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Glyceryl Trinitrate - page 3
Keywords: GTN, Glyceryl trinitrate, Nutrition and Energy
By Einstein10 on 30/12/2009
Level: Foundation Degree
Page Number: 3 of 6 pages: 1 2 3 4 5 6in first-pass metabolism by the liver, therefore saving time and an increased number of active metabolites. These are the reasons why the onset of effect for sublingual tablet is between 1-3 minutes as shown in the table above.
The duration of action of the sublingual tablet is usually very short, approximately 30 minutes. The reason for this is that the GTN tablet is given in a very small dose of between 0.15-0.6 mg. Also because GTN has a very short half-life of about 5 minutes it is quickly metabolised by the liver and after there are no active metabolites left, GTN’s potency decreases with no further affects on the patient.
With regards to a Transdermal patch; although the GTN is absorbed well through the skin because the skin is also a much vascularised organ, it must cross the epidermis, dermis and hypodermis of the skin to reach the general blood circulation. Also the GTN in the Transdermal patch is released via a rate-limiting membrane or matrix which means that the GTN is released steadily over a period of time maintaining a stable blood concentration over a period of 24 hours.
These patches are normally available in 3 dosages. A 5mg patch which releases 5mg/24 hour, a 10mg patch which releases 10mg/24 hours and finally a 15mg patch which releases 15mg/hour. For example in a 15mg patch, the GTN could be released at a rate of 1.6mg/hour. So by the time this 1.6mg is metabolised by the liver another 1.6 mg is released meaning there is a constant supply. This achieves better symptomatic control in a patient from acute angina or prophylaxis. Steady release of the GTN means that it takes between 30-60 minutes to take effect. This method is usually used as a long-term treatment.
Suxamethonium is an analogue of acetylcholine
which binds to nicotinic receptors causing
depolarisation of the motor endplate.
It is used as a muscle relaxant during surgery.
2. Why is suxamethonium not absorbed after oral administration (7 marks)?
Suxamethonium is a short acting, depolarising neuromuscular blocker that is not lipid soluble. This means it is unable to cross the phospholipid endothelium bi-layer of the gut wall by diffusion. Also Suxamethonium contains 2 quaternary amino groups with a charged Nitrogen atom making it a very polar molecule and therefore lipo-phobic. The molecular mass of this drug is 290.401 and molecules larger than 100-200 Daltons are unable to cross cell membranes
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