Hemodynamic instability during anesthesia induction is a persistent clinical challenge, primarily due to the vasodilatory and myocardial depressant effects of commonly used induction agents, such as propofol. These effects often result in acute reductions in systemic vascular resistance and cardiac output, leading to hypotension that can compromise cerebral, coronary, and renal perfusion—complications that are particularly concerning in elderly patients and those with limited cardiovascular reserve. Consequently, vasopressors are often administered during induction to maintain adequate mean arterial pressure and ensure adequate perfusion of vital organs. The most commonly used agents are phenylephrine, ephedrine, and norepinephrine. Each vasopressor has a distinct pharmacologic profile that influences its clinical utility and impact on hemodynamic stability during anesthesia induction.
Phenylephrine, a selective α1-adrenergic agonist, produces rapid vasoconstriction and a predictable increase in systemic vascular resistance, making it highly effective at correcting anesthesia-induced hypotension. Its ease of titration and rapid onset have contributed to its widespread use.
However, phenylephrine lacks β-adrenergic activity; therefore, the increase in afterload may trigger reflex bradycardia and reduce stroke volume, which can lead to decreased cardiac output. Experimental and clinical data demonstrate that, although phenylephrine reliably restores arterial pressure, it can adversely affect tissue perfusion in certain situations due to these cardiac effects (1). This limitation is particularly relevant in patients with compromised ventricular function, for whom maintaining forward flow is critical. In these patients, another vasopressor is preferred for optimal hemodynamic stability if hypotension occurs during anesthesia induction.
Ephedrine differs mechanistically by exerting both direct and indirect α- and β-adrenergic effects. These effects result in an increased heart rate, myocardial contractility, and peripheral vasoconstriction. This mixed activity enables ephedrine to preserve cardiac output more effectively than phenylephrine does, making ephedrine advantageous in situations where hypotension accompanies reduced cardiac performance. A systematic review by Lee et al. found that, although both phenylephrine and ephedrine effectively treat hypotension, ephedrine is associated with higher heart rates and a more favorable cardiac output profile (2). However, because ephedrine depends on endogenous catecholamine release, its effectiveness may decrease with repeated dosing. Its tendency to increase heart rate may also limit its use in patients with ischemic heart disease or arrhythmias.
Norepinephrine offers a pharmacological profile with the advantages of both agents. As a potent α-adrenergic agonist with modest β1-adrenergic activity, norepinephrine increases vascular tone while supporting myocardial contractility. This combination allows for the restoration of arterial pressure with a smaller reduction in heart rate and cardiac output than phenylephrine produces. Clinical evidence indicates that norepinephrine provides effective hemodynamic control with a lower incidence of reflex bradycardia and improved overall cardiovascular stability, making it an excellent vasopressor in many scenarios (3).
Despite the differences between vasopressors, there is no single vasopressor that is optimal for all patients or clinical scenarios. The choice of agent should be guided by the underlying physiologic disturbance and the characteristics of the individual patient. Phenylephrine is appropriate for patients with vasodilatory hypotension and relative tachycardia when increased vascular tone is the primary goal. Ephedrine may be preferred when maintaining cardiac output is essential, though its side effect profile limits its routine use. Norepinephrine is a balanced option that simultaneously supports vascular resistance and cardiac performance, and it is becoming increasingly utilized in contemporary anesthetic practice. These agents provide anesthesiologists with a flexible toolkit to protect hemodynamic stability in patients during induction.
References
- Poterman M, Vos JJ, Vereecke HE, et al. Differential effects of phenylephrine and norepinephrine on peripheral tissue oxygenation during general anaesthesia: A randomised controlled trial. Eur J Anaesthesiol. 2015;32(8):571-580. doi:10.1097/EJA.0000000000000247
- Lee A, Ngan Kee WD, Gin T. A quantitative, systematic review of randomized controlled trials of ephedrine versus phenylephrine for the management of hypotension during spinal anesthesia for cesarean delivery. Anesth Analg. 2002;94(4):. doi:10.1097/00000539-200204000-00028
- Guo L, Xu X, Qin R, et al. Prophylactic Norepinephrine and Phenylephrine Boluses to Prevent Postspinal Anesthesia Hypotension During Cesarean Section: A Randomized Sequential Allocation Dose-Finding Study. Drug Des Devel Ther. 2023;17:1547-1555. Published 2023 May 23. doi:10.2147/DDDT.S406671
