Michael Lohan -- Undergoing Surgery to Fix His Bad Heart

Michael Lohan's life is in the hands of doctors ... who are performing surgery on Mike's heart in an effort to unclog an important artery ... this according to Lohan's rep.

TMZ broke the story ... Lohan had trouble breathing and speaking a couple of weeks ago ... and docs believe the trouble stemmed from Mike's ticker.

Now, the rep tells us ... Lohan is currently undergoing a procedure to "repair a blockage."