Certified Registered Nurse Anesthetist (CRNA) Practice Exam

What is the primary effect of phenylephrine?

1. Vasodilation

2. Vasoconstriction

3. Increased heart rate

4. Bronchodilation

The correct answer is: Vasoconstriction

Phenylephrine primarily acts as a selective alpha-1 adrenergic agonist. When administered, it primarily causes vasoconstriction by stimulating alpha-1 receptors located on vascular smooth muscle. This leads to an increase in peripheral vascular resistance, which subsequently elevates blood pressure. The mechanism of action of phenylephrine is distinct in that its effects are primarily confined to vasoconstriction rather than vasodilation or changes in heart rate. While it is important to recognize that the drug does not have significant intrinsic sympathomimetic activity, its vasoconstrictive properties are exploited in clinical settings, particularly in managing hypotension during surgical procedures. The other options suggest physiological effects that are not characteristic of phenylephrine's pharmacodynamics. For instance, vasodilation would typically involve agonism of beta-2 adrenergic receptors, which is not the case here. Increased heart rate is associated with beta-1 receptor stimulation, which phenylephrine does not promote. Lastly, bronchodilation is a result of beta-2 adrenergic receptor activation, further underscoring that phenylephrine's mechanism is largely restricted to vasoconstriction. Thus, the answer effectively captures the primary pharmacological action of phenylephrine.