Figure 5. Cardiovascular effects of epinephrine and phenylephrine. Epinephrine increases heart rate (HR) by activating beta-1 receptors in the sinoatrial node, the heart's normal pacemaker. It also activates beta-1 receptors on myocardial cells, increasing their contractility and increasing systolic blood pressure (SBP). However, at low doses such as those provided in local anesthetic formulations, it activates beta-2 receptors on systemic arteries, producing vasodilation. This decline in arterial resistance produces a reduction in diastolic pressure (DBP). The sum of these effects results in little change of mean arterial pressure (MAP). In contrast, phenylephrine activates only alpha receptors, increasing arterial resistance and diastolic pressure. Systolic pressure also rises as the heart compensates for this increase in resistance by increasing its contractility and venoconstriction increases venous return (preload). The net effect is an increase in mean arterial pressure, which is sensed in baroreceptors, and a reflex slowing of heart rate supervenes. (Adapted from Westfall et al.11)