Vasopressors

Shock: the problem vasopressors are treating

Shock means the circulation cannot deliver enough oxygen to meet tissue needs. There are four broad categories. Distributive shock occurs when vessels lose tone and dilate excessively, as in sepsis or severe allergic reactions — blood volume is roughly normal, but the container has grown too large. Cardiogenic shock occurs when the heart fails as a pump, as after a large heart attack. Hypovolemic shock occurs when there simply is not enough blood volume, from hemorrhage or severe fluid loss. Obstructive shock occurs when something physically blocks flow, such as a massive pulmonary embolism or fluid compressing the heart.

This classification matters because vasopressors only ever treat the symptom of low pressure — they do not stop bleeding, clear an infection, unblock an artery, or drain fluid around the heart. The cause must be treated in parallel: antibiotics and source control for sepsis, blood products for hemorrhage, clot-directed therapy for embolism, drainage for tamponade. Before assuming a patient needs a vasopressor, volume status must be assessed, since a hypovolemic patient needs fluid or blood first — constricting vessels around an empty circulation can look better on the monitor while organ perfusion still suffers.

Choosing the agent

Norepinephrine is first-line for septic and most undifferentiated shock, constricting vessels while preserving contractility, with a favorable balance between benefit and arrhythmia risk. When higher doses fail to control pressure, vasopressin is usually the second agent added. It is catecholamine-sparing because it works through V1 receptors, a pathway separate from the adrenergic receptors norepinephrine uses. This matters in acidosis, common in severe shock, because acidotic tissue grows less responsive to catecholamines while the vasopressin pathway is not degraded the same way.

Epinephrine fills different core roles: it is the drug of anaphylaxis and cardiac arrest, and also serves as an add-on vasopressor when norepinephrine alone is insufficient, combining vasoconstriction with strong effects on contractility and rate. Phenylephrine is a pure vasoconstrictor with no direct effect on heart rate, useful when a coexisting tachyarrhythmia makes clinicians want to avoid pushing the rate higher. Dopamine now plays a diminished role, carrying meaningfully more arrhythmia risk than norepinephrine at comparable pressor doses.

Dobutamine and milrinone belong in a separate category: they are inotropes, meaning their job is strengthening cardiac contraction rather than constricting vessels, and both can actually lower vascular resistance. They are the mainstay supports in cardiogenic shock, where the problem is a weak pump rather than a dilated circulation. Angiotensin II is a newer option acting through the renin-angiotensin system, sometimes used in distributive shock refractory to standard agents.

Bedside and nursing practice

Vasopressors are titrated to a target mean arterial pressure rather than given as a fixed dose, with the rate adjusted continuously against the patient's response. Central venous administration is strongly preferred because these drugs are intensely vasoconstrictive and tissue-damaging if they leak. Peripheral initiation is often necessary while central access is obtained, done only through a secure line with frequent site checks and a prompt plan to move central.

Extravasation — the drug leaking into surrounding tissue — is a limb-threatening emergency, since the same vasoconstriction that raises pressure can cut off blood flow to tissue around an infiltrated site. Phentolamine, an alpha-blocking agent, is the antidote, injected locally to reverse the constriction and preserve tissue. These lines are never bolused or flushed casually, since a flush can push a concentrated dose into the patient or infiltrated tissue. Infusions run on dedicated lines through pumps programmed under high-alert medication safeguards, since a programming error can swing pressure dangerously within minutes.

Because titration happens minute to minute, an arterial line is often placed for continuous, beat-to-beat monitoring rather than relying on intermittent cuff readings. As the underlying cause resolves and the patient's own vascular tone and cardiac function recover, the infusion is weaned gradually rather than stopped abruptly, watching for rebound hypotension. Throughout, the real target is end-organ perfusion, not the monitor number: clinicians track mentation, urine output, capillary refill, and the trend in serum lactate, reflecting whether tissues are actually getting enough oxygen. An acceptable pressure alongside rising lactate or poor urine output warns that perfusion has not truly improved.