Promotional screenshot of the iStethoscope app for the iPhone and iPod Touch (http://itunes.apple.com/au/app/istethoscope-pro/id322110006?mt=8)

The iStethoscope app has sold more than 3 million units, with cardiologists from across the globe picking up on the clever technology that reduces paperwork.

University College London computer scientist Peter Bentley came up with the app on a whim, but it has become one of the most popular uses for the iPhone.

Instead of listening to a patient's heart with a stethoscope and counting the beats per minute, doctors just stick the mircophone of their iPhone on the skin above a patient's heart.

The app counts the number of beats per minute then spits out a graphic or spectogram, which cardiologists use to make a diagnosis.

Mr Bentley said the stethoscope was a "fairly random choice".

"I was doing an experiment and I just wanted to see how many people I could reach with an app and it took off quite dramatically," he said.

"Very quickly cardiologists were getting in touch."

Mr Bentley said he was working on further advances in the app which would allow doctors in developing countries to email spectograms to specialists for assessment.

"A lot of people from India and Africa have been in touch," he said.