According to Kevin McGran at the Star:
The Leafs said Gustavsson underwent cardiac ablation, which is a non-invasive type of heart surgery.
From MapleLeafs.com's Mike Ulmer:
It sounds worse than it is, but the Monster is having a minor operation to fix an irregular heartbeat. It’s called Ablation, and it’s a non-invasive surgery in which a catheter is used to remove a non-lethal blockage that is interrupting the heart’s normal operation. Ron Wilson said he had the surgery today and walked in and out of the hospital.
He is expected back on the ice in 72 hours.
Courtesy of Down Goes Brown comes this explanation from the Heart Rhythm Society:
Like many cardiac procedures, ablation no longer requires a full frontal chest opening. Rather, ablation is a relatively non-invasive procedure that involves inserting catheters – narrow, flexible wires – into a blood vessel, often through a site in the groin or neck, and winding the wire up into the heart. The journey from entry point to heart muscle is navigated by images created by a fluoroscope, an x-ray-like machine that provides continuous, "live" images of the catheter and tissue.
Once the catheter reaches the heart, electrodes at the tip of the catheter gather data and a variety of electrical measurements are made. The data pinpoints the location of the faulty electrical site. During this "electrical mapping," the cardiac arrhythmia specialist, an electrophysiologist, may sedate the patient and instigate some of the very arrhythmias that are the crux of the problem. The events are safe, given the range of experts and resources close at hand, and are necessary to ensure the precise location of the problematic tissue.
Once the damaged site is confirmed, energy is used to destroy a small amount of tissue, ending the disturbance of electrical flow through the heart and restoring a healthy heart rhythm.