Vasopressors

Vasopressor vs. inotrope

A vasopressor raises blood pressure chiefly by increasing systemic vascular resistance (SVR) through alpha-1 receptor-mediated vasoconstriction. An inotrope raises cardiac output by increasing the force of heart contraction through beta-1 stimulation. Blood pressure equals cardiac output times SVR, so either mechanism raises it, but the cost differs: vasoconstriction raises afterload (the resistance the heart pumps against), while inotropy raises myocardial oxygen demand. Most agents used in shock are not purely one or the other; they sit on a spectrum, and understanding each drug's receptor profile predicts where it falls.

The receptor logic

• Norepinephrine is a potent alpha-1 agonist with modest beta-1 activity. It produces strong vasoconstriction (SVR up) while its mild beta-1 effect keeps cardiac output relatively preserved. It is considered a vasopressor with a small inotropic contribution, and reflex bradycardia from the baroreceptor response often blunts any heart-rate increase. It is the first-line agent for most forms of shock with low SVR.
• Epinephrine activates alpha-1, beta-1, and beta-2 receptors. At the concentrations used in shock, alpha-1 vasoconstriction dominates, but strong beta-1 stimulation also drives heart rate and contractility up substantially, and beta-2 activity can cause some vasodilation in skeletal muscle beds. Epinephrine is both a vasopressor and a potent inotrope/chronotrope, useful when pressure and pump function both need support (cardiac arrest, anaphylaxis), but it raises myocardial oxygen demand and arrhythmia risk more than norepinephrine.
• Phenylephrine is a pure alpha-1 agonist with essentially no beta activity. It is a pressor only — it raises SVR and, through it, blood pressure — without directly increasing heart rate or contractility. With no beta-1 activity to oppose the baroreceptor reflex, phenylephrine often causes reflex bradycardia and can reduce cardiac output, so it is used selectively when tachycardia must be avoided.
• Dopamine is dose-dependent in classic teaching: it stimulates dopaminergic receptors at low concentrations, beta-1 receptors at intermediate concentrations (increasing heart rate and contractility), and alpha-1 receptors at higher concentrations (vasoconstriction). In practice this gives a mixed inotrope/chronotrope/pressor profile, but tachycardia and arrhythmia risk have made it a second-line choice behind norepinephrine.
• Vasopressin is not an adrenergic drug at all — it acts on V1 receptors on vascular smooth muscle, causing vasoconstriction through a separate (non-catecholamine) signaling pathway. It has no direct effect on heart rate or contractility, so it is a pure vasopressor. Because it works through a different receptor system, it is often added to norepinephrine in refractory shock to gain pressor effect without stacking more catecholamine onto already-saturated adrenergic receptors.

Matching the agent to the shock

The physiology of the shock state determines which receptor profile is wanted. In septic (distributive) shock, pathologic vasodilation drops SVR while cardiac output is often normal or high, so a pure or predominant vasoconstrictor (norepinephrine, with vasopressin often added) restores SVR without further burdening the heart. In cardiogenic shock, the problem is failing contractility, not low SVR, so a strong vasoconstrictor without inotropic support can worsen forward flow by raising afterload; agents with inotropic activity, or inotrope-vasopressor combinations, are chosen instead. In anaphylactic shock, epinephrine is preferred because it simultaneously reverses vasodilation, bronchoconstriction, and myocardial depression.

Practical pharmacology points

Vasopressors are given by continuous infusion through a central venous line whenever feasible, because these drugs are extremely potent vasoconstrictors and, if a peripheral IV infiltrates, extravasation into surrounding tissue can cause severe local ischemia and necrosis. If extravasation occurs, the antidote is phentolamine, an alpha-antagonist injected locally to reverse vasoconstriction and restore blood flow. Infusions are titrated against a target MAP (a physician-set goal preserving organ perfusion), guided by continuous blood pressure monitoring (often via arterial line), heart rate, urine output, and perfusion markers such as lactate. Finally, vasopressors treat a symptom — inadequate perfusion pressure — not the cause. They do not replace fluid resuscitation in hypovolemic or distributive shock, and they should be weaned as the underlying pathology (infection, hemorrhage, pump failure) is treated and the patient's own vascular tone and volume status recover.