In classifying ventricular arrhythmias, a fundamental distinction is made between focal and macroreentrant mechanisms. Focal arrhythmias demonstrate early activation at a particular site with centrifugal spread of the impulse from this site of origin. In contrast, macroreentrant rhythms involve contiguous but spatially distributed areas of activation. Despite advances in multielectrode and electroanatomical mapping, the distinction of a focal from a macroreentrant mechanism is sometimes problematic.