ICU vasopressor and inotrope pharmacology — comprehensive (catecholamines, vasopressin, inodilators)

Overview & definition

Vasopressors and inotropes are the haemodynamic backbone of ICU shock management. A vasopressor increases blood pressure predominantly through vasoconstriction (↑ systemic vascular resistance, SVR), while an inotrope increases cardiac output through positive inotropy (↑ stroke volume). Several agents do both — adrenaline is a potent vasopressor and inotrope, and the artificial distinction matters most at the bedside when choosing which drug (or combination) matches the patient's haemodynamic phenotype: a vasodilatory shock (low SVR, high CO) needs a vasopressor; a cardiogenic shock (low CO, high SVR) needs an inotrope; the common mixed shock of sepsis often needs both. [1]

The decision tree hinges on three questions: (1) what is the shock phenotype — vasodilatory, cardiogenic, hypovolaemic, obstructive, or mixed? (2) has the patient been adequately resuscitated — vasopressors are no substitute for intravascular volume (giving noradrenaline to a dry patient worsens peripheral/digital/splanchnic ischaemia without fixing the problem); and (3) which receptor profile best matches the desired haemodynamic change? Understanding adrenergic and vasopressin-receptor pharmacology is therefore non-negotiable for the fellowship exam and for safe practice.[8]

The evidence base has matured through a remarkable sequence of trials — SOAP II (dopamine vs noradrenaline), CAT (adrenaline vs noradrenaline), SOSO (adrenaline vs noradrenaline in septic shock), VASST and VANISH (vasopressin), SEPSISPAM (MAP targets), ADRENAL/APROCCHSS/CORTICUS (corticosteroids), and CLOVERS (early restrictive fluids) — which together establish a clear hierarchy: noradrenaline first-line → add vasopressin → reserve adrenaline for refractory shock → inotrope only when low CO is proven → hydrocortisone for refractory vasopressor-dependent shock. [1]

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Adrenergic receptor physiology — the foundation

Every catecholamine vasopressor/inotrope acts at the three adrenoceptors (α1, α2, β1, β2) plus, for dopamine, the dopaminergic D1/D2 receptors. The fellowship exam expects you to know which receptor each drug hits, what intracellular signal each receptor produces, and the resulting end-organ effect. This is the foundation on which all drug selection rests. [1]

Receptor coupling and intracellular signalling

• Alpha-1 (α1) — Gq → phospholipase C → IP3/DAG → ↑ intracellular Ca²⁺. Located on vascular smooth muscle (arterioles and venules). Effect: vasoconstriction (↑ SVR, ↑ venous return/preload). Also on the radial iris (mydriasis), internal sphincters (urinary retention), and pilomotor smooth muscle. This is the receptor that makes noradrenaline a vasopressor.
• Alpha-2 (α2) — Gi → ↓ cAMP. Presynaptic α2 is an autoreceptor that inhibits noradrenaline release (negative feedback). Postsynaptic α2 on vascular smooth muscle causes vasoconstriction. Clonidine/dexmedetomidine are α2 agonists (sedation + sympatholysis).
• Beta-1 (β1) — Gs → ↑ cAMP → ↑ PKA. Located on the heart (SA node, AV node, atria, ventricles). Effect: positive chronotropy (↑ HR), positive inotropy (↑ contractility), positive dromotropy (↑ AV conduction), and positive bathmotropy (↑ automaticity). Also on the juxtaglomerular cells → ↑ renin release. This is the receptor that makes dobutamine an inotrope.
• Beta-2 (β2) — Gs → ↑ cAMP. Located on bronchial smooth muscle (bronchodilation), vascular smooth muscle of skeletal muscle (vasodilation — lowers SVR), uterine smooth muscle (tocolysis), and hepatocytes (glycogenolysis, gluconeogenesis → hyperglycaemia), and stimulates the Na⁺/K⁺-ATPase (→ ↑ glycolysis and lactate — the mechanism behind adrenaline-induced hyperlactataemia).
• Dopamine D1 — Gs → ↑ cAMP. Located on renal, mesenteric, coronary and cerebral vascular smooth muscle → vasodilation (the historical basis for "renal-dose dopamine", now debunked). Also on renal tubules → natriuresis/diuresis.
• Dopamine D2 — Gi → ↓ cAMP. Located presynaptically and in the chemoreceptor trigger zone → anti-emetic/nausea effect; modulates pituitary hormone release (inhibits prolactin). [1]

Vasopressin receptors

• V1 (V1a) — Gq → IP3/DAG → ↑ Ca²⁺. On vascular smooth muscle → vasoconstriction (splanchnic, renal, cutaneous, coronary). Independent of adrenergic receptors — the critical point, because it works when adrenergic receptors are downregulated and it spares catecholamine dose.
• V2 — Gs → ↑ cAMP. On the renal collecting duct → inserts aquaporin-2 channels → water reabsorption (the basis for desmopressin). This is why high-dose vasopressin can cause dilutional hyponatraemia.
• V3 (V1b) — Gq. On anterior pituitary → ACTH release (minor clinical relevance). [1]

The seven agents — receptor pharmacology at a glance

[1] [2] [8]

Detailed drug profiles

Noradrenaline (norepinephrine) — first-line vasopressor

Noradrenaline is the physiological postganglionic neurotransmitter of the sympathetic nervous system and the exogenous agent of choice for vasodilatory shock. It is a potent alpha-1 agonist (vasoconstriction) with a modest beta-1 effect (mild positive inotropy) and negligible beta-2 action. The result is a reliable rise in SVR, MAP, venous return and a slight rise in stroke volume/CO, with a modest increase in heart rate (less than adrenaline). [1]

• Pharmacokinetics: synthetic L-norepinephrine; onset within 1-2 min, peak effect in ~5 min, half-life 2-3 min (so it is titrated as a continuous infusion and stopped/started at bedside). Metabolised by COMT and MAO (monoamine oxidase — caution/contraindication with MAO inhibitors: hypertensive crisis). Cleared in minutes → no loading dose, just titrate to MAP.
• Pharmacodynamics: alpha-1 vasoconstriction (arterioles + veins) → ↑ SVR + ↑ venous return/preload; mild beta-1 → slight ↑ in CO (less than dobutamine). Net effect: ↑ MAP, ↑ SVR, mild ↑ CO, small ↑ HR, modest ↑ in myocardial O2 demand.
• Dosing: start 0.05-0.1 mcg/kg/min, titrate by 0.02-0.05 every 5-10 min to MAP ≥65 mmHg. Typical range 0.05-1.0 mcg/kg/min; doses above 1.0 mark "high-dose" vasopressor — at this point add a second agent (vasopressin) and consider hydrocortisone.
• Adverse effects: peripheral/digital/mesenteric/coronary vasoconstriction (ischaemia), extravasation necrosis (manage with phentolamine), reflex bradycardia at high doses, hyperglycaemia, ↑ lactate (much less than adrenaline). [1]

Why it is first-line: SOSO (Gordon 2018) and CAT (Myburgh 2008) showed noradrenaline is non-inferior to adrenaline for mortality in septic/critical-care shock, with fewer arrhythmias and less hyperlactataemia. SOAP II showed it has fewer arrhythmias than dopamine. It is the Surviving Sepsis Campaign 2021 first-line recommendation.[1][6][8]

Adrenaline (epinephrine) — third-line, anaphylaxis, arrest

Adrenaline is the adrenal medullary hormone — a non-selective agonist at alpha-1, alpha-2, beta-1 and beta-2 receptors. Its effects are dose-dependent: at low doses beta-2 dominates (vasodilation of skeletal muscle beds, bronchodilation); at moderate doses beta-1 dominates (inotropy + chronotropy); at high doses alpha-1 dominates (vasoconstriction). It is the most versatile but also the most arrhythmogenic and lactatogenic agent. [1]

• Pharmacokinetics: onset within 1-2 min IV, peak 5 min, half-life 2-3 min. Metabolised by COMT/MAO. In anaphylaxis the intramuscular route is preferred (vastus lateralis) — IM absorption is reliable even in shock.
• Pharmacodynamics: alpha-1 (vasoconstriction), beta-1 (inotropy + chronotropy), beta-2 (bronchodilation, vasodilation at low dose, ↑ lactate via Na/K-ATPase stimulation of glycolysis, hyperglycaemia via glycogenolysis/gluconeogenesis).
• Dosing: infusion 0.05-1.0 mcg/kg/min (titrate to MAP); anaphylaxis 0.5 mg (500 mcg) IM repeat 5 min (0.3-0.5 mg); cardiac arrest 1 mg IV every 3-5 min (or 2-2.5 mg endotracheal).
• Adverse effects: arrhythmias (more than noradrenaline — atrial and ventricular), tachycardia, hyperlactataemia (beta-2 stimulation of glycolysis — Type B lactic acidosis, not from tissue hypoperfusion), hyperglycaemia, hypokalaemia, myocardial O2 demand. [1]

When to use it: (1) anaphylaxis (IM 500 mcg, first-line); (2) cardiac arrest (1 mg IV after 3rd shock for VF/pulseless VT, alongside amiodarone); (3) symptomatic bradycardia when atropine/pacing fails; (4) severe refractory septic shock as third-line when noradrenaline + vasopressin insufficient; (5) short-term bridge in profound shock before central access. Why not first-line in septic shock: more arrhythmias than noradrenaline (SOSO, CAT) and the lactate rise confounds sepsis monitoring.[1][6]

Vasopressin — catecholamine-sparing, V1

Vasopressin (antidiuretic hormone, ADH) acts at V1 receptors on vascular smooth muscle to cause vasoconstriction independently of adrenergic receptors — the key principle. In septic shock, endogenous vasopressin levels are paradoxically inappropriately low ("vasopressin deficiency") and adrenergic receptors are downregulated by prolonged catecholamine exposure — both of which make exogenous vasopressin a rational second agent. [1]

• Pharmacodynamics: V1 vasoconstriction (splanchnic, renal, cutaneous, coronary — does NOT cause the same reflex tachycardia as noradrenaline, may even slow HR); V2 water reabsorption (negligible at vasopressor doses, but can contribute to hyponatraemia); V3 ACTH release (minor).
• Dosing: fixed 0.03 U/min (0.03 units/min), NOT titrated. There is no benefit above 0.04 U/min and increasing ischaemia (VANISH used 0.06 U/min in the titrated arm without benefit). Start when noradrenaline >0.25-0.5 mcg/kg/min.
• VASST: no overall mortality benefit vs noradrenaline (the trial was underpowered for that), BUT a pre-specified subgroup of less severe septic shock showed a mortality benefit (26.5% vs 35.7%) — the basis for the catecholamine-sparing role.
• VANISH: early vasopressin (titrated up to 0.06 U/min ± hydrocortisone) did NOT reduce kidney failure-free days vs noradrenaline. No mortality benefit. Confirms vasopressin as catecholamine-sparing adjunct, not a superior agent.
• Adverse effects: digital/mesenteric/splanchnic ischaemia (vasoconstriction — the reason it is NOT titrated above 0.04), hyponatraemia (V2), bradycardia, decreased CO (afterload). Stop vasopressin before stopping noradrenaline during weaning (abrupt withdrawal can cause rebound hypotension from vasopressin deficiency).[3][1]

Dopamine — the historical agent now abandoned

Dopamine is the biochemical precursor of noradrenaline. Its dose-dependent receptor profile is one of the most examinable facts in ICU pharmacology: [1]

• Low dose (1-3 mcg/kg/min) — "renal-dose dopamine": D1 (renal/mesenteric vasodilation) + D2. The historical idea that low-dose dopamine "protects the kidney" in ATN — DEBUNKED. Multiple RCTs (e.g., the Australian ANZICS trial, Bellomo 2000) showed NO benefit in renal outcomes, ATN prevention, dialysis avoidance, or mortality. Do not use "renal-dose dopamine."

• Moderate dose (3-10 mcg/kg/min): β1 (inotropy + chronotropy).

• High dose (>10 mcg/kg/min): α1 (vasoconstriction). [1]

• SOAP II (De Backer 2010): dopamine vs noradrenaline in 1679 patients with shock. No difference in overall mortality (52.5% vs 48.5%) — BUT more arrhythmias (24.1% vs 12.4%, p<0.001), and a pre-specified cardiogenic shock subgroup had HIGHER mortality with dopamine. Conclusion: dopamine is not first-line; it causes harm through arrhythmias.

• Surviving Sepsis 2021: weak recommendation AGAINST dopamine in septic shock.

• Remaining niche uses: symptomatic bradycardia when pacing not immediately available (the chronotropic effect is useful), short-term bridge to pacing, occasional peri-arrest scenarios. Avoid in cardiogenic shock (SOAP II cardiogenic subgroup: ↑ mortality).[2][8]

Dobutamine — first-line inotrope

Dobutamine is a synthetic racemic mixture of beta-1 and beta-2 agonists (predominantly β1). It increases stroke volume and CO with a modest increase in HR, and a mild β2 vasodilation that can lower SVR/BP — hence it is frequently co-administered with noradrenaline. [1]

• Pharmacodynamics: β1 (inotropy > chronotropy — less tachycardia than dopamine/isoprenaline) + mild β2 (vasodilation). Net: ↑ CO, ↑ SV, mild ↑ HR, ↓ SVR (may ↓ BP).
• Dosing: 2.5-20 mcg/kg/min, titrate to cardiac output/lactate/SvO2 (NOT to BP — it often lowers BP).
• Indication in septic shock: add when there is persistent low CO/high lactate/low SvO2 despite adequate MAP and volume — not routinely. Clinical/lactate/SvO2 or POCUS evidence of low CO is the trigger, not just "high noradrenaline."
• Indication in cardiogenic shock: first-line inotrope (± noradrenaline for BP), as a bridge to definitive therapy (revascularisation, mechanical support).
• Adverse effects: tachycardia, arrhythmias (less than dopamine/adrenaline), hypotension (β2), ↑ myocardial O2 demand (ischaemia). Tachyphylaxis in 24-48 h (beta-receptor downregulation). [1]

Milrinone — the inodilator for RV failure and pulmonary HTN

Milrinone is a phosphodiesterase-3 (PDE-3) inhibitor — it prevents the breakdown of cAMP, increasing intracellular cAMP downstream of the beta-receptor. This is its key advantage: it works when beta-receptors are downregulated (chronic HF on beta-blockers, prolonged catecholamine infusion, advanced heart failure). [1]

• Pharmacodynamics: ↑ cAMP in cardiac myocytes → ↑ inotropy; ↑ cAMP in vascular smooth muscle → vasodilation (systemic AND pulmonary). Net: ↑ CO, ↓ SVR, ↓ PVR — "inodilator."
• Dosing: 0.125-0.75 mcg/kg/min infusion. The 50 mcg/kg loading bolus causes significant hypotension — omit in ICU patients already on vasopressors. Renally cleared → reduce dose in renal failure.
• Indications: right heart failure / RV infarct, pulmonary hypertension (reduces PVR), weaning from cardiopulmonary bypass, acute decompensated HF (especially if beta-blocked).
• Adverse effects: hypotension (the main one — often needs noradrenaline concurrently), arrhythmias (esp AF), thrombocytopenia (rare). Half-life 2-3 h (vs minutes for catecholamines) — effects persist after stopping, so the hypotension persists; this is a major ICU pitfall. [1]

Phenylephrine, metaraminol — pure vasoconstrictors

• Phenylephrine — pure α1 agonist, no β effect. Ideal when you need vasoconstriction without tachycardia (AF needing pressor, HOCM, tachyarrhythmia-prone). Drawback: reflex bradycardia reduces CO — avoid in cardiogenic shock. 0.5-5 mcg/kg/min, or 100-200 mcg bolus.
• Metaraminol — mixed α1 (potent, both direct and indirect via noradrenaline release) + mild β1. The ANZ/UK first-line rapid-response and peri-operative pressor (cheaper, longer-acting, usable peripherally short-term). 0.5-2 mg IV bolus or 2-10 mg in 500 mL infusion. Indirect action → tachyphylaxis; not ideal for prolonged shock. [1]

Dose-response relationships — what changes as you titrate

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A crucial exam point: adrenaline and dopamine show a dose-dependent receptor switch (β2 → β1 → α1 for adrenaline; D → β1 → α1 for dopamine), whereas noradrenaline does not — it is α1-dominant across its dose range. This is why low-dose adrenaline can paradoxically lower blood pressure (β2 vasodilation) in a vasodilated patient. [1]

Clinical scenarios — which agent, when

Scenario 1: Septic shock (the commonest)

Phenotype: vasodilatory (low SVR, high or normal CO, warm peripheries), often mixed (myocardial depression in severe sepsis → low CO). [1]

Stepwise escalation: [1]

1. Resuscitate: 30 mL/kg crystalloid (Surviving Sepsis), source control, antibiotics within 1 h. CLOVERS (2023) showed an early restrictive fluid strategy (using vasopressors earlier, less fluid) was safe and feasible — non-inferior to liberal fluids, with fewer positive fluid balance harms.[7]
2. Noradrenaline first-line — titrate to MAP ≥65 mmHg (arterial line).[1]
3. Add vasopressin 0.03 U/min when noradrenaline >0.25-0.5 mcg/kg/min (catecholamine-sparing).[3][1]
4. Add hydrocortisone 200 mg/day (± fludrocortisone 50 mcg) when shock is refractory / vasopressor-dependent. ADRENAL: faster shock reversal, no mortality benefit (so reserve for refractory); APROCCHSS: hydrocortisone + fludrocortisone reduced mortality in severe shock.[4][5]
5. Add dobutamine if proven low CO (POCUS, lactate, SvO2 <65%) — NOT routinely.
6. Adrenaline as third-line vasopressor if still hypotensive.

Scenario 2: Cardiogenic shock

Phenotype: low CO, high SVR (cold, vasoconstricted, oliguric), pulmonary oedema. [1]

• Noradrenaline preferred over adrenaline for BP support (fewer arrhythmias, less lactate) — SOAP II cardiogenic subgroup: dopamine ↑ mortality.
• Dobutamine or milrinone for inotropy. Milrinone preferred in RV failure/pulmonary HTN or if beta-blocked; dobutamine preferred if needing rapid titration (short half-life).
• SWAN-Ganz/pulmonary artery catheter or POCUS to guide — define the haemodynamics, don't guess.
• Mechanical support (IABP, Impella, VA-ECMO) if refractory — drugs alone are a bridge, not a destination. Revascularisation is definitive for ischaemic CS. [1]

Scenario 3: Anaphylaxis

Phenotype: vasodilatory + bronchospasm + angioedema — adrenaline is first-line, not noradrenaline (need the β2 bronchodilation + α1 vasoconstriction). [1]

• Adrenaline 0.5 mg (500 mcg) IM (vastus lateralis), repeat every 5 min.
• Adrenaline infusion 0.05-0.5 mcg/kg/min if refractory.
• Fluids (crystalloid 1-2 L rapidly), H1/H2 blockers (chlorphenamine + ranitidine) and steroids (hydrocortisone 200 mg) are adjuncts — never delay adrenaline. [1]

Scenario 4: RV failure / pulmonary hypertension

Phenotype: high RV afterload, low LV preload/CO, often systemic hypotension. [1]

• Noradrenaline to maintain coronary perfusion (RV perfusion is systemic-pressure-dependent — RV ischaemia is a vicious cycle).
• Milrinone (PDE-3) — reduces PVR + provides inotropy. Often needs noradrenaline concurrently for systemic BP.
• Inhaled nitric oxide / nebulised prostacyclin to selectively reduce PVR without systemic hypotension.
• AVOID pure vasoconstrictors that increase PVR (high-dose noradrenaline, phenylephrine) without RV support. [1]

Scenario 5: Vasoplegia post-cardiac surgery / post-bypass

Phenotype: profound vasodilatory shock after CPB (inflammatory response), often with normal/high CO. [1]

• Noradrenaline first-line.
• Vasopressin second agent (often works well — these patients frequently have vasopressin deficiency from haemodilution).
• Methylene blue 1-2 mg/kg (guanylate cyclase inhibitor — emerging therapy for refractory vasoplegia).
• Hydrocortisone if refractory.
• Angiotensin II (ATHOS-3 paradigm) as rescue. [1]

Scenario 6: Tachyarrhythmia-prone patient needing pressors

• Phenylephrine (pure α1 — no tachycardia) or vasopressin (no adrenergic tachycardia).
• AVOID adrenaline, dopamine, dobutamine (all chronotropic/pro-arrhythmic).
• Noradrenaline is usually acceptable (only mild β1). [1]

Scenario 7: Chronic hypertension / elderly

• Target MAP higher — SEPSISPAM subgroup with chronic hypertension had fewer renal events at higher MAP (80-85 vs 65).[1]

Evidence — the key trials

[1] [2] [3] [1] [1] [4] [5] [6] [7] [1] [1]

Compare: noradrenaline vs adrenaline vs vasopressin

[1]

Compare: dobutamine vs milrinone — the inotrope choice

Compare: dopamine vs noradrenaline (why dopamine is out)

[2] [8]

Compare: MAP targets and the role of corticosteroids

[1] [1] [1]

Clinical pearls

Red flags

Prognosis and context

Quick-reference dosing table

[1]

References

1. [1]Gordon AC, Mason AJ, Thirunavukkarasu N, et al. p53 and Me N Engl J Med, 2018.PMID 29791817
2. [2]De Backer D, Biston P, Devriendt J, et al. Health and medicine in Hapsburg Spain: agents, practices, representations. Introduction Med Hist Suppl, 2009.PMID 20235381
3. [3]Russell JA, Walley KR, Singer J, et al. Visual vignette. Adrenal gland ganglioneuroma Endocr Pract, 2008.PMID 18308669
4. [4]Venkatesh B, Finfer S, Cohen J, et al. Adjunctive Glucocorticoid Therapy in Patients with Septic Shock N Engl J Med, 2018.PMID 29347874
5. [5]Sprung CL, Annane D, Keh D, et al. Antibiotics in urology: new essentials Urol Clin North Am, 2008.PMID 18061025
6. [6]Myburgh JA, Higgins A, Jovanovska A, Lipman J, Ramakrishnan N, D'Amico A, CAT Study Investigators Transcranial direct current stimulation improves recognition memory in Alzheimer disease Neurology, 2008.PMID 18525028
7. [7]Shapiro NI, Douglas IS, Brower RG, et al. Adverse professional life experiences may affect mental health among cardiologists Nat Rev Cardiol, 2023.PMID 36869095
8. [8]Evans L, Rhodes A, Alhazzani W, et al. [Pseudogliomatous osteoporosis due to homozygous mutation of the LRP5 gene] An Pediatr (Engl Ed), 2021.PMID 33775580