Guidant is developing a wireless "advanced patient management" system for defibrillators, and is evaluating whether heartratevariability is a good indicator of attacks.
They quantify your stress level by calculating your heartrateand the minute variations in the time between each beat, a measurement known as heartratevariability (HRV), which has been used to gauge the likelihood of mortality after a heart attack and to help assess certain psychiatric conditions.
If you're curious about your heart-rate variabilityand the other data that can be gleaned from it, I'd recommend the HeartMath Inner Balance for a comprehensive approach.
It measures what HeartMath calls coherence an algorithm applied to heart-rate variability, which the company says can reflect emotional states and stress levels.