Cell Properties & Conduction** Heart cells have four properties: automaticity, excitability, conductivity, contractility [1, 2]. Conduction travels from the SA node (60–100 bpm) through the AV node (40–60 bpm), bundle of His, bundle branches, to Purkinje fibers (20–40 bpm) [1, 3]. Vagal tone slows heart rate (HR); sympathetic activity increases HR and contractility [1]. Resting potential features high internal K+, external Na+; depolarization (Phase 0) is rapid Na+ influx [1, 4].

ECG Basics & Waveforms Leads II and V1 are used for monitoring [2]. Grid: horizontally, small square = 0.04s, large = 0.20s; vertically, small = 0.1 mV, large = 0.5 mV [5].

* P Wave (0.06–0.12s): Atrial depolarization [6].

* PR Interval (0.12–0.20s): Atrial-ventricular conduction [6].