1. A method comprising: obtaining, with a sensing module of a medical device, cardiac electrical signals; processing, with a control module of the medical device, the cardiac electrical signals from the sensing module in accordance with a tachyarrhythmia detection algorithm to monitor for a tachyarrhythmia; detecting, with the control module of the medical device, initiation of a pacing train delivered by a second implantable medical device; determining, with the control module of the medical device, a type of the detected pacing train, wherein determining the type of detected pacing train comprises: computing at least two cycle lengths using at least three most recently detected pacing spikes, estimating a cycle length of the pacing train based on the at least two computed cycle lengths, comparing the estimated cycle length of the pacing train to at least one cycle length threshold, and determining the type of the detected pacing train based on the comparison; modifying, with the control module of the medical device, a tachyarrhythmia detection algorithm based on the type of the detected pacing train; and delivering, with a therapy module of the medical device, a shock therapy based on the tachyarrhythmia detection algorithm.