The most prominent side effects associated with the administration of the p-blockers are those directly attributable to their ability to block p-receptors. Although p-blockers prevent an increase in heart rate and cardiac output resulting from an activation of the autonomic nervous system, these effects may not be troublesome in patients with adequate or marginal cardiac reserve. However, they can be life threatening for a patient with congestive heart failure. Also, because conduction of impulses in the heart may be slowed by p-blockers, patients with conduction disturbances, particularly through the atrioventricular node, should not be treated with p-blockers.
Caution must be exercised in the use of p-blockers in obstructive airway disease, since these drugs promote further bronchoconstriction. Cardioselective p-blockers have less propensity to aggravate bronchoconstriction than do nonselective p-blockers.
p-Blockers potentiate hypoglycemia by antagonizing the catecholamine-induced mobilization of glycogen. The use of p-blockers in hypoglycemic patients is therefore dangerous and must be undertaken with caution. If p-blocker therapy is required, a cardioselective p-blocker is preferred.
Whenever p-blocker therapy is employed, the period of greatest danger for asthmatics or insulin-dependent diabetics is during the initial period of drug administration, since the greatest disruption of the au-tonomic balance will occur at this time. If marked toxi-city does not occur during this period, further doses are less likely to cause problems.
Although the p-blockers produce a number of central effects, it is not clear whether these effects are due to blockade of central p-receptors. After high doses, patients may have hallucinations, nightmares, insomnia, and depression.
Topical application of timolol to the eye is well tolerated, and the incidence of side effects, which consist of burning or dryness of the eyes, is reported to be 5 to 10%.
In spite of the potential seriousness of some of their side effects, p-blockers as a class are well tolerated and patient compliance is good.
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