Is it dangerous for a heart failure patient to abruptly discontinue atenolol?

Depends: If the heart failure is due to coronary disease, then abrupt discontinuation of a beta blocker could be a problem. If there is a compelling reason to hold the beta blocker in other circumstances, it can be held; just remember that beta blockers are important adjuncts to heart failure treatment, so coordinate with the doctors.

Yes: Stopping beta blockers (such as atenolol) can be dangerous due to a 'rebound' effect of unopposed activation of beta receptors. The beta blockers should be gradually discontinued if there is a need to. Thus being said, atenolol is probably not a great beta blocker to treat systolic heart failure; the best are bisoprolol, Carvedilol and metoprolol succinate.