Introduction
Hyperpolarisation-activated and cyclic nucleotide-gated (HCN) cation (Na+/K+) currents are generated by four members of the HCN-channel family – HCN1–4.1 The currents of these channels, often designated as If (funny channel) or Ih (hyperpolarisation-activated cation channels), are activated by membrane hyperpolarisation to produce a graded depolarisation in phase 4 of the cardiac action potential. The most prominent channel in the pacemaker region of the heart, essential to the automaticity of the sinus node, is the HCN4 channel.1 Historically, pathogenic variants in HCN4 have been associated with a sick sinus syndrome phenot
