Antihypertensives: Antiadrenergics – Alpha and Beta-Blockers
Antiadrenergic drugs inhibit or block the activity of the sympathetic nervous system. So the goal of antiadrenergic drug therapy is to suppress stimulation that is not the normal physiologic response to activity or stress, such as increases in heart rate, increased force of myocardial contraction, increased cardiac output, and increased blood pressure. When the nerve impulse is inhibited or blocked at any location along the pathway, the result is lowered blood pressure. Antiadrenergic effects occur when either the alpha or beta receptors are blocked by adrenergic antagonists.
Alpha-1-adrenergic receptor blocking agents dilate blood vessels and decrease peripheral vascular resistance. These include drugs such as prazosin, doxazosin, and terazosin. In addition to treating hypertension, these alpha-1 blocking drugs are also given for BPH (benign prostatic hyperplasia), a condition characterized by obstructed urine flow as the enlarged prostate gland presses on the urethra. Alpha-1 blocking agents relax muscles in the prostate and urinary bladder, which decreases urinary retention and improves urine flow. One adverse effect is orthostatic hypotension with palpitations, dizziness, and syncope, 1-3 hours after the first dose or an increased dose. This is called the first-dose phenomenon, and therefore it is best if first doses or an increased dose be taken at bedtime. Another adverse effect, after taking long-term or higher doses, is the development of sodium and fluid retention, requiring diuretic therapy also.
Alpha-2 receptor agonists are centrally acting, causing less norepinephrine to be released, which leads to decreased cardiac output, decreased heart rate, lower peripheral vascular resistance, and lower blood pressure. Clonidine and methyldopa are both alpha-2 receptor agonists given for hypertension.
Beta-adrenergic blocking agents prevent the beta-adrenergic receptors from responding to the hormone epinephrine (adrenaline). The drugs are given to decrease heart rate, decrease cardiac output, decrease the force of myocardial contraction, and decrease renin release from the kidneys. Beta blockers are mainly given for cardiovascular disorders, such as angina, hypertension, cardiac tachydysrhythmias, myocardial infarction, and heart failure. They are also given for glaucoma and migraines. Common side effects of beta blockers include fatigue, cold hands or feet, and weight gain. Less common side effects include trouble sleeping, depression, and shortness of breath.
Beta blockers are generally not given to patients with asthma or COPD because of the risk of triggering severe asthma attacks. In patients with diabetes, beta blockers may block signs of low blood sugar, such as rapid heartbeat. If a patient with diabetes or pulmonary disorders needs to take beta blockers, the cardioselective beta blockers are preferred, such as metoprolol (lopressor) and atenolol. Cardioselective just means that they have more effect on beta-1 receptors and cause less bronchospasm and impairment of glucose metabolism. Nonselective beta blockers are those drugs that block both beta-1 and beta-2 receptors. Examples of nonselective blocking agents include propranolol, nadolol, and timolol. Labetalol is an alpha-beta-blocking agent, often given for hypertensive emergencies.
Important education for patients taking beta blockers includes… counting their pulse daily and reporting if it is under 50 for several days in a row, reporting weight gain, shortness of breath, excessive fatigue, ankle edema, fainting spells, or difficulty breathing. Also, do not stop taking a beta blocker abruptly, as it can result in chest pain, a heart attack, or other heart problems.
Let’s take a look at some questions for review…
A patient with a history of COPD is prescribed propranolol to treat hypertension. In this situation, the nurse should assess for the presence of
- Respiratory depression
If you chose 3, bronchoconstriction, you’re right! Propranolol is a nonselective beta blocker, which has a risk of bronchoconstriction. A cardioselective beta blocker is preferred because they cause less bronchospasm.
Let’s try another one.
Which of the following instructions should be given to the hypertensive patient before taking his first dose of prazosin?
- Take your medication with a glass of milk.
- Take your medication on an empty stomach.
- Take your medication at bedtime.
- Take your medication in the morning after meals.
If you chose 3, take the medication at bedtime, you’re correct! Prazosin is an alpha-1 adrenergic blocking agent that dilates blood vessels, and can result in the first-dose phenomenon of orthostatic hypotension with dizziness and possibly syncope, and should therefore be taken at bedtime.
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