Techniques for determining whether a lead related condition exists based on analysis of a cardiac electrical signal associated with a non-sustained tachyarrhythmia (NST) are described. In some examples, the techniques include determining the duration of intervals between consecutive cardiac events, e.g., R-R intervals, during an NST. The techniques may further include determining one or more metrics based on the durations of the intervals during the NST. Examples of metrics include an average, a minimum, a maximum, a range, a median, a mode, or a mean. A lead related condition is identified based on the values of the one or more metrics, e.g., by comparison to respective thresholds. In some examples, an alert is provided or a therapy modification is suggested if a lead related condition is identified.