Emergency Vasopressors

Emergency vasopressors raise blood pressure by constricting blood vessels and sometimes increasing heart rate and contractility. They are given when low blood pressure threatens organ perfusion and fluids alone have not restored it. Septic, anaphylactic, neurogenic, anaesthetic and cardiogenic shock each favour different drugs. Vasopressors are not a substitute for fluids, source control or oxygen.

Classification. Catecholamines (noradrenaline, adrenaline, dopamine); selective alpha agonists (phenylephrine, metaraminol); vasopressin and analogues; angiotensin II; ephedrine; methylene blue (adjunct).

Mechanism. Alpha-1 stimulation causes vasoconstriction. Beta-1 stimulation raises heart rate and contractility. Vasopressin acts on V1 receptors. Angiotensin II acts on AT1 receptors.

Pharmacokinetics. Onset is within seconds by infusion and the half-life is 2 to 3 minutes for catecholamines, so doses are titrated every few minutes. Vasopressin has a half-life of 10 to 20 minutes.

Targets. Aim for a mean arterial pressure of 65 mmHg or more, adjusted to the patient (higher in chronic hypertension). Judge by urine output, lactate, skin perfusion and mental state.

Administration. Use a central line when possible. In emergencies, start through a large peripheral vein or intraosseous line at low concentration and move to central access soon. Use dedicated lines and infusion pumps.

Extravasation. Stop the infusion, aspirate, and infiltrate phentolamine 5 to 10 mg in 10 mL of sodium chloride 0.9% into the area.

• First-line. Noradrenaline in septic, cardiogenic and most vasodilatory shock; adrenaline in anaphylaxis and arrest; phenylephrine, metaraminol or ephedrine in anaesthetic hypotension.
• Alternative. Vasopressin added when noradrenaline reaches 0.25 micrograms/kg/minute; adrenaline when noradrenaline is unavailable.
• Specialist. Angiotensin II, methylene blue and combinations in refractory shock.
• Emergency. Shock with MAP under 65 mmHg despite fluids, cardiac arrest, anaphylaxis, neurogenic shock.

Selection. Choose by cause. Combine agents rather than escalate a single drug to high dose. Reassess volume, rhythm and cardiac function.

Contraindications. No absolute contraindications in life-threatening shock; relative: uncorrected hypovolaemia, severe arrhythmia, phaeochromocytoma crisis (without alpha blockade), mesenteric ischaemia.

Serious adverse effects.

• Arrhythmia, tachycardia, myocardial ischaemia, hypertensive overshoot.
• Limb, gut and kidney ischaemia, digital necrosis, skin necrosis from extravasation.
• Lactic acidosis, hyperglycaemia and tachyphylaxis (adrenaline).
• Bradycardia, reflex slowing (phenylephrine); hyponatraemia (vasopressin).

Major interactions. Monoamine oxidase inhibitors, tricyclic antidepressants and linezolid potentiate responses; beta-blockers reduce effect of adrenaline and cause unopposed alpha effects; halothane raises arrhythmia risk; alkaline solutions inactivate catecholamines in the same line.

Common prescribing errors.

• Starting without correcting volume or finding the cause.
• Wrong concentration or unit (micrograms/kg/minute versus micrograms/minute).
• Stopping an infusion suddenly.
• Giving through a small peripheral vein for long periods.
• Using dopamine as first choice.

Special groups.

• Pregnancy: use the usual adult drugs; phenylephrine or ephedrine in obstetric anaesthesia; left uterine displacement.
• Breastfeeding: compatible after the acute episode.
• Children: weight-based; adrenaline and noradrenaline infusions use paediatric tables.
• Older adults: lower starting doses and greater arrhythmia risk.
• Kidney: no adjustment; ischaemia risk.
• Liver: titrate to effect.
• East Africa: noradrenaline may not be stocked everywhere, and adrenaline, ephedrine and dopamine may be the practical options; infusion pumps and central lines may be limited, so prepare a written dilution chart and use burettes or syringe drivers; anaphylaxis needs intramuscular adrenaline first.

Monitoring. Continuous ECG and arterial or frequent cuff blood pressure, lactate, urine output, peripheral perfusion and infusion site every hour.

Referral and escalation. MAP under target on two agents, rising lactate, arrhythmia, limb ischaemia, or need for specialist agents or mechanical support.

Comparison with alternative classes. Vasopressors restore perfusion pressure quickly but do not fix cardiac output or the cause. Inotropes support the heart. Fluids correct volume. Steroids support refractory septic shock.