The main risk is the "Good Samaritan" situation. A patient suffering from angina that has the nitro in the pocked and just pass by when he see som one with something that can looks like a heart attack....
Nitro isn't available over the counter here, it's prescribed by a Doctor who knows the patients background. If one has Nitro without the Doctors knowledge, then I agree it is a dangerous thing.
I would not dare to use it!!
The problem is that unless that you have an scanner, I think there is no way to distinguish an ischemic from an hemorrhagic stroke and, if it is the second, and you administer nitro the chances to survive are lower.
By the way.... speaking about the heart attack it is also a very sensitive point, unless you know the medical background of the patient... to separate a HA from a tear in the aorta it is not that easy...
Jesus.
Now that's a million dollar question. Even during a 'real' heart attack, nitro is not likely to help because the blockage is total. Clots are a jelly like substance which will expand with the vessel. The best solution is clot busting medication and I'm not sure if paramedics carry such things with them.