The power of PACs.

An elderly woman was admitted for palpitation, light-headedness, and syncope. Telemetry strips revealed numerous episodes of supraventricular tachycardia (SVT) as well as premature beats and pauses. Analysis of the effects of spontaneous premature atrial contractions (PACs) on both the sinus discharge rate and on the SVTs revealed strict relationships that helped decipher the electrophysiological mechanisms of the arrhythmias and offered a rational approach to treatment. Invasive electrophysiological evaluation was not needed. A hitherto unreported phenomenon was the predictable termination of reentrant SVTs by spontaneous PACs. This instructional exercise shows the power of PACs in triggering and terminating arrhythmias, as well as their ability to serve as diagnostic tools in arrhythmia analysis.