Vasopressors & Inotropes

Shock is the state where blood pressure and flow fall so low that organs stop getting the oxygen they need. Blood pressure depends on two things: how hard the vessels squeeze (systemic vascular resistance) and how much blood the heart pumps (cardiac output). Different shocks fail differently — septic shock dilates the vessels; cardiogenic shock weakens the pump.

Vasopressors and inotropes are the emergency levers. They borrow the body’s own fight-or-flight (adrenergic) signaling to squeeze vessels, strengthen the heart, or both — buying time while the underlying cause is treated. They are high-alert, titrated to effect, usually in an ICU.

These drugs act on the same adrenergic receptors beta-blockers oppose, but in the opposite direction — they *stimulate* them. Alpha-1 (α1) receptors on blood vessels cause vasoconstriction (↑ resistance, ↑ BP). Beta-1 (β1) receptors on the heart increase rate and contractility (↑ cardiac output). Beta-2 (β2) relaxes vessels and airways.

Each drug is really a *recipe* of receptor effects. Norepinephrine (mostly α1, some β1) is the first-line pressor in septic shock — strong squeeze, modest cardiac boost. Phenylephrine is pure α1 (squeeze only). Dobutamine is mostly β1 — a pure inotrope for the failing pump, but it can lower BP. Epinephrine hits α and β hard (arrest, anaphylaxis). Dopamine’s effect shifts with the dose. Knowing the recipe tells you what each drug does and what to watch for.

The very vasoconstriction that saves a patient centrally becomes destructive if the drug leaks out of the vein. Extravasation of a vasopressor clamps down the local vessels so hard that the tissue loses its blood supply and can die (necrosis). This is why vasopressors are preferably given through a central line, and why the antidote — phentolamine, an α-blocker injected around the site — must be known.

Systemically, too much α1 squeeze can raise the workload on the heart and cause reflex effects and arrhythmias; too much β1 can cause tachyarrhythmias and ischemia. Every dose is a balance between enough perfusion pressure and too much.