Pacemaker, ArtificialHeartHeart RateBiological ClocksHeart BlockSinoatrial NodeSick Sinus SyndromeHeart FailureEquipment FailureHeart DiseasesAtrioventricular BlockBradycardiaHeart Defects, CongenitalHeart TransplantationHeart AtriaHyperpolarization-Activated Cyclic Nucleotide-Gated ChannelsFetal HeartCircadian RhythmHeart ValvesElectrodes, ImplantedInterstitial Cells of CajalSuprachiasmatic NucleusDevice RemovalTime FactorsAction PotentialsAtrioventricular NodeEquipment SafetyMyocardiumArrhythmias, CardiacHeart Conduction SystemCyclic Nucleotide-Gated Cation ChannelsPeriodicityElectrocardiographyMyocardial Contraction