An implantable medical device includes a voltage measurement circuit to measure a potential difference between implanted electrodes in a thorax of a living being, the potential difference resulting from an electrical P-wave cardiac signal. The implantable medical device also includes a processing unit to calculate a vector corresponding to the P-wave cardiac signal, the vector comprising a magnitude and a direction, and derived from measured potential differences and orientations defined by locations of the implanted electrodes. The implantable medical device further includes a monitoring unit to track a rotation of the vector corresponding to the P-wave cardiac signal. In various implementations, the monitoring unit may use the rotation to detect an inter-atrial block condition.