Objectives: We have observed sensing errors in third generation implantable cardioverter-defibrillators that appear to be caused by variation in the R wave amplitude during sinus rhythm, particularly after premature beats. The purpose of this study was to quantify spontaneous R wave variability during sinus rhythm and to determine whether abrupt changes in cycle length further augment R wave amplitude variability.
Background: Pacemaker sensing algorithms presume a relatively constant R wave signal to establish a sensing threshold. The concept of a fixed sensing threshold is not as applicable in third-generation cardioverter-defibrillators, which depend on automatic gain amplifiers to rapidly detect ventricular fibrillation. These devices may be susceptible to undersensing during sinus rhythm if significant variability in R wave signal characteristics occurs.
Methods: Twelve patients with combination bradycardia pacing cardioverter-defibrillators were studied. The device used (Cadence, Ventritex) allowed recording of real time, telemetered electrograms from the sensing lead system. Measurements were made of the maximal range of the R wave amplitude during sinus rhythm and in response to abrupt changes in heart rate produced by premature atrial and ventricular stimuli.
Results: The maximal range in R wave amplitude during sinus rhythm was 1.7 +/- 1.3 mV, or 23.7 +/- 19.2% of the mean R wave amplitude. The R wave amplitude variability increased with abrupt changes in cycle length, with a range of 2.8 +/- 1.5 mV, or 38.8 +/- 18.3% of the mean R wave amplitude (p < 0.05 compared with sinus rhythm). In most patients, R wave amplitude and coupling interval demonstrated an inverse proportional relation.
Conclusions: There is substantial variability in the R wave amplitude during sinus rhythm measured by permanent ventricular sensing lead systems, and this variability is further augmented by abrupt changes in cycle length. This phenomenon may explain the occurrence of undersensing of sinus rhythm in implantable cardioverter-defibrillators with automatic gain sense amplifiers.