Can I be sued?

This is very unlikely. In English law, for someone to be held liable it would have to be shown that the intervention had left someone in a worse situation than if there had been no intervention. In the case of a cardiac arrest under discussion (i.e. someone is technically dead following a cardiac arrest) it is very unlikely that this would arise. No case brought against someone who tried to provide first aid has been successful in the UK, where the courts have tended to look favourably on those who try to help others (UK Resuscitation Council 2010).
In the workplace, someone using an AED will be shielded by the Employer’s Liability Insurance against any litigation if the person dies (Resuscitation Council, 2013).