Beta Blockers Reassessing Pharmacological Differences April, 1998 â blockers have grown steadily in number and application since Highlights the introduction of propranolol in 1965. This edition will • â blockers are considered first line agents along with compare the efficacy, safety, and cost of commonly prescribed thiazide diuretics in patients with uncomplicated HTN, â blockers. Carvedilol (Coregâ), the latest addition to the due to proven ↓ morbidity & mortality and low cost. newly evolving class of combination á-â blockers is also • Cardioselectivity may improve safety and tolerability, reviewed. however, this is not guaranteed. Comparative Pharmacology: • Metoprolol SR combines cardioselectivity with well- Despite a common chemical structure, â blockers possess a documented efficacy in angina, myocardial infarction number of properties that differentiate one agent from another. and CHF, and provides excellent 24h BP control. Advantages and disadvantages of these properties are often of • â blockers, unless contraindicated, are often preferred in more theoretical than clinical significance. patients with stable or unstable angina, supraventricular arrhythmia, and previous MI. Cardioselective â blockers include acebutolol • Carvedilol may be beneficial in patients with heart (Sectralâ/Monitanâ), atenolol (Tenorminâ), and metoprolol failure who are refractory to other drug therapies. (Lopresorâ/Betalocâ). These agents preferentially block â1 receptors found primarily in cardiac muscle, adipose tissue, and agents. Cardioprotective efficacy has been associated with a â the kidney. They have less effect on â2 receptors found in blocker’s degree of lipophilicity (see MI). pulmonary and vascular smooth muscle and the pancreas. Hypertension: Cardioselective agents should theoretically cause less coldness â blockers have a well established role in the treatment of of the extremities and less bronchospasm. Unfortunately, hypertension. They are still considered first line agents along selectivity is dose-dependent and varies widely among patients1 with thiazides in the management of uncomplicated so this property cannot be relied upon to consistently provide hypertension because of clinically proven reductions in improved safety and reduced side effects. Non-cardioselective morbidity and mortality.2 At equipotent doses, all â blockers agents are generally required if used to treat essential tremor or are equally effective in reducing blood pressure.1 â blockers prevent migraine. may be particularly effective antihypertensives in younger Pindolol (Viskenâ), oxprenolol (Trasicorâ), and acebutolol patients with high renin activity. The elderly may also benefit have intrinsic sympathomimetic activity (ISA). These agents from â blockers especially when used in combination with act as partial agonists and cause smaller reductions in heart rate diuretics. â blockers may be preferred in patients with and cardiac output. The clinical significance of these effects angina, supraventricular arrhythmia, previous MI, has yet to be clearly demonstrated.1 Agents with ISA should migraine, anxiety, or other comorbid conditions where â be avoided in patients post-MI, or with CHF. blockers are of benefit. Acebutolol, pindolol, and propranolol (Inderalâ) also have a Optimal antihypertensive therapy requires consistent BP local anaesthetic effect or membrane stabilizing activity reduction over a 24 hour period. Although several agents may (MSA). Supratherapeutic doses are required to produce this be given once daily (acebutolol, atenolol, nadolol, pindolol, effect so it is usually not clinically relevant.1 timolol), care should be taken to ensure adequate 24 hour control is maintained. Once daily atenolol may not control Adverse CNS effects are often thought to be related to â 19,25 blood pressure adequately during the early morning hours. blockers’ lipophilicity, as fat-soluble agents cross the blood- Sustained release (SR) metoprolol, given once daily combines brain barrier more readily. However, frequency and severity of 25,26 excellent 24 hour BP control with cardioselectivity. CNS adverse effects may not be solely related to lipophilicity as fatigue, mood changes, depression, and nightmares have â blockers, particularly in low doses, may be better tolerated been reported with all â blockers1 including water-soluble than commonly believed. Results of TOMHS (Treatment of Mild Hypertension Study) indicated acebutolol was associated with the greatest overall improvement in quality of life measures when compared with agents from 4 other classes of avoided as they do not appear to have any benefical effect in antihypertensives (amlodipine, enalapril, chlorthalidone, and HF. 15 Initially, â blockers may worsen symptoms of HF but doxazosin).17 Adverse effects are further outlined in Table 2. improvement is usually seen with long term therapy (i.e. after 1-2 months). Patients should be stabilized on a regimen of Angina: ACE inhibitors, diuretics, and/or digoxin before starting â â blockers appear to be the most efficacious agents for the blockers. Initial doses should be very small, 1/10-1/20th the treatment of stable and unstable angina. They are the only antihypertensive dose, and titrated up gradually over a period of antianginal agents shown to improve clinical outcomes in several weeks (eg. metoprolol 6.25mg BID x 7 days; double patients with myocardial ischemia.3 Their negative dose weekly till 50 mg BID or adverse affects appear ).15,16 chronotropic and inotropic effects decrease heart rate and Approximately 15% of patients will not tolerate â blocker contractile force, reducing myocardial oxygen demand and therapy, developing low output syndrome with bradycardia, relieving angina. All â blockers improve exercise tolerance hypotension, edema, and progressive heart failure. Patients and reduce signs and symptoms of angina regardless of intolerant to â blockers tend to have right-sided failure with differences in ISA and selectivity.1 Combinations of a â ascites, jugular venous distension, tricuspid regurgitation, blocker and a long-acting dihydropyridine calcium antagonist increased right filling pressure,and EF of <20%. â blockers are are useful in some patients. â blockers should be avoided in best avoided in this population.15 vasospastic angina as uncompensated alpha receptor 4 stimulation may result in further vasoconstriction. α1 -â âlockers: â â Myocardial Infarction (MI): Secondary prevention Labetalol (Trandate ) was the first blocker to combine both â α Many studies in the last decade have shown the benefit of â non-selective and vasodilatory 1 adrenergic blockade in a blockers in prevention of recurrent MI and other related single drug. Its â:α blockade ratio is ≈ 3:1. Labetolol IV is complications, acutely and in the postinfarction period. Results indicated in various hypertensive emergencies. from major trials show a reduction in overall mortality of more â â 6-9 Carvedilol (Coreg ) is a more potent non-selective blocker than 20-30%. â blockers with moderate to high and α1 antagonist. Unlike other â blockers, carvedilol also lipophilicity (timolol, metoprolol, propranolol) have shown 7-9 possesses several unique cardioprotective, anti-ischemic, and marked reductions in sudden cardiac death. It is antioxidant properties.22 Many recent studies lend support postulated that these agents enter the brain rapidly and increase to the use of carvedilol in HF.23 Most impressive of these, the vagal tone. This prevents ventricular fibrillation associated US Carvedilol Heart Failure Study was terminated early after a with the increased sympathetic drive and reduced vagal tone 10 6 month follow-up showed a 65% reduction in overall mortality during an acute MI. â blockers appear underused in survivors 5 in carvedilol vs. placebo treated patients. All study patients of acute MI. Studies looking at pindolol and oxprenolol were stabilized on diuretics, ACE inhibitors, and digoxin. Until 11,12 â showed no effect on mortality. As both agents have ISA, further trials are completed, carvedilol serves as a promising blockers with this property may not be appropriate for use post- 1 alternative in HF patients refractory to other therapy. Although MI. carvedilol is a potent antihypertensive, it is not currently Arrhythmia: approved in Canada for the treatment of hypertension. â All blockers have some inherent (Class II) antiarrhythmic Carvedilol is generally well tolerated. Effective doses are activity. They slow conduction through both the SA node and lower then comparable doses of â blockers because of its â the AV node. All blockers appear beneficial in treatment of combined mechanisms of action. Carvedilol’s â blocking supraventricular arrhythmias, regardless of MSA, ISA, or activity reduces reflex tachycardia and fluid retention usually 1 â â selectivity. Sotalol (Sotacor ) may be the preferred seen with α blockers. Complaints of cold extremities, blocker for arrhythmias because it has both Class II and Class bradycardia, and reduced cardiac output seen with pure â III antiarrhythmic activity. Caution is required as sotalol also blockers are minimized by carvedilol’s vasodilatory effect.21 has pro-arrhythmic potential. Adverse effects are more common early in therapy and are Heart Failure (HF): often dose-related. Common complaints include dizziness, The role of â blockers in HF has yet to be clearly established. headache, and orthostatic hypotension as well as fatigue and 20 In the past, â blockers were considered contraindicated due to asthenia. Unlike