Guidant is developing a wireless "advanced patient management" system for defibrillators, and is evaluating whether heart rate variability is a good indicator of attacks.

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While there are many heart rate apps available, Inner Balance is monitoring heart rate variability (HRV) - a measurement of beat-to-beat changes that occur within heart rhythms and a sophisticated and highly sensitive observation of emotional fluctuations.

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They quantify your stress level by calculating your heart rate and the minute variations in the time between each beat, a measurement known as heart rate variability (HRV), which has been used to gauge the likelihood of mortality after a heart attack and to help assess certain psychiatric conditions.

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The Zen Index uses heart-rate variability to quantify stress levels in a simplified manner, according to the company.

WSJ: Review of Tink�� and HeartMath Inner Balance Sensor for iOS

It measures what HeartMath calls coherence an algorithm applied to heart-rate variability, which the company says can reflect emotional states and stress levels.

WSJ: Review of Tink�� and HeartMath Inner Balance Sensor for iOS

For the next 17 months, the scientists tracked various metrics of emotional well-being, such as heart-rate variability and levels of stress hormone.

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If you're curious about your heart-rate variability and the other data that can be gleaned from it, I'd recommend the HeartMath Inner Balance for a comprehensive approach.

WSJ: Review of Tink�� and HeartMath Inner Balance Sensor for iOS

Heart-rate variability, says Dr Stein, is a good index of the degree of control an individual has over his autonomic nervous system (which controls many of the internal organs).

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