Vasopressors and Inotropes Pharmacology
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Direct answer
Vasopressors and inotropes are drugs that raise blood pressure or cardiac contractility by acting on adrenergic and non-adrenergic receptors. Noradrenaline is the first-line vasopressor in septic shock, while dobutamine is the usual choice for low cardiac output with adequate pressure. Selection depends on the haemodynamic problem, so the receptor profile matters more than the drug name.
What you must remember
- Noradrenaline: predominantly alpha-1 with beta-1 activity, and first-line in septic shock with a mean arterial pressure target of 65 mmHg or higher.
- Adrenaline: acts on alpha, beta-1 and beta-2 receptors, and is the drug of choice in anaphylaxis and cardiac arrest, with second-line use in septic shock.
- Dopamine: dose-dependent, acting on dopaminergic receptors at low dose, beta-1 at moderate dose and alpha-1 at high dose.
- Dobutamine: predominantly beta-1 inotrope with some beta-2 vasodilation, used for low output, and it can lower blood pressure.
- Vasopressin: a V1 receptor agonist that works independently of adrenergic receptors, added to noradrenaline in refractory vasodilatory shock.
- Milrinone: a phosphodiesterase-3 inhibitor that raises cAMP, giving an inodilator effect with a risk of hypotension.
- Phenylephrine: a pure alpha-1 agonist, useful when tachyarrhythmia limits catecholamine use.
Matching the agent to the haemodynamic problem
The first question is whether the defect is low pressure, low flow or both. The table maps each drug to its receptor action and the setting where it earns its place.
| Agent | Receptor action | Typical use |
|---|---|---|
| Noradrenaline | Alpha-1 greater than beta-1 | First-line in septic shock |
| Adrenaline | Alpha, beta-1 and beta-2 | Anaphylaxis, cardiac arrest, second-line in sepsis |
| Dopamine | Dose-dependent dopaminergic, beta-1 and alpha-1 | Reserved for specific indications |
| Dobutamine | Beta-1 predominant, some beta-2 | Low output with adequate blood pressure |
| Vasopressin | V1 receptor | Add-on in refractory septic shock |
| Milrinone | Phosphodiesterase-3 inhibition | Inodilator for low output; hypotension risk |
Cold peripheries with low output and adequate pressure point to dobutamine or milrinone. Low pressure with a warm, vasodilated patient points to noradrenaline.
Vasopressin is usually layered on top of noradrenaline rather than substituted for it. Bedside response is judged by perfusion markers such as urine output, mental state and lactate clearance, not by the pressure reading alone.
Where the exam tries to catch you
Pharmacology questions in NEET-PG hinge on receptor selectivity and dose dependence.
- Trap: choosing dopamine for renal protection in shock. Correct: low-dose dopamine does not protect the kidney, and guidelines advise against using it for that purpose.
- Trap: choosing dobutamine when blood pressure is low. Correct: its beta-2 vasodilation can drop pressure further, so a vasopressor such as noradrenaline comes first.
- Trap: using adrenaline as the first-line septic shock drug. Correct: noradrenaline is first-line in septic shock, and adrenaline is kept for refractory cases.
Frequently asked questions
Which vasopressor is first-line in septic shock?
Noradrenaline is the first-line vasopressor in septic shock, titrated to a mean arterial pressure target of 65 mmHg or higher. Vasopressin may be added when noradrenaline doses rise, and adrenaline is a second-line option.
What is the main indication for dobutamine?
Dobutamine is mainly used for low cardiac output with adequate blood pressure, such as cardiogenic shock or low output after cardiac surgery. It is predominantly a beta-1 agonist that increases contractility, though it can lower blood pressure through beta-2 vasodilation.
Why is dopamine no longer used for renal protection?
Low-dose dopamine does not protect the kidney in acute kidney injury, which is why guidelines advise against it for that purpose. At higher doses it causes tachycardia and arrhythmias, so dopamine is reserved for particular indications.
What is the mechanism of action of milrinone?
Milrinone is a phosphodiesterase-3 inhibitor that raises intracellular cAMP, increasing contractility and causing vasodilation. It is an inodilator, so it can cause hypotension and needs dose reduction in renal impairment because it is renally cleared.
How does vasopressin differ from noradrenaline in vasodilatory shock?
Vasopressin acts on V1 receptors and works independently of adrenergic receptors. It is typically added to noradrenaline when vasodilatory shock does not respond adequately, and it is less likely to provoke tachyarrhythmias than catecholamines.