|MadSci Network: Medicine|
Glycerol trinitrate and other nitrate-containing compounds such as isosorbite mononitrate/dinitrate are very useful in the treatment of heart conditions: in particular, they are most commonly used to treat angina pectoris and acute heart failure. Angina pectoris is the crushing central chest pain that some people suffer when they exercise due to the heart muscle being starved of oxygen. This is usually caused by narrowing of the coronary arteries (by atherosclerosis) that leads to inadequate blood supply to the heart muscle. Nitrates, in simple terms, cause vasodilatation (dilatation of blood vessels) in both the coronary vessels (blood vessels that supply the heart) and more importantly in the systemic venous circulation. The effect is 2-fold: first dilatation of the coronary vessels immediately increases blood supply to the heart muscle and relieves the "oxygen- starvation" (known as ischaemia), but more importantly dilatation of the systemic venous circulation leads to a reduction of blood returning to the heart (reducing venous return) and hence lessen the workload of the heart muscle (by the Starling's Law). The immediate combined effect is that the heart's oxygen demand drops (due to venous dilatation) and its oxygen supply increases (due to coronary dilatation). So the “mismatch” is reduced and the heart muscle ischaemia is temporarily relieved. Glycerol trinitrate (GTN) is often supplied as a spray under the tongue (sub-lingual), since this route is fast, easy to self-administer and can by-pass the liver inactivation. The same effect of decreasing venous return to the heart by systemic venous vasodilatation is also exploited in the treatment of acute heart failure but in this case GTN is usually set-up as an intra-venous infusion instead of sub-lingual spray. I hope this helps:-) Joshua Chai Student Doctor School of Clinical Medicine University of Cambrige, UK
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