# INTRAVENOUS INFUSION infusion of drugs. LOADING DOSE PLUS IV INFUSION: ONE-COMPARTMENT MODEL The...

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### Transcript of INTRAVENOUS INFUSION infusion of drugs. LOADING DOSE PLUS IV INFUSION: ONE-COMPARTMENT MODEL The...

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Mosul University College of pharmacy

Lec 7 Biopharmaceutics

INTRAVENOUS INFUSION

The main advantage for giving a drug by IV infusion is:

1. The IV infusion allows precise control of plasma drug concentrations to fit

the individual needs of the patient.

2. Drugs with a narrow therapeutic window (eg, heparin), IV infusion

maintains an effective constant plasma drug concentration by eliminating

wide fluctuations between the peak (maximum) and trough (minimum)

plasma drug concentration.

3. The IV infusion of drugs, such as antibiotics, may be given with IV fluids

that include electrolytes and nutrients.

4. The duration of drug therapy may be maintained or terminated as needed

using IV infusion.

The plasma drug concentration-versus-time curve of a drug given by constant IV

infusion is shown in the figure 1 .

Because no drug was present in the body at zero time, drug level rises from zero

drug concentration and gradually becomes constant when a plateau or steady-state

drug concentration is reached. At steady state, the rate of drug leaving the body is

equal to the rate of drug (infusion rate) entering the body. Therefore, at steady

state, the rate of change in the plasma drug concentration:

= 0

Rate of drug input (infusion rate) = Rate of drug output (elimination rate)

2

A pharmacokinetic equation for infusion may be derived depending on whether the

drug follows one- or two-compartment kinetics.

ONE-COMPARTMENT MODEL DRUGS

The pharmacokinetics of a drug given by constant IV infusion follows a zero-order

input process.

The change in the amount of drug in the body at any time (

) during the

infusion is the rate of input minus the rate of output.

B ……1

Where DB is the amount of drug in the body, R is the

infusion rate (zero order), and k is the elimination rate

constant (first order).

Integration of Equation 1 and substitution of

DB = CpVD gives:

……2

As the drug is infused, the value for time (t) increases

in Equation 2. At infinite time, t = , approaches

zero, and Equation 2 reduces to Equation 4

……3

………4

……..5

Steady-State Drug Concentration ( ) and Time Needed to Reach

There is no net change in the amount of drug in the body, D B, as a function of

time during steady state i.e, the rate of drug leaving the body is equal to the rate of

drug entering the body (infusion rate) at steady state.

Figure 2: Plasma drug concentrations versus

time profiles after IV infusion. IV infusion is

stopped at steady state (A) or prior to steady

state (B). In both cases, plasma drug

concentrations decline exponentially (first

order) according to a similar slope.

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Whenever the infusion stops either at steady state or before steady state is reached,

the log drug concentration declines according to first-order kinetics with the slope

of the elimination curve equal to .

The time required to reach the steady-state drug concentration in the plasma is

dependent on the elimination rate constant of the drug for a constant volume of

distribution, as shown in Equation 4.

For a zero-order elimination process, if the rate of input is greater than the rate of

elimination, plasma drug concentration will keep increasing and no steady state

will be reached. This is a potentially dangerous situation that will occur when

saturation of metabolic process occurs.

Drug solution is infused at a constant or zero-order rate, . During the IV infusion,

the drug concentration increases in the plasma and the rate of drug elimination

increases because rate of elimination is concentration dependent (ie, rate of drug

elimination ). C p keeps increasing until steady state is reached, at which

time the rate of drug input (IV infusion rate) equals the rate of drug output

(elimination rate).

The time for a drug whose is

6 hours to reach at least 95% of

the steady state plasma drug

concentration will be , or

5 x 6 hours = 30 hours.

If the drug is given at a more rapid infusion rate, a higher steady-state drug level

will be obtained, but the time to reach steady state is the same

At steady state, the rate of infusion

equals the rate of elimination.

Therefore, the rate of change in

the plasma drug concentration is

equal to zero.

Table 1: number of 𝑡 s to reach a fraction of 𝐶𝑠𝑠

Figure 3: Plasma level-time curve for IV infusions given at

rates of R and 2R, respectively.

4

……….6

Equation 6 shows that the steady-state concentration is dependent on the

volume of distribution, the elimination rate constant, and the infusion rate. Altering

any one of these factors can affect steady-state concentration.

Examples 1: An antibiotic has a volume of distribution of 10 L and a of 0.2 hr-1.

A steady-state plasma concentration of 10 µg/mL is desired. The infusion rate

needed to maintain this concentration can be determined as follows. Equation 6 can

be rewritten as

g/ml (10)(1000ml)(0.2 hr-1) = 20 mg/h

Assume the patient has a uremic condition and the elimination rate constant has

decreased to 0.1 hr -1

. To maintain the steady-state concentration of 10 µg /mL, we

must determine a new rate of infusion as follows.

R = (10 mg/ml)(10)(1000ml)(0.1 hr -1

) = 10 mg/h

When the elimination rate constant decreases, the infusion rate must decrease

proportionately to maintain the same . However, because the elimination rate

constant is smaller (ie, the elimination t 1/2 is longer), the time to reach will be

longer.

Example 2: An infinitely long period of time is needed to reach steady-state drug

levels. However, in practice it is quite acceptable to reach 99% (ie, 99%

steady-state level). Using Equation 6, we know that the steady state level is

and 99% steady-state level is

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99%

Substituting into Equation 2 for , we can find the time needed to reach steady

state by solving for .

Take the natural logarithm on both sides:

Notice that in the equation directly above, the time needed to reach steady state is

not dependent on the rate of infusion, but only on the elimination half-life. Using

similar calculations, the time needed to reach any percentage of the steady-state

drug concentration may be obtained. Table 1.

Intravenous infusion may be used to determine total body clearance if the infusion

rate and steady-state level are known, as with Equation 6 repeated here:

6

because total body clearance, Cl T, is equal to ,

………….7

Example 3: A patient was given an antibiotic (t 1/2 = 6 hr) by constant IV infusion

at a rate of 2 . At the end of 2 days, the serum drug concentration was 10

mg/L. Calculate the total body clearance for this antibiotic.

The total body clearance may be estimated from Equation 7. The serum sample

was taken after 2 days or 48 hours of infusion, which time represents 8 x t 1/2,

therefore, this serum drug concentration approximates the .

INFUSION METHOD FOR CALCULATING PATIENT ELIMINATION

HALFLIFE

Equation 2 is arranged to solve for :

……….2 Since

Substituting into Equation 2;

Rearranging and taking the log on both sides,

7

………8

Where is the plasma drug concentration taken at time is the

approximate steady-state plasma drug concentration in the pa

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