Intravenous medicines

Last updated: Tuesday, July 12, 2022

The intravenous (IV)  route provides a rapid way of administering drugs, fluids, blood products and parenteral nutrition. There are two basic methods:

Administration of a direct intravenous injection
1.  Direct intravenous injection is the administration of a small volume of drug solution into an entry port sited in a vein. Injections can also be made via existing IV infusion administration lines (or ‘giving sets’). The technique of direct injection is sometimes called an ‘IV push’. The term ‘IV bolus’ is widely used as well but is potentially misleading because it implies that the injection can be given instantly over a few seconds whereas most injections should normally be administered over 2 to 5 minutes.

Direct injection gives therapeutic levels of a drug quickly, but there can be problems with this technique. For example, injections may damage a vein, particularly if the drug is irritant (e.g. phenytoin). In addition, it is impractical if a drug needs to be given slowly or in a large volume.

2.  Intravenous infusion involves IV administration over a longer time period often using larger volumes. An administration set (‘IV line’ or ‘giving set’) connects the bag or syringe of drug solution to an entry point in a patient’s vein as in the image. The infusion may be intermittent (e.g. metronidazole 500mg in 100ml saline over 20 minutes every eight hours) or continuous over 24 hours. Continuous infusion is indicated for drugs with a short half-life (e.g. glyceryl trinitrate) and/or when a constant therapeutic drug level is required (e.g. insulin). Infusions require a drug to be stable in the specified diluent for the duration of administration. They also require the drug to be compatible with the plastic used to make the IV line and infusion bag. 

An administration set connects a bag of infusion fluid to a patient
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‘Y-site’ is the point where two IV lines containing different solutions join to run down the same line into a patient forming a ‘Y’ shape. One infusion may be described as being ‘piggybacked’ onto the other.

The choice of intravenous administration method depends on the drug (e.g. indication, volume of infusion fluid, rate of administration, degree of irritancy, pharmacokinetics) and patient factors (e.g. age, availability of IV access, concurrent disease such as presence of heart failure). 

Intravenous injections and infusions may be given via peripheral or central veins

Peripheral lines are usually inserted in the hand or arm. They can become blocked or dislodged easily and, because the veins are quite narrow, they are not usually used for irritant drugs (e.g. amiodarone), concentrated solutions in fluid-restricted patients (e.g. potassium) and vasoconstricting drugs (e.g. adrenaline). Central lines are preferred for these situations and are inserted into larger veins such as the superior vena cava. Central lines can be used to administer any intravenous medicine.

Some drugs may cause pain on injection because of their tonicity, pH, or irritancy (e.g. erythromycin). The vein may become red and inflamed; this is called phlebitis. If a medicine liable to cause phlebitis must be given peripherally, then the peripheral line should go into as large a vein as possible.
Common clinical problems
  • Calculating the rate of administration of a drug (e.g. phenytoin).
  • Advising on the volume and type of infusion fluid suitable for diluting a drug (e.g. vancomycin).
  • Determining how best to administer an IV drug in fluid restricted patients (e.g. potassium).
  • Choosing between a peripheral or central intravenous line (e.g. dopamine, amiodarone).
  • Advising on the safety of mixing drugs in the same syringe, IV bag or line (e.g. cefuroxime and metronidazole)