Dopamine

Dopamine is a catecholamine precursor of noradrenaline and a dose dependent agonist of dopaminergic, beta 1 and alpha 1 receptors. It is used as an infusion in shock with bradycardia or low cardiac output when other agents are unavailable. Low dose renal protection does not work and is not advised.

Classification. Catecholamine; inotrope and vasopressor.

Mechanism. Low doses (about 2 to 5 micrograms/kg/min) stimulate dopamine receptors and cause mesenteric and renal vasodilation. Intermediate doses (5 to 10 micrograms/kg/min) stimulate beta 1 receptors and release noradrenaline from nerve endings. High doses (over 10 micrograms/kg/min) cause alpha 1 mediated vasoconstriction.

Pharmacodynamics. Onset within 5 minutes; effect lasts less than 10 minutes. Dose bands overlap, so patients vary. More tachycardia and arrhythmia than noradrenaline.

Pharmacokinetics. Inactive orally. Half life about 2 minutes. Metabolised by monoamine oxidase and catechol O methyltransferase; about 25% is converted to noradrenaline in nerve terminals. Does not cross the blood brain barrier.

Hypotension and shock with bradycardia or low cardiac output, adult.

• Start 2 to 5 micrograms/kg/min IV.

• Increase by 1 to 4 micrograms/kg/min every 10 to 30 minutes against blood pressure, perfusion and rhythm.

• Usual range 5 to 20 micrograms/kg/min; maximum 50 micrograms/kg/min (labelled), though doses over 20 usually add toxicity rather than benefit.

• Septic shock: noradrenaline is preferred; dopamine is an alternative only in selected patients with low risk of tachyarrhythmia and with bradycardia or low cardiac output.

Symptomatic bradycardia unresponsive to atropine, adult. 5 to 20 micrograms/kg/min as a bridge to pacing. This matches the option of an adrenaline infusion.

Children, shock. 2 to 20 micrograms/kg/min IV or intraosseous, titrated to effect.

Renal dose dopamine. Not advised: no proved benefit for kidney function.

Contraindications. Phaeochromocytoma; ventricular fibrillation or untreated tachyarrhythmia; hypersensitivity to sulphite (some products).

Warnings. Extravasation causes necrosis. Tachyarrhythmias, hypertension and ischaemia at higher doses. Correct hypovolaemia first. Reduced effect in patients with depleted noradrenaline stores. Cardiac index may fall at high doses from afterload rise.

Adverse effects. Tachycardia, ectopic beats, chest pain, hypertension, headache, nausea, vomiting, peripheral ischaemia, dyspnoea.

Interactions. Monoamine oxidase inhibitors: reduce dopamine starting dose to one tenth of usual. Tricyclic antidepressants, linezolid and oxytocic drugs: hypertensive crisis. Phenytoin IV: hypotension and bradycardia. Beta blockers: antagonise the effect. Halogenated anaesthetics: arrhythmia. Alpha blockers antagonise high dose effect.

Special groups.

• Pregnancy: use only if benefit outweighs risk; may reduce uterine blood flow.

• Breastfeeding: no data; short term critical use acceptable.

• Children and neonates: dose by weight; watch pulmonary and peripheral perfusion.

• Older adults: start at the lowest dose.

• Kidney and liver impairment: no adjustment.

Extravasation. Infiltrate phentolamine 5 to 10 mg in 10 mL 0.9% sodium chloride around the site promptly.

Formulations. Injection 40 mg/mL and 200 mg/5 mL concentrate; premixed bags.

Preparation. For example 200 mg in 250 mL 5% glucose or 0.9% sodium chloride (800 micrograms/mL). Central line preferred; large peripheral vein only briefly. Do not mix with sodium bicarbonate or other alkaline solutions. Use a pump.

Baseline. ECG, blood pressure, volume status, potassium.

Monitoring. Continuous ECG and arterial pressure, urine output, peripheral perfusion, lactate, infusion site. Reduce or stop for heart rate over 140/min in adults or ventricular ectopy.

Weaning. Reduce gradually; abrupt cessation can cause hypotension.

Common prescribing errors. Using low dose infusion for kidney protection; confusing micrograms/kg/min with micrograms/minute; running through a small peripheral vein; failing to reduce the dose with monoamine oxidase inhibitors.