Trigeminy is a term used to define a heart rhythm where two normal heartbeats are followed by a premature beat, then (usually) a compensatory pause, with the cycle repeated over and over. In text form the rhythm could be described as “BEAT beat-skip …pause… BEAT beat-skip …pause… BEAT beat-skip …pause… ”
The term trigeminy is used to describe both PACs and PVCs, but is much more commonly used to describe PVCs, and is usually assumed to be referring to premature ventricular contractions, where if the premature contractions are originating in the atria, they will often be described as “trigeminal PACs.”
On the electrocardiogram (ECG or EKG) below you can see trigeminal PVCs, a repeating pattern of two normal sinus beats followed by a premature ventricular contraction.
Thanks to Lisa33167 for the EKG