Re: Why do heart patients sometimes need nitroglycerine tablets?

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