This will sound odd to most of you, but it's a serious question. A friend on the Cardiac Athletes forum asked recently whether anyone with an ICD (pacemaker) has ever been jolted while swimming open water. ICDs are set to give you a shock if the pulse gets too slow or too squirrly. You know like, "CLEAR! ZAP!", except there's no one shouting "clear", just the unannounced zap. The jolt is like a punch in the chest and takes the wind out of you for a few seconds. He's a bit nervous about swimming in a lake with this thing. I have an artificial aortic valve, but not an ICD.