Outline the pharmacology of intravenous metoprolol.
The use of beta-blockers peri-operatively has been topical for the last few years. There have been conflicting results with large studies, but the evidence has strongly suggested that stopping a patient on long term beta blocker therapy can cause harm.
We are often faced with patients who, for various surgical and hospital reasons, have missed their regular beta blocker dose. This question asks about the pharmacology of intravenous metoprolol, which is one of the more commonly used beta blockers in a theatre environment. We ask this question because we want you to consider why and when you would give this drug, and what the effects of giving this might be.
A. Metoprolol is a non-selective beta antagonist. TRUE/FALSE
Beta blockers can be categorised as non selective or cardio-selective. The cardio-selective are more potent at blocking beta-1 receptors than beta-2, and are therefore much less likely to trigger bronchospasm. This effect can be overcome with higher doses and even cardio-selective beta blockers can become non-selective at higher doses.
B. The metabolism of metoprolol is reliable and predictable. TRUE/FALSE
Metoprolol is a racemic mixture and exhibits stereo selective metabolism when administered orally. It is dependent on oxidation via CYP2D6. CYP2D6 is absent in approximately 8% of the caucasian population.
C. The intravenous dose of metoprolol is the same as the oral dose. TRUE/FALSE
The oral bioavailability of Metoprolol is 50%. This can go up to 70% if there are repeated doses. The biological half life has a large range but in normal metabolisers is 3-7 hours.
D. Metoprolol is not used in congestive heart failure or ischaemic heart disease, due to its effects on contractility. TRUE/FALSE
Metoprolol is used in coronary vascular disease. Why is this?
E. Metoprolol can be used in Raynaud’s due to the vasodilatory effects. TRUE/FALSE
The beta 2 receptors can be activated and cause bronchodilation. They can also cause vasodilation in the vessels in the gut, skeletal muscle and the kidneys.