion undergo a zero-order input process.
• During this process, the drug enters the bloodstream at a constant rate.
• Drug elimination is typically a first-order process for most drugs.
• The rate of input minus the rate of output represents the change in drug amount within the body(dDB/dt).
• A steady-state level is reached, where the rate of drug input equals the rate of drug output.

Steady-State Equation

• dDB/dt = R - k DB
• R is the rate of drug input (infusion rate) and k is the elimination rate constant.
• Cp = R/Vd K

Steady State Considerations

• At steady-state the rate of drug input (R) equals the rate of drug output (k DB).
• R = k. DB and R = k .Cp .Vd
• Cp=R/Vd.K
• At steady-state, Cp = Css
• Css = R/k.VD
• Css = R/Cl
• Once infusion stops, or when a steady-state has been reached, the drug concentration declines according to first order kinetics.
• This decline will have a slope on the elimination curve equal to k/2.3.

Example Calculation

• A desired steady-state theophylline plasma concentration of 15 mg/L is needed.
• The average half-life of theophylline is 4 hours and the apparent volume of distribution is 25 liters.
• To calculate the rate of infusion needed to reach this concentration, the equation R = k . Css. VD must be used
• R = 0.17/hr x 15 x 25
• R = 63. 75 mg/hr

Steady-State Drug Concentration

• During drug administration, the plasma drug concentration increases and the elimination rate also increases.
• The rate of elimination is concentration-dependent.
• Cp keeps increasing until a steady-state condition is reached where drug input (IV infusion rate) equals drug output (elimination rate)

Steady-state Drug Concentration Continued

• At steady-state, the resulting plasma drug concentration is directly related to the rate of infusion and inversely related to the body clearance of the drug.
• The therapeutic activity is observed when the drug concentration is close to the desired plasma concentration - the required steady-state drug concentration.

Time to Reach Steady-State

• The time to reach steady-state can be determined from the time taken to reach half the steady state concentration.

• Css = R/VD.k = R/Clearance

• At (t½); time to reach half the steady-state, Cp = Css/2

• Cp = [R/Vd K] (1−e−kt half) = Css/2

• taking the natural log on both sides

• T1/2 = 0.693/k

Additional Example

• A patient with septicemia was administered IV infusion of an antibiotic that had an elimination half-life of 6 hours.
• The infusion rate was 2 mg/hr.
• After 48 hours, the serum drug concentration was 10 mg/L.
• The problem requires determining overall body clearance for the drug.

Calculating Patient Elimination Half-life

• The half-life can be calculated from a mathematical expression that describes the elimination rate constant.
• Cp =[ R/Vd K ] ( 1-e-kt)

Additional Example

• A patient is given an IV infusion of a specific antibiotic at 15 mg/hr.
• The concentration is measured at 8 and 24 hr with a reading of 5.5 and 6.5 mg/L respectively.
• The elimination half-life is calculated using the following equations.

Loading Dose plus IV Infusion

• To quickly achieve a therapeutic concentration, a loading dose of drug using rapid intravenous injection is followed by a slower maintenance dose
• At this condition, the total drug concentration is dependent on IV bolus (dose) and the infusion dose.
• The concentration using a IV bolus is described using:
• C1 = Loading Dose (DL) / Vd x e^-kt=Coe^-kt
• The concentration using IV infusion at infusion rate R is :
  • C2= [R/ Vd. K] (1-e^-kt)
• The total concentration is sum of bolus and infusion: Cp= C1+C2

Loading Dose and Steady State

• If the loading dose represents the amount of drug in the body at steady-state(DL) then DL=Css.Vd, since Css=R/k.Vd.
• Therefore, DL=R/K

Example Calculation

• A certain anesthetic drug is administered using IV infusion at a rate of 2 mg/hr, has an elimination rate constant of 0.1/hour and a volume of distribution(Vd) of 10L
• The physician wants a drug level of 2 ug/mL right away
• DL=Css.Vd= 2ug/mL x 10 L x 1000= 20mg.Alternatively DL=R/K=2/0.1=20mg

Advantages of IV Infusion

• Immediate therapeutic effect due to rapid drug/fluid delivery to target sites.
• Applicable when patients cannot tolerate oral drugs.
• Reduces pain and irritation from drugs administered intramuscular/subcutaneously.