Programmed Atrial Stimulation and Rapid Atrial Pacing in Patients with Sinus Pauses and Sinoatrial Exit Block

Abstract

Although Eyster and Evans (1915) and Levine (1916) described second-degree sinoatrial block in ECG recordings many years ago they realized that other possible sinoatrial conduction disturbances eluded analysis, primarily because electrical activity in the sinus node cannot be directly recorded in vivo. In 1973, Strauss et al. proposed that the pre- mature atrial stimulation technique could be used to derive the sinoatrial conduction time. In the application of this technique a programmable stimulator is used to elicit variably coupled atrial premature depolarizations during spontaneous sinus rhythm. The last undisturbed spontaneous sinus cycle (A₁A₁), premature cycle (A₁A₂) and return cycle (A₂A₃) are analyzed and the normalized return cycles (A2 A3/A1 A1) are plotted as a function of the normalized premature cycles (A₁A₂/A₁A₁) (Figure 1). Late atrial premature cycles are followed by compensatory return cycles. Atrial premature depolarizations elicited earlier in atrial diastole are followed by less than compensatory return cycles. The duration of the less than compensatory A₂A₃ cycle is determined by the retro- grade conduction time from the atrium to the sinus node (A₂SAN₂), the sinus node return cycle (SAN₂SAN₃) and the antegrade sinoatrial conduction time (SAN₃A₃). The estimation of the sinoatrial conduction time is based on the assumption that the sinus node return cycle (SAN₂SAN₃) equals the spontaneous sinus node cycle (SAN₁SAN₁) or the corresponding spontaneous atrial cycle (A₁A₁) (Figure 1). Hence, the difference between the atrial return cycle (A₂A₃) and the spontaneous atrial cycle (A₁A₁) should equal the retrograde conduction time (A₂SAN₂) for A₂ and the antegrade conduction time (SAN₃A₃) for the subsequent sinus node depolarization (SAN₃). The difference between A₂A₃ and A₁A₁ computed for atrial premature depolarizations falling in the latest third of the reset zone (zone II) is used to derive a mean value for the estimated antegrade and retrograde sinoatrial conduction time (SACT_A+R) (Strauss et al., 1976). It is also assumed that the sum of the A2SAN2 + SAN3A3 equals a value that is twice the normal antegrade sinoatrial conduction time (SAN₁ A₁). For this reason many investigators have divided SACT_A+R by 2 to obtain a value that approximates the antegrade sinoatrial conduction (Dhingra et al., 1975; Breithardt et al., 1976; Jordan et al., 1977). Since antegrade and retrograde conduction times may not be equal, (Bonke et al., 1969; Klein et al., 1973; Miller and Strauss, 1974) we have chosen to express our sinoatrial conduction times as the total value, i.e. SACT_A+R.

Supported in part by the U.S. Public Health Service Grants HL 19216, 05736, 15190,07101,01613 and RR 30 from the General Clinical Research Centers Program of the Division of Research Resources, National Institutes of Health, and by a Research Career Development Award (1-K04-HL-00268) to Dr. Strauss, by an American Heart Association Teaching Scholar Award to Dr. Scheinman, and by a Medical Research Council of Canada, Fellowship Award to Dr. Benditt.