Pharmacology for Anaesthesia and Intensive Care

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Section IIICardiovascular drugs

varied. Theβ 2 effects may drop the systemic vascular resistance so that the increase

in cardiac output is insufficient to maintain blood pressure. Myocardial oxygen

delivery may decrease significantly when tachycardia reduces diastolic coronary

filling time and the reduced diastolic blood pressure reduces coronary perfusion.

Some coronary vasodilatation occurs to attenuate this.


Respiratory – it is a potent bronchodilator and inhibits histamine release in the

lungs, improving mucous flow. Anatomical dead space and ventilation perfusion

mismatching increases, which may lead to systemic hypoxaemia.


Central nervous system – isoprenaline has stimulant effects on the CNS.


Splanchnic – mesenteric and renal blood flow is increased.


Metabolic – itsβeffects lead to a raised blood glucose and free fatty acids.

Kinetics

When administered orally it is well absorbed but extensive first-pass metabolism

results in a low oral bioavailability, being rapidly metabolized by COMT within the

liver. A significant fraction is excreted unchanged in the urine along with conjugated

metabolites.

Dobutamine

Dobutamine is a direct-acting synthetic catecholamine derivative of isoprenaline.

β 1 effects predominate but it retains a small effect atβ 2 -adrenoceptors.

Presentation and uses

Dobutamine is presented in 20 ml water containing 250 mg dobutamine and sodium

metabisulphite or in 5 ml water containing 250 mg dobutamine and ascorbic acid. It

is used to augment low cardiac output states associated with myocardial infarction,

cardiac surgery and cardiogenic shock (dose range 0.5–20μg.kg−^1 .min−^1 ). It is also

used in cardiac stress testing as an alternative to exercise.

Effects


Cardiovascular – its main actions are direct stimulation ofβ 1 -receptors resulting in

increased contractility, heart rate and myocardial oxygen requirement. The blood

pressure is usually increased despite a limited fall in systemic vascular resistance

viaβ 2 stimulation. It may precipitate arrhythmias including an increased ventric-

ular response rate in patients with atrial fibrillation or flutter, due to increased AV

conduction. It should be avoided in patients with cardiac outflow obstruction (e.g.

aortic stenosis, cardiac tamponade).


Splanchnic – it has no effect on the splanchnic circulation although urine output

may increase following a rise in cardiac output.