Circumvention of Maximum Tracking Limitations with a Rate Modulated Dual Chamber Pacemaker

A patient having high grade AV block with intact sinus node function is presented in whom DDDR pacing provided the benefit of preventing 2:1 pacemaker block in response to exercise-induced sinus tachycardia. In paired treadmill tests with the patient blinded as to pacing mode, she was able to exercise longer (7.5 vs 6.6 METS) when programmed in DDDR than in DDDO. This is attributable to circumvention of 2:1 pacemaker block which had resulted in abrupt onset of fatigue and SOB (shortness of breath) when the sinus rate exceeded the maximum tracking rate of 130/min. Outpatient ambulatory electrocardiographic monitoring confirmed this phenomenon during relatively strenuous activity. The theoretic advantages of dual chamber rate modulated pacing compared to the DDDO and VVIR modes are discussed.