Vasopressors and Inotropes in Shock
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Direct answer
Norepinephrine is the first-line vasopressor for distributive and most undifferentiated shock, started at 0.01 to 0.05 micrograms per kg per minute and titrated to a MAP of 65 mmHg; vasopressin 0.03 units per hour is added as the catecholamine-sparing second agent, and hydrocortisone enters once shock persists on escalating doses. Epinephrine is the drug of anaphylaxis, cardiac arrest and refractory septic shock; dobutamine or milrinone support the low-output phenotype once pressure is restored.
What you must remember
- Doses worth carrying into the exam: norepinephrine 0.01 to 1 microgram per kg per minute (usual range 0.05 to 0.5), epinephrine 0.01 to 0.5, phenylephrine 0.1 to 3, dobutamine 2.5 to 20, milrinone 0.125 to 0.75 microgram per kg per minute, vasopressin fixed at 0.03 units per hour.
- SOAP II settled norepinephrine over dopamine — similar mortality but significantly fewer arrhythmias; "renal-dose dopamine" protects nothing and is obsolete.
- MAP target is 65 mmHg; SEPSISPAM showed no outcome gain from pushing to 80 to 85 mmHg, only more atrial fibrillation — except perhaps in chronic hypertension with prior stroke signals, where individualisation is allowed.
- Vasopressin is not titrated: fixed 0.03 units per hour, added when norepinephrine exceeds roughly 0.25 to 0.5 micrograms per kg per minute, and usually weaned first as shock resolves.
- Terlipressin, given as 0.5 to 2 mg boluses, is widely used in Indian ICUs — first-line with albumin in hepatorenal syndrome and a practical bolus vasopressor where infusions are difficult.
- Angiotensin II (ATHOS-3) rescues a minority with refractory distributive shock; start around 20 nanograms per kg per minute and titrate.
- Dobutamine for low cardiac output with adequate MAP; milrinone where beta-adrenergic desensitisation or beta-blockade matters — but reduce the dose in renal impairment and expect vasodilation.
Choosing by phenotype
Sequence the drug to the physiology. Distributive septic shock: volume plus norepinephrine, vasopressin second, hydrocortisone 200 mg per day when the dose creeps up, epinephrine third-line. Cardiogenic shock after infarct: norepinephrine is still the preferred pressor over dopamine, with dobutamine added once MAP allows, and an early cardiology conversation about revascularisation — the IABP-SHOCK II trial demoted routine balloon pumping. Obstructive shock from massive pulmonary embolism: norepinephrine to hold coronary perfusion, cautious fluids (a distended right ventricle does not want more preload), and reperfusion therapy. Anaphylaxis: intramuscular epinephrine 0.5 mg, then an infusion, with aggressive volume — no norepinephrine-first hedging. Right ventricular failure with sepsis: prefer norepinephrine over pure alpha agents, because phenylephrine raises pulmonary vascular resistance and can crush the right ventricle that dobutamine or milrinone is trying to unload. Vasoplegia after cardiopulmonary bypass: vasopressin, and methylene blue 1 to 2 mg per kg as the salvage idiom. Two housekeeping rules complete the answer: norepinephrine may run peripherally for hours when a central line is pending, and no vasopressor replaces finding the source.
Where students slip
The exam trap is a table without physiology. "Which vasopressor in septic shock?" earns full marks only with SOAP II cited and dopamine explicitly rejected. Candidates forget vasopressin is fixed-dose — titrating it marks the answer down. Milrinone chosen in a hypotensive anuric patient ignores its vasodilation and renal clearance. "Add dobutamine for low BP" conflates pressure with flow: dobutamine in an under-filled patient drops pressure. And the Indian viva staple: know terlipressin's place — hepatorenal syndrome, variceal bleeding, and as bolus vasopressin — because it is asked precisely because India uses it generously.
Frequently asked questions
What is the first-line vasopressor in septic shock?
Norepinephrine, 0.01 to 1 microgram per kg per minute titrated to MAP 65 mmHg; dopamine is reserved for exceptional bradycardic situations.
Why is vasopressin added rather than titrated?
It acts as a catecholamine-sparing second agent at a fixed 0.03 units per hour; higher doses add ischaemia without benefit.
What MAP target does evidence support?
65 mmHg for most patients; SEPSISPAM found targeting 80 to 85 mmHg offered no survival benefit and increased atrial fibrillation.
Which inotrope when blood pressure is adequate but output is low?
Dobutamine 2.5 to 20 micrograms per kg per minute, or milrinone 0.125 to 0.75 where vasodilation and renal function permit.
What role does terlipressin have in Indian practice?
Boluses of 0.5 to 2 mg for hepatorenal syndrome with albumin, variceal bleeding, and as a bolus alternative to vasopressin infusion.
Which vasopressors suit massive pulmonary embolism with shock?
Norepinephrine for pressure with careful volume, plus urgent reperfusion — avoid phenylephrine, which worsens pulmonary vasoconstriction.