Primary negativity does not predict dominant pacemaker location: implications for sinoatrial conduction

Activation sequence maps derived during normal sinus rhythm from extracellular potentials in the canine right atrium exhibit widely separated sites of origin. The objectives of this study were to characterize the distribution of pacemakers within the right atrium and to determine the relationship of pacemaker action potentials to sites of earliest surface activation as well as to local extracellular electrograms. The right atria of six adult mongrel dogs were rapidly excised under deep pentobarbital sodium anesthesia and perfused with 95% O2-5% CO2 Krebs-Henseleit solution. Action potentials from the epicardial surface were recorded throughout the region bounded by the crista terminalis laterally and the atrial septum medially. Simultaneously, unipolar extracellular electrograms were recorded from 250 endocardial sites. The earliest pacemakers preceded the earliest electrogram by 63 +/- 34 ms; the latest pacemakers followed the earliest electrogram by 71 +/- 40 ms. Primary negativity in the extracellular electro gram did not predict the site of the earliest or dominant pace maker and in some cases was associated with the latest pace makers. We conclude that primary negativity and/or the sites of earliest activation reflect the point at which the impulse engages atrial myocardium, not the site of earliest pacemaker activity. As such, early extracellular activation appears to represent sites of exit from a relatively insulated sinus node.