Sympatholytic drugs
Sympatholytic drugs include several different types of drugs, but all reduce blood pressure by inhibiting or blocking the sympathetic nervous system. They’re classified by their site or mechanism of action and include:
- central-acting sympathetic nervous system inhibitors (clonidine and methyldopa)
- alpha-adrenergic blockers (doxazosin, phentolamine, prazosin, and terazosin)
- mixed alpha- and beta-adrenergic blockers (carvedilol and labetalol)
- norepinephrine depletors (guanadrel, guanethidine, and reserpine’these are rarely used).
Pharmacokinetics
Most sympatholytic drugs are absorbed well from the GI tract, distributed widely, metabolized in the liver, and excreted primarily in urine.
Pharmacodynamics
All sympatholytic drugs inhibit stimulation of the sympathetic nervous system, causing dilation of the peripheral blood vessels or decreased cardiac output, thereby reducing blood pressure.
Pharmacotherapeutics
If blood pressure fails to come under control with beta-adrenergic blockers and diuretics, an alpha-adrenergic blocker, such as prazosin, or a mixed alpha- and beta-adrenergic blocker, such as labetalol, may be used. If the patient fails to achieve the desired blood pressure, the physician may add a drug from a different class, substitute a drug in the same class, or increase the drug dosage.
Warning!
Adverse reactions to sympatholytics
Alpha-adrenergic blockers
- Hypotension
Central-acting drugs
- Depression
- Drowsiness
- Edema
- Liver dysfunction
- Numbness, tingling
- Vertigo
Guanadrel
- Difficulty breathing
- Excessive urination
- Fainting
- Orthostatic hypotension
Guanethidine
- Decreased heart contractility
- Diarrhea
- Fluid retention
- Orthostatic hypotension
Reserpine
- Abdominal cramps, diarrhea
- Angina
- Blurred vision
- Bradycardia
- Bronchoconstriction
- Decreased libido
- Depression
- Drowsiness
- Weight gain
- Fatigue
- Hypotension
Drug interactions
Sympatholytic drugs can create these drug interactions:
- Carvedilol taken with antidiabetics may result in increased hypoglycemic effect.
- Carvedilol taken with calcium channel blockers may result in increased conduction disturbances.
- Carvedilol taken with digoxin may result in increased digoxin levels.
- Carvedilol taken with rifampin decreases carvedilol levels.
- Clonidine plus tricyclic antidepressants may increase blood pressure.
- Clonidine taken with CNS depressants may worsen CNS depression.
- Reserpine taken with diuretics or other hypotensive agents can increase the hypotensive effects of reserpine.
- Reserpine taken with cardiac glycosides can lead to cardiac arrthymias.
Sympatholytic drugs can also produce significant adverse reactions.