At low-dose epinephrine infusion (0.01-0.03 mcg/kg/min), which receptors are activated and what is the net effect on heart rate, contractility, SVR, and blood pressure?

At low-dose epinephrine infusion (0.01-0.03 mcg/kg/min), which receptors are activated and what is the net effect on heart rate, contractility, SVR, and blood pressure?

• A. B1 and B2; increased heart rate and contractility; decreased SVR; overall blood pressure increases
• B. Alpha and Beta; increased heart rate and contractility; SVR increases; blood pressure decreases
• C. Beta-1 only; increased heart rate; SVR unchanged; blood pressure unchanged
• D. Alpha-1 and Beta-2; decreased heart rate; SVR increases

Answer: (A)

At low-dose epinephrine, the dominant action comes from beta receptor activation rather than alpha. Epinephrine stimulates beta-1 receptors in the heart, increasing heart rate (chronotropy) and contractility (inotropy), which raises cardiac output. It also stimulates beta-2 receptors in the vasculature, causing vasodilation in skeletal muscle and a decrease in systemic vascular resistance. The net result is higher cardiac output with lower SVR, so systolic blood pressure tends to rise due to the increased CO, while diastolic pressure may stay the same or fall slightly from the vasodilation. This is why low-dose infusion is described as producing increased heart rate and contractility, decreased SVR, and overall an increase in blood pressure. As the dose increases, alpha-1 effects become more prominent, shifting the pattern toward vasoconstriction and higher SVR.

At low-dose epinephrine, the dominant action comes from beta receptor activation rather than alpha. Epinephrine stimulates beta-1 receptors in the heart, increasing heart rate (chronotropy) and contractility (inotropy), which raises cardiac output. It also stimulates beta-2 receptors in the vasculature, causing vasodilation in skeletal muscle and a decrease in systemic vascular resistance. The net result is higher cardiac output with lower SVR, so systolic blood pressure tends to rise due to the increased CO, while diastolic pressure may stay the same or fall slightly from the vasodilation. This is why low-dose infusion is described as producing increased heart rate and contractility, decreased SVR, and overall an increase in blood pressure. As the dose increases, alpha-1 effects become more prominent, shifting the pattern toward vasoconstriction and higher SVR.