EFFECT OF RATE AND COUPLING INTERVAL ON ENDOCARDIAL-R WAVE AMPLITUDE VARIABILITY IN PERMANENT VENTRICULAR SENSING LEAD SYSTEMS

Objectives. We have observed sensing errors in third generation implan table cardioverter-defibrillators that appear to be caused by variatio n in the R wave amplitude during sinus rhythm, particularly after prem ature beats. The purpose of this study was to quantify spontaneous R w ave variability during sinus rhythm and to determine whether abrupt ch anges in cycle length further augment R wave amplitude variability. Ba ckground. 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 cardioverte r-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 combinati on bradycardia pacing cardioverter-defibrillators were studied. The de vice used (Cadence, Ventritex) allowed recording of real time, telemet ered 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 atri al and ventricular stimuli. Results. The maximal range in R wave ampli tude 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 sinu s rhythm). In most patients, R wave amplitude and coupling interval de monstrated an inverse proportional relation. Conclusions. There is sub stantial variability in the R wave amplitude during sinus rhythm measu red by permanent ventricular sensing lead systems, and this variabilit y is further augmented by abrupt changes in cycle length. This phenome non may explain the occurrence of undersensing of sinus rhythm in impl antable cardioverter-defibrillators with automatic gain sense amplifie rs.