Alpha-2 adrenergic agonists: Nursing pharmacology

Alpha-2 adrenergic agonists are medications that oppose the effects of the sympathetic nervous system, ultimately decreasing smooth muscle contraction.

These medications include clonidine, guanfacine, and methyldopa, which can be administered orally, and clonidine also transdermally, to treat hypertension.

In addition, clonidine can also be used orally to treat attention deficit hyperactivity disorder, or ADHD for short, and less frequently, through epidural infusion for pain management in cancer.

Finally, methyldopa can also be administered intravenously to treat hypertensive crises. Once administered, alpha-2 adrenergic agonists act in the brainstem.

Normally, neurons in the brainstem carry signals to the spinal cord nuclei, where they synapse and release norepinephrine to stimulate the sympathetic nervous system.

This triggers the fight or flight response, which results in smooth muscle contraction, in turn leading to an increase in the heart rate and blood pressure, as well as decreasing gastrointestinal motility and urination by contracting the sphincters.

Now if we zoom into this synapse, we can see that some of the norepinephrine turns around and binds to alpha-2 adrenergic receptors.

This inhibits further release of norepinephrine and serves as a mechanism of negative feedback control.

So, alpha-2 adrenergic agonists work by stimulating these receptors, therefore blocking the downstream sympathetic response, and ultimately decreasing blood pressure.

Some common side effects include nausea, vomiting, constipation, and dry mouth. Some clients may also experience bradycardia, while clonidine can also cause rebound hypertension when stopped abruptly, as well as CNS depression.

On the other hand, guanfacine and methyldopa can also cause orthostatic hypotension and sedation.

In addition, methyldopa can also cause granulocytopenia, thrombocytopenia, and hemolytic anemia, as well as hepatotoxicity and hepatic necrosis.

As far as contraindications go, these medications should be used with caution during pregnancy and breastfeeding, in children and elderly clients, as well as in clients with cardiovascular, cerebrovascular, or renal disease.

In addition, clonidine has a boxed warning against epidural use for obstetrical, postpartum, or perioperative pain management, due to the risk of hypotension and bradycardia.

Epidural administration is also contraindicated in clients with bleeding disorders or who are taking anticoagulants; and it can’t be administered above the C4 dermatome.

Finally, methyldopa is contraindicated in clients with active hepatic disease, as well as in those taking MAO inhibitors.

Regarding interactions, clonidine increases the risk of bradycardia when combined with verapamil or diltiazem, and could cause life-threatening elevations of blood pressure if taken with beta-blockers.

On the other hand, methyldopa can increase lithium toxicity, the hypertensive effect of MAO inhibitors, and the risk of hypotension and CNS toxicity of levodopa.

Now, when caring for a client with hypertension who has been prescribed transdermal clonidine, first perform a baseline assessment including vital signs, especially blood pressure and heart rate.