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Specific Anticholinergic Drugs
Atropine is a tertiary amine. The naturally occurring levorotatory form is active, but the commercial mix-ture is racemic (Figure 13–1).
As a premedication, atropine is administered intra-venously or intramuscularly in a range of 0.01–0.02 mg/kg, up to the usual adult dose of 0.4–0.6 mg. Larger intravenous doses up to 2 mg may be required to completely block the cardiac vagal nerves in treat-ing severe bradycardia. Atropine sulfate is available in a multitude of concentrations.
Atropine has particularly potent eff ects on the heart and bronchial smooth muscle and is themost efficacious anticholinergic for treating brady-arrhythmias. Patients with coronary artery disease may not tolerate the increased myocardial oxygen demand and decreased oxygen supply associated with the tachycardia caused by atropine. A derivative of atropine, ipratropium bromide, is available in a metered-dose inhaler for the treatment of broncho-spasm. Its quaternary ammonium structure significantly limits systemic absorption. Ipratropium solution (0.5 mg in 2.5 mL) seems to be particularly effective in the treatment of acute chronic obstructive pulmonary disease when combined with a β-agonist drug (eg, albuterol). The central nervoussystem effects of atropine are minimal after the usual doses, even though this tertiary amine can rapidly cross the blood–brain barrier. Atropine has been associated with mild postoperative memory deficits, and toxic doses are usually associated with excit-atory reactions. An intramuscular dose of 0.01– 0.02 mg/kg reliably provides an antisialagogue effect. Atropine should be used cautiously in patients with narrow-angle glaucoma, prostatic hypertrophy, or bladder-neck obstruction.
Scopolamine, a tertiary amine, differs from atropine by the addition of an epoxide to the heterocyclic ring.
The premedication dose of scopolamine is the same as that of atropine, and it is usually given intramus-cularly. Scopolamine hydrobromide is available as solutions containing 0.3, 0.4, and 1 mg/mL.
Scopolamine is a more potent antisialagogue than atropine and causes greater central nervous system effects. Clinical dosages usually result in drowsiness and amnesia, although restlessness, diz-ziness, and delirium are possible. The sedative effects may be desirable for premedication but can interfere with awakening following short procedures. Scopol-amine has the added virtue of preventing motion sickness. The lipid solubility allows transdermal absorption, and transdermal scopolamine has been used to prevent postoperative nausea and vomiting. Because of its pronounced ocular effects, scopol-amine is best avoided in patients with closed-angle glaucoma.
Glycopyrrolate is a synthetic product that differs from atropine in being a quaternary amine and having both cyclopentane and a pyridine moieties in the compound
The usual dose of glycopyrrolate is one-half that of atropine. For instance, the premedication dose is 0.005–0.01 mg/kg up to 0.2–0.3 mg in adults. Gly-copyrrolate for injection is packaged as a solution of 0.2 mg/mL.
Because of its quaternary structure, glycopyr-rolate cannot cross the blood–brain barrierand is almost devoid of central nervous system and ophthalmic activity. Potent inhibition of salivary gland and respiratory tract secretions is the primary rationale for using glycopyrrolate as a premedication. Heart rate usually increases after intravenous—but not intramuscular—administration. Glycopyrrolate has a longer duration of action than atropine (2–4 h vs 30 min after intravenous administration)
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