Activation mapping from the coronary sinus may be limited by anatomic variations.

The purpose of this study was to examine the anatomic relationship between the mitral annulus (MA) and the coronary sinus (CS). Fifty consecutive hearts of 31 men and 19 women, 63.5 +/- 13.7 years of age, were examined at autopsy. MA was divided perpendicularly to the annular plane into an anteromedial block and a posterolateral block by sectioning from the CS ostium through the center of MA. The posterolateral block was subdivided radially into five equal sections at 36 degrees, 72 degrees, 108 degrees, 144 degrees, and 180 degrees. The distance from the ventricular endocardium under MA to the nearest wall of CS (D) was measured in each cross-section. D measured 9.7 +/- 2.3, 10.9 +/- 3.3, 10.2 +/- 3.6, 9.2 +/- 3.4, and 8.2 +/- 2.9 mm at 36 degrees, 72 degrees, 108 degrees, 144 degrees, and 180 degrees, respectively. D at 72 was significantly longer than at 144 degrees and 180 degrees (P < 0.01). Likewise, D at 108 degrees was significantly longer than at 144 degrees and 180 degrees (P < 0.05). The population was divided according to the morphology into five patterns. "Type C," the pattern that separated in the middle section and then reapproximated, was more common (66%) than any other pattern. D was confirmed to be longest at the level of the 72 degrees section, corresponding to a left posterolateral free-wall location. The potential mapping in CS would be easily modified by this anatomic feature. When mapping activation from the CS, the electrophysiological data should be interpreted in light of these anatomic findings.