Infusions

Mark Tomlin

Consultant Pharmacist: Critical Care

Southampton University Hospitals NHS Trust

Characteristics of IV therapy

• Half-Life ?

• What Dose ?

• Intermittent Infusion ?

• How Often ?

• With Loading Dose ?

• Continuous Infusion ?

• Effect of organ dysfunction ?

Concentration – time

profile of an IV bolus

Con

cent

ratio

n

Time

Concentration – time

profile of an IV bolus

Con

cent

ratio

n

Time

An exponential decay

Cpt = Cp

0 x e-kt

Half-life

Con

cent

ratio

n

Time

10

5t

t

Described by a constant decay factor or:

Half-life - t

• Defined as the time taken for the concentration, of a drug, in the blood, to halve.

• We usually dose at the half-life• If the half-life is less than 4 hours then the

medicine is usually administered by infusion.• After 5 half-lives, the drug is completely

eliminated so it describes the duration in the blood

• Prescribe mg/kg/hr or μg/kg/min to remind staff about response delays after dose adjustments

Characteristics of IV therapy

• Half-Life ?

• What Dose ?

• Intermittent Infusion ?

• How Often ?

• With Loading Dose ?

• Continuous Infusion ?

• Effect of organ dysfunction ?

Problems with an IV

bolus

Con

cent

ratio

n

Time

MTC – Minimum Toxic Concentration

24hours

Problems with an IV

bolus

Con

cent

ratio

n

Time

MEC – Minimum Effective Concentration

24hours

Characteristics of IV therapy

• Half-Life ?

• What Dose ?

• Intermittent Infusion ?

• How Often ?

• With Loading Dose ?

• Continuous Infusion ?

• Effect of organ dysfunction ?

An Intermittent

Infusion

Con

cent

ratio

n

Time

MEC – Minimum Effective Concentration

24hours

End Infusion

MTC – Minimum Toxic Concentration

Characteristics of IV therapy

• Half-Life ?

• What Dose ?

• Intermittent Infusion ?

• How Often ?

• With Loading Dose ?

• Continuous Infusion ?

• Effect of organ dysfunction ?

How often ? – Intermittent bolus

MEC

MTC

2412

How often ? – adjacent bolus

MEC

MTC

2412

How often ?- accumulating bolus

MTC

MEC

2412

Characteristics of IV therapy

• Half-Life ?

• What Dose ?

• Intermittent Infusion ?

• How Often ?

• With Loading Dose ?

• Continuous Infusion ?

• Effect of organ dysfunction ?

Accumulating bolus – with Loading Dose or not?

Concentration

Time

MTC

MEC

LD

How long?

Characteristics of IV therapy

• Half-Life ?

• What Dose ?

• Intermittent Infusion ?

• How Often ?

• With Loading Dose ?

• Continuous Infusion ?

• Effect of organ dysfunction ?

Short Half-life ?Concentration

Time

MTC

MEC

10 mins

Short Half-life = InfusionConcentration

Time

MTC

MEC

10 mins

Characteristics of IV therapy

• Half-Life ?

• What Dose ?

• Intermittent Infusion ?

• How Often ?

• With Loading Dose ?

• Continuous Infusion ?

• Effect of organ dysfunction ?

Reduced elimination

Con

cent

ratio

n

Time

MEC

MTC

Reduced elimination

Con

cent

ratio

n

Time

MEC

MTC

Extended interval dosing with poor renal perfusion

Concentration

Time