Presumably a single lead would still be able to detect certain arrhythmias? I.e. if there are no QRS complexes, then there may be something wrong with the person wearing the device. There are benign explanations for why the signal might be weak but also lethal causes like Vfib -- with a few additional, I don't think it's implausible for a device to be able to make the decision to ask the user "hey, are you okay?" and then potentially seek help if the user doesn't confirm that they are OK within some set period.