Dose range is 0 01-0-3 micrograms kg-1 per minute. Epinephrine is a potent 01, 02 and a agonist. The cardiovascular effects of epinephrine depend on dose. At lower doses (0 005-0 02 micrograms kg-1 per minute which is the equivalent of 1 ml h-1 of 5mg epinephrine in 50 ml saline), pi stimulation predominates, that is, increased contractility, heart rate, and hence cardiac output. There is some stimulation of 02 receptors causing bronchodilation and vasodilatation in certain vascular beds (skeletal muscle). Systemic vascular resistance may fall, which explains the reduction in diastolic blood pressure that is sometimes seen. However, blood pressure tends to rise because 01 effects predominate. Epinephrine also enhances the rate of myocardial relaxation, to allow increased diastolic filling time.
Alpha stimulation becomes more predominant with increasing doses leading to vasoconstriction, which increases systolic blood pressure. There is also visceral bed vasoconstriction at these higher doses. There is a more marked increase in myocardial oxygen consumption than is seen with dobutamine. Other effects include a fall in plasma potassium and a rise in serum glucose. Although its vasoconstrictor effects tend to limit its use, epinephrine is useful in low cardiac output failure unresponsive to dobutamine. This is because increasing cardiac output, but not blood pressure, in severe hypotension may not succeed in improving flow to vital organs that depend on a critical perfusion pressure (the kidneys, coronary arteries, and brain). The indications for and disadvantages of epinephrine are given in Box 6.3.
Box 6.3 Epinephrine (adrenaline) indications and disadvantages
• Low cardiac output states especially if severe hypotension Disadvantages
• Tachycardia and arrythmias
• Myocardial ischaemia
• Splanchnic vasoconstriction
• Hypokalaemia and hyperglycaemia
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