What is the mechanism of action of vasopressors (e.g. norepinephrine, epinephrine, vasopressin) and inotropes (e.g. dobutamine, milrinone)?

Mechanism of Action of Vasopressors and Inotropes

Vasopressors: Core Mechanisms

Vasopressors increase mean arterial pressure primarily through peripheral vasoconstriction by acting on adrenergic receptors and other vascular targets. 1

Norepinephrine (First-Line Agent)

• Acts on both α1-adrenergic receptors (causing vasoconstriction) and β1-receptors (providing modest cardiac stimulation), making it the preferred first-line vasopressor for most shock states. 1, 2
• Increases systemic vascular resistance while maintaining cardiac output, with lower risk of adverse events compared to other catecholamines. 2, 3
• The α1-mediated peripheral vasoconstriction is the dominant effect, redistributing blood flow from extremities to vital organs. 4

Epinephrine (Dual Action)

• Acts on both α-adrenergic and β-adrenergic receptors with a three-fold mechanism: direct myocardial stimulation (positive inotropic action), increased heart rate (positive chronotropic action), and peripheral vasoconstriction. 5
• At low doses, β2-mediated vasodilation decreases systemic vascular resistance and diastolic blood pressure, but at higher doses α1-mediated vasoconstriction predominates, increasing diastolic pressure. 5
• However, epinephrine is associated with increased mortality and marked worsening of cardiac and renal biomarkers in cardiogenic shock, independent of prior cardiac arrest. 6
• Onset of blood pressure increase occurs within 5 minutes, with offset within 20 minutes after stopping infusion. 5

Vasopressin (Non-Adrenergic)

• Causes vasoconstriction by binding to V1 receptors on vascular smooth muscle, coupled to the Gq/11-phospholipase C pathway, resulting in intracellular calcium release. 7
• At therapeutic doses (0.01-0.1 units/minute), increases systemic vascular resistance and mean arterial pressure while reducing norepinephrine requirements. 7
• The pressor effect reaches peak within 15 minutes and fades within 20 minutes after stopping infusion, with no evidence of tachyphylaxis. 7
• Can be added when norepinephrine alone is inadequate to achieve target blood pressure. 1

Dopamine (Dose-Dependent Effects)

• At low doses (<3 μg/kg/min): acts on δ-receptors with potential renal effects. 4
• At moderate doses (3-5 μg/kg/min): β-adrenergic inotropic effects predominate. 4
• At high doses (>5 μg/kg/min): α-adrenergic vasopressor effects dominate. 4
• Associated with increased mortality and more arrhythmic events compared to norepinephrine in cardiogenic shock patients. 4

Inotropes: Core Mechanisms

Inotropes enhance cardiac output through improved myocardial contractility, primarily via β1-adrenergic receptor stimulation or phosphodiesterase inhibition. 1, 4

Dobutamine (β1-Selective)

• Stimulates β1-receptors to increase cardiac contractility while also relaxing vascular smooth muscle to reduce afterload. 4
• The major caveat is that dobutamine causes severe hypotension due to its vasodilatory effects, limiting its use in hypotensive patients. 4
• Should be reserved for patients with persistent low cardiac output and hypotension related to left ventricular systolic dysfunction, used at the lowest doses for the shortest duration with progressive titration. 4
• Can be added when tissue perfusion remains inadequate despite adequate mean arterial pressure. 1

Milrinone (Phosphodiesterase-3 Inhibitor)

• Increases intracellular calcium levels, enhancing myocardial contractility and cardiomyocyte relaxation through phosphodiesterase-3 inhibition. 4
• Causes both arterial and venous vasodilation, which reduces preload and afterload but can lead to hypotension. 4
• Has a longer half-life than dobutamine, making it less titratable in the face of hypotension risk. 4

Levosimendan (Calcium Sensitizer)

• Acts as an inodilator with a unique mechanism distinct from traditional inotropes. 4
• The SURVIVE study showed reduced mortality in cardiogenic shock patients receiving levosimendan compared to dobutamine. 4
• Bolus dosing (12 μg/kg over 10 minutes) is not recommended in hypotensive patients (systolic blood pressure <90 mmHg). 4

Critical Clinical Principles

Target Blood Pressure

• The latest recommendations suggest a mean arterial pressure objective of 65 mmHg, though this may need individualization based on baseline blood pressure and comorbidities. 4, 1
• In pulmonary arterial hypertension patients, systolic systemic arterial pressure goals are higher to maintain systemic vascular resistance greater than pulmonary vascular resistance. 4

Essential Precautions

• Vasopressors should never be used before correcting hypovolemia, as uncorrected hypovolemia is an absolute contraindication. 1
• All vasoactive agents require continuous monitoring including ECG, arterial blood pressure, and tissue perfusion assessment. 1
• Inotropes cause sinus tachycardia, may induce myocardial ischemia and arrhythmias, and carry long-standing concerns about increased mortality. 4
• High-dose dopamine can cause tachyarrhythmias and should be avoided. 1

Combination Therapy Strategy

• When norepinephrine alone is inadequate, adding a non-catecholamine vasopressor (vasopressin or angiotensin-II) is reasonable rather than escalating to high-dose catecholamines. 1, 2
• In critically ill pulmonary hypertension patients, replacement-dose vasopressin can offset the SVR drop from dobutamine. 4
• There is no mortality difference whether norepinephrine is combined with dobutamine or levosimendan. 6