Described herein are methods, devices, and systems for improving R-wave detection sensitivity and positive predictive value, and for improving arrhythmia detection accuracy. Certain embodiments involve determining whether to classify a potential R-wave as a false R-wave (or more specifically, an over-sensed P-wave) by determining a measure of magnitude of a first portion of the signal corresponding to a first window following the potential R-wave, determining the measure of magnitude of a second portion of the signal corresponding to a second window following the first window, and classifying the potential R-wave as a false R-wave if the measure of magnitude of the second portion of the signal is at least a specified extent larger (e.g., at least 3 times larger) than the measure of magnitude of the first portion of the signal. Certain embodiments also involve adjusting an R-wave marker for a potential R-wave that is classified as a false R-wave.