Uses of β Adrenergic Blocking Drugs

USES

1. Hypertension

β blockers are relatively mild antihypertensives. All agents, irrespective of associated properties, are nearly equally effective. They are one of the first choice drugs because of good patient acceptability and cardioprotective potential.

2. Angina Pectoris

All β blockers benefit angina of effort. Taken on a regular schedule they decrease frequency of attacks and increase exercise tolerance. High doses, however, may worsen angina in some patients by increasing ventricular size and reducing coronary flow.

3. Cardiac Arrhythmias

β blockers suppress extrasystoles and tachycardias, especially those mediated adrenergically (during anaesthesia, digitalis induced)—may be used i.v. for this purpose. They control ventricular rate in atrial fibrillation and flutter, but only occasionally restore sinus rhythm. Esmolol is an alternative drug for paroxysmal supraventricular tachycardia.

4. Myocardial Infarction (MI)

In relation to MI, β blockers have been used for two purposes:

a.  Secondary prophylaxis of MI: There is now firm evidence of benefit. Longterm use after recovery from MI has been found to decrease subsequent mortality by 20%.

o   By preventing reinfarction

o   By preventing sudden ventricular fibrillation at the second attack of MI.

High risk patients (those who had large infarcts) should be put on β blockers (if there are no haemodynamic contraindications) for at least 2 years. β blockers with partial agonistic action are less suitable for this purpose.

b. Myocardial salvage during evolution of MI: Administered i.v. within 4–6 hours of an attack followed by continued oral therapy. β blockers—

·        May limit infarct size by reducing O₂ consumption—marginal tissue which is partially ischaemic may survive.

·        May prevent arrhythmias including ventricular fibrillation.

However, β blockers can be given to only those patients not in shock or cardiac failure and who have heart rate > 50/min with not higher than first degree heart block (PR interval < 0.24 sec). In megatrials such therapy has been found to reduce mortality by 20–25%.

5. Congestive Heart Failure

Although β blockers can acutely worsen heart failure, several studies have reported beneficial haemodynamic effects of β₁ blockers over longterm in selected patients with dilated cardiomyopathy. Introduced gradually and maintained for long term, these drugs retard the progression of CHF and prolong life. The benefit may result from antagonism of deleterious effects of sympathetic overactivity on myocardium. Overactivation of cardiac β₁ receptors has been found to exert toxic effects on the heart by accelerating myocyte apoptosis and promoting functionally unfavourable remodeling. Certain β₁ blockers, used appropriately along with other measures, is now established as standard therapy for most mild to moderate CHF patients. However, they should not be given to patients with marked fluid retention and to those requiring i.v. vasodilators or i.v. inotropic drugs (see Ch. 37).

6. Dissecting Aortic Aneurysm

β blockers help by reducing cardiac contractile force and aortic pulsation.

7.   Pheochromocytoma

β blockers may be used to control tachycardia and arrhythmia, but should

never be administered unless an α blocker has been given before, otherwise dangerous rise in BP can occur. They suppress cardiomyopathy caused by excess CAs.

8. Thyrotoxicosis

Propranolol rapidly controls sympathetic symptoms (palpitation, nervousness, tremor, fixed stare, severe myopathy and sweating) without significantly affecting thyroid status. It inhibits peripheral conversion of T₄ to T₃ and is highly valuable during thyroid storm. Major use, however, is preoperatively and while awaiting response to antithyroid drugs/ radioactive iodine.

9. Migraine

Propranolol is the most effective drug for chronic prophylaxis of migraine.

10. Anxiety

Propranolol exerts an apparent antianxiety effect, especially under conditions which provoke nervousness and panic, e.g. examination, unaccustomed public appearance, etc. This is probably due to blockade of peripheral manifestations of anxiety (palpitation, tremor) which have a reinforcing effect. It is largely ineffective in anxiety neurosis, but may benefit somatic symptoms.

11. Essential Tremor

Nonselective β blockers have now an established place in treating essential tremor. However, they do not benefit parkinsonian tremor.

12. Glaucoma

Ocular β blockers are widely used for chronic simple (wide angle) glaucoma; also used as adjuvant in angle closure glaucoma (see below).

13. Hypertrophic  Obstructive  Cardiomyopathy

The subaortic region is hypertrophic. Forceful contraction of this region under sympathetic stimulation (exercise, emotion) increases outflow resistance which has incapacitating haemodynamic consequence. β blockers improve c.o. in these patients during exercise by reducing left ventricular outflow obstruction, though they have little effect while at rest.