An implantable cardiac device is programmed to detect and classify premature atrial contractions (PACs) and administer responsive pacing therapy. The responsive pacing therapy is in the form of an atrial extrastimulus, which is intended to preempt initiation of a reentrant tachycardia. The atrial extrastimulus is timed to occur late enough after a PAC to ensure atrial capture, but early enough that the resulting atrial depolarization does not conduct through the AV node to the ventricles if the PAC has already done so. If both of these criteria cannot be met, the device may be configured to inhibit the atrial extrastimulus.