Cardiac: Adrenergic Antagonists (Sympathetic Antagonists) (Beta Blockers)

Question 1

Metoprolol (Lopressor)

Onset: immediate (IV)
Peak effects: 20 min (IV)
Duration: 5-8 hours
Half-life: 3-4 hours

MOA: causes a reduction in HR, systolic BP, and cardiac output. Has selective effects on beta-1 adrenergic receptors. Inhibits tachycardia, particularly following an acute MI. Is effective in reducing incidence of v-fib and chest pain in MI pts

Answer 1

Class: selective beta-blocker (Class II antiarrhythmic)

Indications: used in pts with suspected or definite acute MI who are hypertensive, and do not have any contraindications, can also be considered in the following:

• stable, narrow-complex tachycardias if rhythm remains uncontrolled or unconverted by adenosine or vagal maneuvers or if SVT is recurrent
• controlling ventricular rate in pts with Atrial Fibrillation or Atrial Flutter
• certain forms of polymorphic VT (associated with acute ischemia, familial long QT syndrome, and catecholaminergic)

Contraindications: HR less than 45 bpm, systolic BP less than 100mmHg, CHF, first-degree heart block with PRI greater than .24 second (only in ACS pts), second-degree heart blocks, third-degree heart blocks, shock (both early and late signs), asthma, bronchospastic disease

Precautions: any of the following indicates immediate discontinuation: signs and symptoms of CHF, bradycardia, shock, heart block, bronchospasm

Side effects: bradycardia, hypotension, lethargy, CHF, dyspnea, wheezing, and weakness

Interactions: should not be administered to pts who have received intravenous calcium channel agents. Should be administered with caution to pts taking antihypertensive agents

Route: IV

Dosage: 5 mg every 2 minutes, up to 15 mg

Question 2

Labetalol (Trandate, Normodyne)

Onset: 2-5 min (IV)
Peak effects: 5-15 min (IV)
Duration: 2-4 hours (IV)
Half-life: 3-8 hours

MOA: a non-selective beta-adrenergic antagonist that shows no preference for beta-1 or beta-2 receptors. It blocks alpha-1 receptors, thus causing peripheral vasodilation. It is a potent agent for decreasing BP in hypertensive crisis. It lowers BP by decreasing cardiac output by beta-1 blocking properties (to a limited degree), and by causing peripheral vasodilation through blocking alpha-1 adrenergic receptors.

Answer 2

Class: Nonselective beta-blocker (Class II antiarrhythmic)

Description: a nonselective beta-blocker and a selective alpha-1 blocker

Indications: acute management of hypertensive crisis

Contraindications: pts with bronchial asthma, CHF, heart block, bradycardia, cardiogenic shock

Precautions: immediately discontinue with signs and symptoms of decongestive heart failure, bradycardia, shock, heart block, or bronchospasm. Anticipate postural hypotension; the pt should be supine during administration.

Side effects: bradycardia, hypotension, lethargy, CHF, dyspnea, wheezing, and weakness

Interactions: should not be administered to pts who have received IV calcium channel blockers. Use with caution to pts taking antihypertensive agents.

Route: IV/IO

Dosage: can be given two ways:

(1) Initial dose: 20 mg slow IVP, subsequent doses: 40 mg every 10 minutes up to 300 mg
(2) add 500 mg to 250 ml of D5W. This gives you a concentration of 2.0 mg/ml. This should be administered at a rate of 2 mg/min, to a max dose of 300 mg