The direct administration of drugs into veins is the only route where bioavailability considerations are not relevant. This route provides an almost instantaneous response with controllability of the rate of drug input into the body. This route is especially suitable for those drugs which cannot be absorbed adequately from the gastrointestinal tract or tissues depots (e.g., intramuscular administration) or where there is a significant first pass effect upon oral administration. The drugs which would be intolerably painful in the subcutaneous or muscle tissues by virtue of their irritant properties may be injected slowly into a vein without much difficulty, e.g., nitrogen mustard in cancer chemotherapy.
There are however, several disadvantages with the use of the intravenous route. A drug administered intravenously cannot be recalled, whereas some such measures can be taken with other routes. Rapid intravenous injection may evoke catastropic effects in the circulatory and the respiratory systems due to the transient wave of concentrated solute suddenly reaching the myocardium and the chemoreceptors in the aortic arch and carotid sinus. Intravenous injections should, therefore, be administered slowly, preferably over a period of one minute or more, during which time the blood completes its circulation. The possibility of anaphy-lactoid reactions is much greater than with any other route of administration.
The tonicity of solution is also important since hypotonic or hypertonic solution can cause hemolysis or agglutination of erythrocytes. The damage of the vascular wall also leads to local reactions, especially after prolonged infusions.
The possibility of microbiologic contamination and pyrexia due to pyrogens is a serious concern in the use of intravenous administration.
The intravenous route is especially suitable when a rapid response is required, as in the treatment of epileptic seizures, acute asthmatic attacks, cardiac arrhythmias, etc. The fluctuation of plasma concentration is generally very small if a drug is administered by slow intravenous infusion, as is employed for lidocaine, theophylline, and many antibiotics. A caution is needed for drugs with poor water solubility which can precipitate resulting in thrombosis, an removal of drug from circulation and deposition of the precipitate in various tissues resulting in reduced apparent bioavailability. In addition, drugs which bind to plasma proteins extensively may show altered response depending on rate of injection since the initial binding and concentration at site of action can vary significantly.
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If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.