Innate collateral segments are predominantly present in the subendocardium without preferential connectivity within the left ventricular wall

Key points Innate collateral arteries provide the biophysical substrate for arteriogenesis. Their distribution and morphology predestine tissue areas salvageable by collateral flow. Fluorescent episcopic cryomicrotome imaging resulted in a three-dimensional representation of the coronary network, in which collateral segments were automatically identified. Innate collateral segments are predominantly present in the subendocardium without preferential connectivity within the left ventricular wall of the dog heart and are preferentially oriented perpendicular to the long axis of the heart in the outer layers and parallel to this axis at the subendocardium. These results suggest that collateral segments are maintained without local hypoxia but because of heterogeneity in pressure gradients in the arterial tree. The high density and long-axis orientation of collateral arteries in the subendocardial region provide the substrate for arterial plexus formation and indicate the need for three-dimensional perfusion assessment in clinical perfusion imaging. Abstract Functional collateral vessels often stem from outward remodelling of pre-existing connections between perfusion territories. Knowledge of the distribution and morphology of innate collateral connections may help in identifying myocardial areas with protection against risk for ischaemia. The coronary network of six healthy canine hearts was investigated with an imaging cryomicrotome. Innate collateral connections ranged from 286 to 1015 μm in diameter. Left ventricular collateral density (number per gram of tissue) was about five in the subendocardium vs. 2.5 in the mid-myocardium (P < 0.01) and 1.3 in the epicardium (P < 0.01). Subendocardial collateral connections were oriented parallel to the long axis of the heart. For the major coronary arteries, five times more intracoronary than intercoronary connections were found, while their median diameter and interquartile range were not significantly different, at 96.1 (16.9) vs. 94.7 (18.9) μm. Collateral vessels connecting crowns from sister branches from a stem are denoted intercrown connections and those within crowns intracrown connections. The number of intercrown connections was related to the mean tissue weight of the crowns (y = 0.73x − 0.33, r2 = 0.85, P < 0.0001). This relation was likewise found to describe intercoronary connections. The median collateral diameter and length were independent of the tissue volumes bridged. We conclude that connectivity and morphology of the innate collateral network are distributed with no preference for intra- or intercrown connections, independent of stem diameter, including epicardial arteries. This renders all sites of the myocardium equally protected in case of coronary artery disease. The orientation of subendocardial collateral vessels indicates the longitudinal direction of subendocardial collateral flow.