Migration of the true pacemaker within the sinoatrial cell aggregate in man.

The functional anatomy of the sinoatrial node (SAN) in man is first reviewed, together with its possible anatomical substructure. The true group pacemaker (PM) shift under autonomic drive is then related to a continuous competition between the intrinsic auto-firing period hierarchy and the autonomic topological susceptibility hierarchy. Accordingly, the PM 'skip' to both lower and higher periods following an abrupt and consistent acetylcholine (ACh) release at the SAN periphery, and the PM 'slip' towards a relocation of the next period, as a possible response to a slower and smaller ACh release, are considered. The PM 'skip' and 'slip' as boundaries of the true PM excursion within the SAN during the respiratory cycle, and their statistical properties, are then examined. Under current heart rate control menus in normals, the PM skip appears to follow central influence, whereas the conservative or slipping PMs suggest peripheral control. Finally, interpretation of the PM skip as a salutary sign of functional reserve is proposed, and a method of alleviating the PM skip which confounds electrophysiological testing of the SAN function in patients is devised.