All dual chamber pacemakers are set with an upper rate limit so that if the atria beat abnormally fast, the ventricles will not be paced too fast. This is accomplished by simulation of Mobitz type I (Wenkebach) second degree AV block.
This is the 3 lead ECG of a patient with a dual chamber pacemaker who went into atrial flutter. Note the varying intervals beween the flutter waves and the pacer spikes and how every fourth paced beat is preceded by an increased cycle length.