MANSLAUGHTER, VOLUNTARY - In order for someone to be found guilty of voluntary manslaughter the government must prove that the person killed another person; the person acted in the heat of passion; and heat of passion was caused by adequate provocation.
Heat of passion may be provoked by fear, rage, anger or terror. Provocation, in order to be adequate, must be such as might naturally cause a reasonable person in the passion of the moment to lose self-control and act on impulse and without reflection.
http://www.lectlaw.com/def2/m012.htm
This is what I had always understood to be second degree murder in the American system.
The above site states that the difference between first and second degree is only in the sentence and it's up to the prosecution's discretion whether to charge with first or second degree - seems a tad confusing