Figure 1.

Microreentry in response to premature action potentials in older, nonuniform anisotropic bundle. Results were obtained at basic stimulus rate of 171/min; a premature stimulus was introduced every 10^th beat at intervals shown in box above each group of waveforms. Drawings at upper left shows locations where each waveform was recorded. Drawing at lower right shows perimeter of reentrant circuit, indicated by solid lines with arrows. Open elongated triangle indicates initial decremental longitudinal conduction to block. Shaded region denotes surrounding areas in which extracellular potentials waveforms were recorded at 16 sites; at each of these peripheral sites, local excitation occurred after that of sites confined to perimeter of reentrant circuit for the corresponding area. This confirmed that the circuit that initiated reentry was limited to region marked by solid lines with arrows. Action potentials measured during regular paced beats at sites above location of stimulus (region A) were greater in duration than those measured in region B. Preparation was from 64-year-old patient with markedly dilated right atrium secondary to underlying atrial septal defect. Used with permission from Spach et al.¹¹