FRCA Notes

Dobutamine & Dopexamine

  • Dobutamine is a direct-acting synthetic catecholamine derivative of isoprenaline with predominant β1 effects, although it still retains some β2 effects
  • Dopexamine is a synthetic catecholamine analogue of dopamine

Presentation

  • Racemic mixture
    • Both enantiomers are β1 full agonists, and the (+) enantiomer has some β2 activity
    • The (+) enantiomer is an ɑ1 receptor competitive antagonist, whereas the (-) enantiomer is a partial agonist; the net effect is some partial agonist activity
  • 250mg in 20ml water + sodium metabisulfite; or
  • 250mg in 5ml water + ascorbic acid

Uses

  • To augment cardiac output in low cardiac output states e.g. MI, cardiac surgery and cardiogenic shock (0.5 - 20μg/kg/min)
  • As an alternative to exercise in stress testing

Pharmacodynamics

  • Positive chronotropy, inotropy and myocardia oxygen requirement
  • BP normally maintained despite some reduction in SVR
  • May precipitate arrhythmias
  • Should not be used in those with cardiac outflow obstruction (aortic stenosis, tamponade)
  • No effect on splanchnic circulation

Pharmacokinetics

  • Only available for IV administration
  • VD 0.2L/kg
  • Rapidly metabolised by COMT to inactive metabolites
  • Metabolites are conjugated and excreted in urine
  • Half-life 2mins

Presentation

  • Presents as a 10mg/ml solution of pH 2.5 for IV use

Uses

  • Used to improve cardiac output and mesenteric perfusion (0.5 - 6μg/kg/min)

Pharmacokinetics

  • Rapidly cleared from the blood with a half-life of 7mins

Pharmacodynamics

  • Stimulates β2 and D1 receptors
    • Minimal effect at β1 and D2 receptors
    • No effect at alpha receptors

  • Bronchodilation via β2 effect

  • Cardiovascular
    • Reduces afterload due to peripheral β2 effect, with some positive inotropy due to β1 effects
    • Small increase in coronary blood flow and no change in myocardial oxygen extraction
    • Rarely precipitates arrhythmias

  • Splanchnic
    • Increased renal and mesenteric blood flow via β2 effects (increased CO/reduced SVR)
    • Increased urine output as a consequence

  • Can cause nausea/vomiting, tremor and headache