Direct acting sympathomimetic amine with potent alpha1 agonist actions.
Causes a rapid rise in systemic vascular resistance and BP. No effect on beta receptors
Phenylephrine presentation and uses
Presented as a clear solution containing 10mg in 1mL. Bolus doses of 50-100mcg are used IV. Continuous infusion of 20-100mcg per minute are used too.
It is used to increase SVR associated with spinal anaesthesia or systemically administered drugs.
CVS - Raises SVR and BP in may result in a reflex bradycardia (BRR response).
CNS - nil
Renal - RBF decreases
Rapid rise in BP lasts about 5-10 minutes.
Metabolised by MAO
Highly potent synthetic catecholamine with actions at B1 and B2 receptors. Has no alpha effects
Isoprenaline Presentation and uses
Clear solution containing 1mg/mL for IV infusion.
Used IV to treat severe bradycardia associated with complete heart block of beta-blocker overdose
CVS - Stimulation of B1 receptors increases heart rate, myocardial contractility, automaticity and cardiac output.
Effects on BP are varied. B2 effects may drop SVR so that the increase in CO is not enough to maintain MAP.
Respiratory - Potent bronchodilator and inhibits histamine release in the lungs. V/Q mismatch will increase.
CNS - stimulant effects
Splanchnic - mesenteric and RBF increased
Metabolic - B effects = increased blood glucose
Low bioavailability due to extensive first pass metabolism.
Rapidly metabolised by COMT in the liver.
Direct acting synthetic catecholamine derivative of isoprenaline. B1 effects predominate but has a small affect at b2 receptors
Dobutamine presentation and uses
Presented in 20mL of water containing 250mg dobutatmine and sodium metabisulfite.
Used to increase cardiac output in patients with heart failure or cardiogenic shock
Dose range 0.5-20mcg/kg/min
CVS - Main actions are direct stimulation of b1 receptors = increased contractility, heart rate, myocardial 02 requirement. Blood pressure usually increased due to increased CO (more than b2 activation).
May precipitate arrhythmias due to increased AV conduction
Only administered IV. Rapidly metabolized by COMT in the liver to inactive compounds which are excreted in the urine. Half life 2 min.
Ephedrine presentation and uses
Clear solution for injection containing 30mg/mL.
Used IV to treat hypotension associated with anaesthesia. Can also be used to treat bronchospasm
Ephedrine mechanism of action
Has both direct and indirect sympathomimetic actions.
Activates alpha and beta receptors directly (b1, B2, alpha1) but also inhibits the actions of MAO on noradrenaline and decreases uptake plus increases release from vesicles. This causes increased norad present in the synapse for longer periods of time.
Due to its indirect actions it is prone to tachyphylaxis as norad stores in vesicles deplete.
CVS - increases CO, HR, BP, myocardial 02 consumption. May precipitate arrhythmias
Respiratory - bronchodilation
Renal - RBF and GFR decreased
Interactions - should be used in extreme caution in patients taking MAOI
Not metabolised by MAO or COMT so has longer duration of action and elimination half life of four hours.
Metaraminol mechanism of action
Synthetic amine with both direct and indirect sympathomimetic actions. Acts mainly via alpha 1 receptor but retains some beta adrenoreceptor activity
Also stimulates noradrenaline release from vesicles
Metaraminol presentation and uses
Clear solution, 10mg/mL. Used as IV bolus 0.5-1mg or infusion 0.5-10mg/hr.
used for hypotension associated with anaesthesia, haemorrhage etc
CVS - Increase SVR. Often causes a decreased CO due to increased SVR with little beta stimulation
Milrinone mechanism of action
Selective PDE III inhibitor.
Decreases hydrolysis of cAMP leading to increased IC concentration which in turn increases calcium concentration in myocardium and vascular smooth muscle.
Milrinone preparation and uses
Yellow solution containing 1mg/mL. Stored at room temp. Should be diluted before administering.
Used for short term management of cardiac failure
CVS - increased inotropy and vascular smooth muscle dilatation = increased cardiac output
Decreased pulmonary artery pressures and SVR
Can decrease platelet aggregation