Even 'return false' would best 92% accuracy.

I suspect it's probably worse than that in reality. From a quick search on state of the art ECG results (the full system of leads attached all over your torso) it looks like around 90 percent specificity (True negative rate) and under 50 percent sensitivity (true positive rate). So it's only pretty good at ruling out heart attacks, but still misses them sometimes. But is pretty bad when it comes to false alarms. I think they use it along with multiple other tests and consideration of symptoms in triage at the hospital.