CLINICAL USES OF CATECHOLAMINES
The clinical uses of catecholamines are based on their actions on bronchial smooth muscle, blood vessels, and the heart. Epinephrine is also useful for the treatment of allergic reactions that are due to liberation of hista-mine in the body, because it produces certain physio-logical effects opposite to those produced by histamine. It is the primary treatment for anaphylactic shock and is useful in the therapy of urticaria, angioneurotic edema, and serum sickness.
Epinephrine also has been used to lower intraocular pressure in open-angle glaucoma. Its use promotes an increase in the outflow of aqueous humor. Because epi-nephrine administration will decrease the filtration an-gle formed by the cornea and the iris, its use is con-traindicated in angle-closure glaucoma; under these conditions the outflow of aqueous humor via the filtra-tion angle and into the venous system is hindered, and intraocular pressure may rise abruptly.
The vasoconstrictor actions of epinephrine and nor-epinephrine have been used to prolong the action of lo-cal anesthetics by reducing local blood flow in the re-gion of the injection. Epinephrine has been used as a topical hemostatic agent for the control of local hemor-rhage. Norepinephrine is infused intravenously to com-bat systemic hypotension during spinal anesthesia or other hypotensive conditions in which peripheral resist-ance is low, but it is not used to combat the hypotension due to most types of shock. In shock, marked sympa-thetic activity is already present, and perfusion of or-gans, such as the kidneys, may be jeopardized by norepi-nephrine administration.
Dopamine is used in the treatment of shock owing to inadequate cardiac output (cardiogenic shock), which may be due to myocardial infarction or conges-tive heart failure. It is also used in the treatment of septic shock, since renal circulation is frequently com-promised in this condition. An advantage of using dopamine in the treatment of shock is that its in-otropic action increases cardiac output while dilating renal blood vessels and thereby increasing renal blood flow.
Because they increase the force of the heartbeat, all three catecholamines may produce an excessively rapid heart rate. Palpitations produced by epinephrine and isoproterenol are accompanied by tachycardia, whereas those produced by norepinephrine usually are accom-panied by bradycardia owing to reflex slowing of the heart. Headache and tremor are also common. Epinephrine is especially likely to produce anxiety, fear, and nervousness.
The greatest hazards of accidental overdosage with epinephrine and norepinephrine are cardiac arrhyth-mias, excessive hypertension, and acute pulmonary edema. Large doses of isoproterenol can produce such excessive cardiac stimulation, combined with a decrease in diastolic blood pressure, that coronary insufficiency may result. It also may cause arrhythmias and ventricu-lar fibrillation. Tissue sloughing and necrosis due to se-vere local ischemia may follow extravasation of norepi-nephrine at its injection site.