What is the effect of phenylephrine on cardiac output?

Phenylephrine is a potent selective alpha-1 adrenergic agonist primarily used as a vasopressor. Its primary mechanism of action is to cause vasoconstriction, leading to an increase in systemic vascular resistance (SVR).

When phenylephrine is administered, it causes an immediate increase in vascular tone, resulting in elevated blood pressure due to increased SVR. However, the increase in resistance can lead to a decrease in cardiac output. This occurs because the heart faces increased afterload, which is the pressure the heart must work against to pump blood into the circulation. As a result, the heart may not be able to fill adequately or may eject less blood, ultimately leading to a reduction in cardiac output.

This understanding is essential for managing patients, especially in cases where maintaining appropriate cardiac output is critical, such as during anesthesia or in certain shock states. In such scenarios, the use of phenylephrine must be carefully monitored to balance blood pressure and cardiac performance.