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Beta blockers should not be given to patients with bronchospasm or asthma or to those with a history of obstructive airways disease. This contra-indication generally applies even to those beta-blockers considered to be cardioselective. However, cardioselective beta-blockers may be used with extreme caution when there is no alternative treatment. Other contra-indications include metabolic acidosis, cardiogenic shock, hypotension, severe peripheral arterial disease, sinus bradycardia, and second or third degree AV block; caution should be observed in first degree block. Although beta blockers are used in the management of heart failure, they should not be given to patients with uncontrolled heart failure and treatment should be begun with great care, starting with a low dose and cautiously titrating upwards. Patients with phaeochromocytoma should not be given beta blockers without alpha-adrenoreceptor blocking therapy as well.
Beta blockers may block the symptoms of hyperthyroidism and of hypoglycaemia. They may unmask myasthenia gravis. Psoriasis may be aggravated. Beta blockers may increase the number of attacks of chest pain in patients with Prinzmetal's angina; this occurs especially with non-cardioselective beta blockers, which should be avoided. Beta blockers increase sensitivity to allergens and also the severity of anaphylactoid reactions; patients with a history of anaphylaxis to an antigen while taking beta blockers.
After withdrawal of beta blockers has sometimes resulted in angina, myocardial infarction, ventricular arrythmias and death. Patients on long term treatment with a beta blocker should have their medication stopped gradually over a period of 1 to 2 weeks. In patients undergoing surgery,
beta blockers may reduce the risk of hypotension, the decision to withdraw or continue therapy depends on individual patient risk. If beta blockers are withdrawn, this should take place at least 24 to 48 hours before surgery; if they are continued, atropine may be given to counter increases in vagal tone and anaesthetics causing myocardial depression, such as ether, cyclopropane, and trichloroethylene, are best avoided. It is of greatest importance that the anaesthetist is aware that beta blockers are being given.
Use of beta blockers in pregnancy shortly before delivery has occasionally resulted in bradycardia and other adverse effects such as hypoglycaemia and hypotension in the neonate. Many beta blockers are distributed into breast milk.
Similar precautions apply when beta blockers are used as eye drops since systemic absorption can occur.
Special care should be taken in patients whose cardiac reserve is poor. Heart failure should be satisfactorily controlled before started carvedilol. Carvedilol has beta-blocking activity, so withdrawal should be gradual in patients with ischaemic heart disease. As with other beta-blockers, carvedilol may mask the symptoms of hyperthyroidism and early signs of acute hypoglycemia may be masked in patients with diabetes mellitus; so be careful in using Carvedilol.
Patients taking carvedilol should be warned not to drive or operate machinery if they experience dizziness.
