Sympathomimetics

These are alpha and beta agonists. The therapeutic effect is from the local stimulation of alpha-1 receptors, which causes local vasoconstriction of skin and mucosa; blood vessels in skeletal muscles do not have these receptors.

Of great concern to dentists are the systemic effects of vasoconstrictors. Stimulation of beta-receptors causes vasodilation of skeletal muscle (beta-2) and stimulation of the heart (beta-1). A general rule is that alpha-1 causes constriction of muscles, and beta-2 causes muscles to relax. Also, beta-1 affects the heart, beta-2 and alpha-1 affect the blood vessels.

The results of vasoconstrictors’ systemic effects are as follows:

-Decreased peripheral resistance (beta-2 receptor)

-increased heartrate/ tachycardia (beta-1 receptor)

-increased heart contractility (positive inotropic effect) (not mediated by receptors)

-for epinephrine (at “low dose” rate), mean blood pressure is unchanged (systolic BP increases [beta-1 receptor], diastolic BP decreases [beta-2 receptor]).

-for the other vasoconstrictors, mean BP increases (because there are less beta-2 effects)

-Cardiac output (C.O.) increases

Beta-1 effects also increase the irritability of the myocardium, thus increasing the danger of arrhythmia and fibrillation.

In summary:

-The net beta effect is good for counteracting cardiovascular depression of LA’s, but we don’t want too much beta stimulation due to cardiac irritability.

-Pure alpha agonists (phenylephrine) have no direct cardiac effects, but raises peripheral resistance and blood pressure, causing a reflex bradycardia of long duration; therefore we see hypertension and bradycardia. Also, a pure alpha agonist can cause too much vasoconstriction and produce tissue necrosis; thus, pure alpha agonists are generally not desirable.

Category: Pharmacology Notes