Beta-Blockers in the treatment of congestive heart failure
Introduction Beta Blockers
Beta blockers are considered the primary drugs for the pharmacological management of CHF. These drugs provide their beneficial effect by decreasing the excessive activity of the sympathetic nervous system which is characteristic of CHF. Increased sympathetic nervous system activity is a neurohormonal compensation involving the renin-angiotensin system that occurs in CHF patients in order to increase cardiac output of the failing heart and maintain blood pressure. While initially beneficial, these compensations place an ultimately damaging degree of stress on the already failing heart.
As their name suggests, the clinically useful beta blockers used to treat CHF bind to beta-adrenergic receptors on the myocardium, blocking the effects of norepinephrine and epinephrine. By this mechanism, beta blockers normalize sympathetic activity reducing heart rate, cardiac contraction force, and angina. These beta blockers are specifically known as beta-1 cardioselective blockers. Another class of beta blockers, beta-2 blockers, are bronchoconstrictors, and thus provide no real clinical value.
Two FDA approved beta blockers commonly used to treat CHF are Carvedilol and Metoprolol, although there are many others that may be used with the discretion of the treating physician.
- Carvedilol is taken orally, with dosages ranging from 3.125 mg taken twice daily to a maximal dose of 50 mg taken twice daily. The half-life of carvedilol is 7-10 hours and it is metabolized extensively and excreted in feces via bile with less than 2% excreted unchanged in the urine.
- Metoprolol is taken orally starting at dosages of 12.5-25 mg once a day, which can be doubled every two weeks up to 200 mg daily. Metoprolol has a half-life of 3-7 hours and is mostly metabolized by the liver.
A primary potential side effect of beta blockers is bradycardia or an abnormally slow heart rate, which may exacerbate the failing heart’s ability to pump enough blood. Additionally, symptoms of fatigue, weakness, and dizziness are also side effects of beta blockers.
While beta blockers may allow patients to participate in therapy less inhibited by angina and with reduced stress on the heart, exercise programs must be prescribed with extreme caution and monitored carefully. Beta blockers may render conventional aerobic workload guidelines inappropriate as they typically decrease maximal heart rate, and consequently have the potential to mislead the therapist prescribing an exercise program. Heart rate and blood pressure should be monitored frequently, and the patient should be assessed for signs of fatigue, dizziness, and weakness prior to the initiation of the therapy session and monitored throughout.
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