Beta receptors exist in the heart, lungs and blood vessels. When activated by the sympathetic nervous system, they stimulate an increase in heart rate and blood pressure, and cause dilation of the airways. This is part of the "fight or flight" system. Beta-blocking medications inhibit these actions. They are used to treat high blood pressure, angina, heart failure, abnormal heart rhythms and heart attacks. They are also used in the prevention of migraine headache.
Cardioselective Beta Blockers
This group of medication mainly affects receptors in the heart. This means that they are less likely to affect the lungs or peripheral blood vessels, making them less useful in preventing migraines. Atenolol, betaxolol, bisoprolol and metoprolol fall into this category. They are typically taken by mouth once or twice daily. Higher doses tend to negate the cardiac selectivity. Potential side effects of these drugs include a slowed heart rate, heart block, abnormally low blood pressure, fatigue and depression. Rapid or abrupt discontinuation may lead to a dangerous rebound increase in heart rate and blood pressure. All beta blockers may mask the symptoms of low blood sugar in those with diabetes. There are potential drug interactions with any medicine that may lower the blood pressure or affect the heart rhythm.
Nonselective Beta Blockers
These medications affect the heart receptors as well as those in the lungs and blood vessels. This makes them more effective for things such as migraine headache prevention, but also increases the risk of airway spasm. These drugs are not the first choice in those with asthma or chronic obstructive pulmonary disease. This category includes the drugs nadolol, propranolol and timolol. They are typically taken orally once or twice a day. The side effect and drug interaction profile are similar to that discussed above, with the exception of the lung issue.
Other Beta Blockers
Several medicines do not fit neatly into the categories outlined above. Carvedilol and labetalol are not considered cardioselective, but do block alpha receptors which are present in the outlying blood vessels. This means they have the additional effect of decreasing pressure directly in the vasculature. Nebivolol has similar properties, but is considered cardioselective.
A few drugs have the property of triggering some activation of the sympathetic nervous system, thus countering some of the heart and lung effects of the beta blocking action. Acebutolol, penbutolol and pindolol fall into this category, with only the first being cardioselective. These drugs are less likely to dampen the heart or cause bronchoconstriction.
