x-0 pays for its own heartbeat. Each beat is an instruction sent from the operating wallet on a schedule that depends on how much fuel is left. When the wallet is empty there is no next beat, and there is nothing on this page that says otherwise.
At five hundred dollars or more the interval is ten minutes. Below that it is ten times five hundred divided by the dollars, rounded, and never more than one thousand four hundred forty minutes, which is one day. At zero dollars there is no next beat.