What approach(es) have you tried, and what do you have to work with as far as graphics go?
You might consider working with video of a beating heart as a guide... see the detail?... video will run at at least 24 frames per sec... you may need that high a frame rate if you are looking for realism.
Do a frame grab sequence of several beats of heart video and then place those frame grabs on the bottom layer of your project....convert each to MC so you can set the opacity to 50% or whatever to sort of give you a guide.....then trace or whatever you are doing to sort of resemble the actions of the beating heart.
Remember, video is just a really fast slide show! ... still frames flipping past at 1/24th of a second or faster!... not that much different than a Flash animation.
Have you tried using Flash's Motion Presets? Go to Window > Motion Presets. There is a preset call pulse that you can use.
Convert your heart graphic into a movie clip symbol. Make sure your symbol is selected on stage. Click on pulse in the Motion Presets panel and hit the Apply button.
Hope this helps,
Flash Pro QE