Situations and certain drugs do induce panic attacks, as well as past medical histories like Nathan discussed.
Of course , you want to treat them, so the best thing to do is understand how the body reacts inside when this happens.
Under certain drugs, situations, etc., the CO2 level will become very sensitive. The more CO2 that you take in , the harder it will feel to breath. In rooms packed with people or airplanes, alot of CO2 is given off......anyone sensitive to it can't stay in the room too long.....they panic and have to leave. Once in fresh air they can start to calm down and get rid of all the CO2. Valium was an excellent choice to take. Any of the benzo's can help calm you down. Canada sells sublinqual Lorazapam. This would be excellent for you to carry with you until you realize what is causing the attack exactly. Pop it under your tongue and in 5 minutes it's absorbed. But you have to get rid of the CO2 first. If you feel it coming on again I'd get outside for fresh air first. If it passes a little, but your still shaky....then take a benzo, such as the valium you took before.
Hope this helps some.