9.1.1 Beta-Adrenergic Antagonists. Beta blockers are widely used in the management of cardiovascular disorders, including hypertension, angina pectoris, and cardiac arrhythmias. These drugs decrease the heart rate and the cardiac output, decrease blood pressure, and can decrease IOP. Although thiazide-type diuretics are used as initial therapy for most patients with hypertension, beta blockers are commonly used for stage 2 hypertension or other compelling indications.1 Systemic hypertension and glaucoma often coexist in patients, and glaucoma patients frequently use systemic cardiovascular medications.2 Beta-adrenergic blocking drugs for long-term therapy of systemic hypertension are listed in table 9.1.
Adrenergic effects are triggered by activation of alpha and beta receptors. Stimulation of alpha-1 receptors causes contraction of smooth muscle, which accounts for vasoconstriction associated with adrenergic activity. Beta-adrenergic receptors have been classified as beta-1 and beta-2. Increased cardiac contractility has been associated with beta-1 receptor stimulation, whereas increased bronchodilation and vasodilation have been associated with beta-2 receptor stimulation. Beta receptors can be blocked by several drugs, which have different degrees of selectivity for beta-1 and beta-2 receptors. "Cardioselective" adrenergic antagonists are those drugs that preferentially block beta-1 receptors.
Beta blockers vary in their intrinsic sympathomimetic activity, which is the ability of the drug itself to stimulate beta receptors, even in the absence of catecholamine. Some beta blockers also have direct action on cell membranes, which is described as a membrane-stabilizing, local-anesthetic, and quinidine-like effect. The local-anesthetic potency of propranolol is about equal to that of lidocaine, whereas timolol and atenolol are almost devoid of this property.
In 1967, Phillips et al.3 reported that propranolol reduced IOP in seven patients with glaucoma following administration of 10 mg intravenously or 5 to 40 mg by mouth. Cote and Drance4 in 1968 described the ocular hypotensive effect of 20 to 50mg/day of orally administered propranolol in 26 patients with open-angle glaucoma. Topical administration of propranolol also reduced IOP;5,6 however, membrane-stabilizing activity caused significant corneal anesthesia,6 which prevented topical use of this drug. The subsequent search for beta blockers without adverse effects led to the development of topically administered timolol.7
After oral administration of propranolol to ocular hypertensive patients, reduction of IOP occurs within 1 hour, reaching a maximum at 3 hours and lasting at least 7 hours (figure 9.1).8 The reduction of IOP is greater in patients with higher initial measurements compared with lower initial measurements.4,8 Propranolol causes a reduction of IOP that is comparable to that of orally administered acet-azolamide.9 Although the response may tend to decrease with time, long-term reduction of IOP can be achieved with systemic propranolol.10
Systemic beta blockers other than propranolol decrease IOP. Oral administration of practolol reduces IOP,11 although topical use of the drug is limited by an im-munologically mediated oculomucocutaneous syndrome.12,13 Atenolol, a cardio-selective beta-1 blocker without membrane-stabilizing activity, lowers IOP after oral administration.11,14,15 Other beta blockers that have been found to lower IOP include pindolol (a beta blocker with intrinsic sympathomimetic activity),16 ox-prenolol (a nonselective beta blocker),17 and nadolol (a long-acting, nonselective beta blocker).18 The mechanism of IOP reduction after both systemic and topical administration of beta blockers is decreased aqueous production.
In patients already using systemic beta blockers, topical administration of beta blockers may be less effective compared with addition of these drops to patients not taking systemic medications. In patients treated with oral propranolol, the response
Table 9.1 Systemic Beta-Adrenergic Blockers
Nonselective Beta Blockers
Nadololb Corgard Propranololb
Inderal Propranolol long actingb
Inderal LA Timololb Blocadren
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Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...