α-Agonists

α-Agonists

1. Phenylephrine is a direct-acting α₁-agonist at typical clinical doses (40 to 200 mcg/min infusion), with some β-receptor activity at higher doses or with concurrent α-blockade. Phenylephrine causes both arterial and venous vasoconstriction. This dual action increases venous return and mean arterial blood pressure, frequently resulting in reflex bradycardia. Phenylephrine maintains cardiac output in patients with normal myocardial function but may decrease cardiac performance in the presence of myocardial ischemia. Phenylephrine has a short duration of action, which makes it easily titratable.
2. Clonidine is a centrally acting antihypertensive with relative selectivity for α₂-adrenoreceptors. Its actions include reducing sympathetic tone, increasing parasympathetic activity, reducing anesthetic and analgesic requirements, causing sedation, and decreasing salivation. It can be administered intravenously, intramuscularly, orally, transcutaneously, and into the intrathecal and epidural spaces.
3. Dexmedetomidine is a selective α₂-adrenoreceptor agonist currently approved for the short-term (<24 hours) intravenous sedation of mechanically ventilated patients in an intensive care setting. Its action on presynaptic receptors inhibits the release of norepinephrine, and its activation of postsynaptic α₂-receptors in the CNS inhibits sympathetic activity. These effects decrease blood pressure and heart rate. Dexmedetomidine offers potential advantages over other sedatives, including lack of respiratory depression, lower rates of associated delirium, and a decreased incidence of hypotension.