FIGURE 7.

Cardiac alternans occur at faster pacing cycle lengths. (A) Representative optical action potentials, and (B) calcium transients from a Langendorff-perfused guinea pig heart. Slower PCL (200 ms) produces uniform action potentials and calcium transients (left), while a faster PCL (140 ms) results in cardiac alternans (right). Traces were acquired from two different regions of interest (solid line—left ventricle, dotted line—right ventricle) and discordant alternans were readily observable in the calcium signal. Four sequential beats show regional differences in alternans in calcium duration maps. (C) Relative occurrence of cardiac alternans in different species (mouse, rat, guinea pig). Epicardial dynamic pacing applied to the left ventricle for all studies (n = 5–18 independent heart studies per species).