P wave tracking above the maximum tracking rate in a DDDR pacemaker.
Dual-chamber, rate-modulated pacemakers have recently become available. These devices, whose rate response is determined by the combined input from the intrinsic atrial rate and the sensor-driven rate, have led to some unexpected upper rate behaviors. In the case presented, with the maximum sensor pacing rate at 150 ppm, multiple episodes of apparent P wave tracking occurred above the maximum P wave tracking rate of 100 ppm. This behavior is explained by P waves that inhibit sensor-driven atrial output; inhibition may result in variable maximum tracking rates that are equal to the current sensor-driven rate. This effect appears to have minimal physiological consequence. Its importance lies in its recognition as normal DDDR function so that searches for pacemaker malfunction will not be made.
Higano
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Division of Cardiovascular Diseases and Internal Medicine, Mayo Clinic, Rochester, Minnesota 55905.
Hayes
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D000328:Adult; D006327:Heart Block / Q000628:therapy; D006801:Humans; D008297:Male; D010138:Pacemaker, Artificial