iPhone heart monitor is a creative way to use iPhone sensors
9to5mac.com/2008/09/29/iphone-heart-monitor-is-a-creative-way-to-use-iphone-sensors
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
This one belongs in the "what will they think of next category". Some enterprising programmers managed to make a heart monitor out of the microphone in the iPhone headset.
While this is a cool idea and fun for most, we wouldn't recommend it just yet for those with heart conditions.