The "smart" escape interval: an innovative approach to cardiac pacing

This study attempts to improve cardiac pacing by utilizing a new strategy based on integrative rather than substitutive stimulation. This type of cardiac stimulation does not compete with the sinoatrial oscillator and can position the stimulated beat at the expected time. This can be achieved by locking the pacemaker's escape interval to the sinus cycle, making it a variable parameter continuously updated on the preceding cycle. When the rate and rhythm are regular, the pacemaker's activity is completely "concealed," although the escape interval continues to be updated. When a sudden conduction block occurs, the pacemaker positions the stimulated beat where we would expect the natural beat to be. When the natural heart rate goes outside the locking field by being too low, the pacemaker loses the lock and returns to its free-running frequency. However, an alternative choice is to pass to "overdrive" stimulation: this occurs when the escape interval is locked in phase to the sinoatrial oscillator, but is set to have a period equal to one-half or one-third of the sinus cycle. When applied to sinus bradycardia, this can multiply the natural heart rate by 2 or 1.5, but the rate responsiveness is maintained because it is locked to a natural "sensor." When applied to bicameral pacing, the phase-locked pacemaker marks a jump from atrial triggered stimulation to a "true" atrial synchronous stimulation. In this novel situation, the ventricular stimulator remains relatively independent with respect to the atrial rhythm while the tracking of the sinusal drive is ensured. This feature could prove useful in reducing both the incidence of pacemaker mediated tachycardias and effects such as myopotential interference.