What does nitroglycerin do during a heart attack?

Reduce pain: There is actually some debate about the use of nitroglycerine in the setting of all heart attacks. It is known that nitroglycerine dilates the veins and to a lesser extent the arteries and sort of "unloads" the heart. As such it will make it easier for the heart to pump under the stressful circumstances of a heart attack. However, when the blood pressure is low, it is not a good idea.

Dilation: Nitrates like Nitroglycerin cause the arteries to dilate so more blood flows (higher rate). This is to try to mitigate the lack of blood flow to the affected part of the heart through increased flow in the affected arteries and in collateral circulation.